Gwent Group
Advanced Materials systems

PRINTING THICK FILM HYBRIDS

Alan Hobby

Applications Manager
DEK Printing Machines Ltd.
March 1997

1. Introduction.

2. Fundamentals of Screen Printing.

3. Printing Machines.

4. A Typical Print Set up.

5. Special Techniques.

 

1. INTRODUCTION

The manufacturing process which is characteristic of thick film hybrid production is the screen printing process. Many other processes are used during thick film hybrid manufacture - resistor trimming, component placement, wire bonding, soldering etc. - and these are used in many other circuit manufacturing techniques. In none of these other techniques, however, is screen printing used to directly form the circuit elements.

A commonly held opinion of screen printing is “It’s a black art”. There are, however, some people who treat it without total success as an exact science. The truth is somewhere between these two extremes but tending towards an exact science. To screen print successfully the technician needs to understand the fundamentals of the process, to work as near to conditions of good practice as possible and to be prepared to make sensible adjustments to obtain the best results, not simply to work to some pre-ordained settings and hope for the best.

Screen printing has several advantages over other methods. The tooling to produce the pattern is relatively cheap and simple, the machinery need not be complex, and the surface on which the print is to be delineated need not be flat. The two principal advantages, however, are that the thickness of the print is greater than those given by other methods and that the thickness is controllable. These two features enable the desired electrical performance of the thick film to be achieved.

2. FUNDAMENTALS OF SCREEN PRINTING

Four items are essential in order to screen print. These are a printing medium (thick film paste), a screen to define the required patterns, a surface onto which the print will be made (the substrate) and a squeegee to force the paste through the screen.

Figure 1 shows a schematic arrangement of the various parts. The screen is held above the substrate, paste is applied to the screen and the squeegee travels over the screen, pressing it down into contact with the substrate pushing the paste through the screen, thus depositing paste onto the substrate surface.

This is not the place to say any more about pastes and substrates than they should be formulated for screen printing and be reasonably flat respectively. Their choice should be made after discussion with the material manufacturer. However, some discussion of screens and squeegees is appropriate.


Fig 1. The basic screen print process.

2.1 Screens

A screen is composed of three sections, a stencil which defines the pattern to be printed, a mesh which supports the stencil, and a frame which supports the mesh. It is convenient to examine these in reverse order.

2.1.1 Frames

For thick film applications screen frames are nearly always made of metal and usually of aluminium alloy. While wood and plastic frames are used for industrial and fine art screen printing, these materials are not sufficiently stable for thick film applications. The load supported by each side of even a small screen frame when the mesh is attached to it is in the order of 50 kg and so the strength and stability of metal is necessary. Screen frames are usually cast or of welded construction and machined to flatness tolerances of typically ±0.15 mm.


Fig 2. Image distortion.

In graphic art or industrial applications it is quite common to image the screen within 30 or 40 mm of the edge of the frame. Figure 2 shows, however, that the image will be distorted.

A simple calculation will show that the image width will increase from 240 mm to some 240.104 mm (+0.04%). Precision will often demand therefore, that a ratio of screen size to image size of 1.5 to 1 or better still 2 to 1 is adopted, in which the case the image on the above screen would be increased in size by 0.03% or 0.02%.

2.1.2 Mesh

Three types of mesh material are commonly available, these being nylon, polyester and stainless steel. Nylon monofilament is strong and resilient and so can be thoroughly abused. It would be an ideal screen material if it were stable. Unfortunately, because it can absorb water, it is affected by changes in humidity. It is also affected by changes in temperature and so can only be used for the least critical applications such as printing the manufacturer’s name onto the finished hybrid. Its ability to stretch makes it possible to print onto irregular surfaces, for instance, into the bottom of low walled package cavities, but such use is rare.

Polyester is very much more stable than nylon, it is resilient and sufficiently flexible to conform with the normal irregularities found in the surface of thick film circuits. Polyester is the most common mesh used for industrial screen printing and can often be used for thick film printing.

Stainless steel is stronger and more stable than either nylon or polyester. It has a much higher modulus of elasticity but a lower elastic limit, so it is easily damaged by denting or coining. Being harder than nylon or polyester it is less susceptible to wear and so could give a longer life. However, it will almost certainly be accidentally broken long before it becomes worn.

Tables 1, 2 and 3 show the dimensional characteristics of typical nylon, polyester and stainless steel meshes respectively. The number of threads per centimetre and per inch, thread diameter, mesh opening, open area or open surface and cloth thickness are reasonably self explanatory but one or two comments may be in order. Nylon and polyester mesh counts have suffixes of S, T, HD and HD Super and these relate to the thread diameter. The thinnest is “S” or “Serigraphic grade”, i.e. for screen printing while “T” means “Textile grade”. The thick thread version, HD has heavy duty applications, while the yet thicker HD Super version is even heavier duty. In spite of the “Serigraphic” name, the most frequently used of these mesh grades is T.

It is quite common to talk about, say, “a 145 stainless steel screen”. This clearly lacks precision. While most thick film manufacturers would assume that this refers to 145 threads per inch, it could equally refer to 145 threads per centimetre. In either case, it could be of ultra thin, standard or heavy duty wire. To help to resolve this area of doubt, Bopp, the major manufacturer of stainless steel mesh, use a designation of two figures, the first specifying the mesh opening, the second the wire diameter, both in microns. Whichever method of specifying mesh is used, it helps if both the purchaser and the supplier agree.

Two rules of thumb are useful to help choose a mesh. The first is that the minimum line width which can be printed with a given mesh is three times the mesh thread diameter. Clearly therefore, narrow lines cannot be printed with large thread diameter mesh.

(From a purely practical point of view, it is unwise to use mesh with wire diameters less than 24 mm, i.e. below SD 53/24, the ultra thin 325 stainless steel. The mesh becomes both very fragile and expensive and an especially good reason is necessary for the use of these meshes. SD 53/24 will meet the great majority of fine line production requirements.)

The second rule is that the mesh opening should be at least three times the particle size of the paste being printed. In normal thick film applications, the paste particles are unlikely to be more than 5 or 10 mm in size and so the finest meshes can be used. However, solder paste has particles typically in the range of 25-70 mm and this clearly restricts the choice of mesh quite severely.

The open area of the mesh has a great effect on the passage of paste through the screen. For ease of passage, meshes having the greatest percentage open area should be selected. Open area is calculated as:

Equation 1

The cloth thickness is approximately twice the thread diameter. In the case of nylon and polyester meshes it is always a little less than two diameters however. If the mesh were simply woven it would be unstable. After weaving, it is passed between heated rollers to weld the crossovers together, thus holding the filaments in place. This has the effect of reducing the mesh thickness to a little under two threads diameter. In the case of stainless steel however, no such welding is necessary. The wires are deformed during weaving and so hold themselves in position.


Fig 3. Range of mesh thickness

Figure 3 shows that the range of thickness which could be produced by the weaving process varies from two to three thread diameters. In practice, it is normal to find that the mesh thickness is a little thicker than two wire diameters but much less than three. There is rather more variation in stainless steel mesh thickness than in those of nylon or polyester. This is caused not by large variations in wire diameter but by the variations in hardness or stiffness encountered in stainless steel wires.

The remaining columns, theoretical colour volume, weight and recommended screen tension must now be considered. Weight can be dismissed very rapidly as being not relevant to screen printing. Woven meshes have many applications such as sieving, filtration and reinforcement as well as the manufacture of printing screens and the weight may be of interest to other users. The theoretical colour volume as expressed in the tables is the volume of colour (i.e. ink or paste) which will be printed through one square metre of the mesh. It is calculated by multiplying the percentage open area by the cloth thickness (cm) by 100 x 100 cm and so can be used for estimating paste usage. While this may be useful for industrial users it is more useful for thick film users to consider this to represent wet print thickness. The number is the same but the units are different of course. Cloth thickness in microns multiplied by the percentage open area gives the theoretical wet print thickness (in microns) which will be obtained from the mesh alone. It must be remembered that the figure is theoretical. In practice, the thickness will probably be some 10-20% less than predicted as some ink will remain trapped in the mesh. Second, it is for wet print thickness. Third, any emulsion on the screen or other prints on the substrate will increase the print thickness.

The recommended screen tension is the tension necessary to stretch the mesh sufficiently to cause the screen to peel away from the substrate after printing but not to be stretched so much that damage is likely. Figure 4 shows Bopp’s force versus elongation curve for stainless steel mesh.


Fig 4. Mesh elongation vs tension (Courtsy G. Bopp & Co.)

The yield point or elastic limit is at an elongation of 1% so that if a piece of stainless steel wire is stretched by less than 1% and released, it will return to its original length. If it is stretched by more than 1% and released, it will remain somewhat stretched. If the mesh is elongated by 0.9% there is still 0.1% in reserve before the elastic limit is reached. If it is elongated by 0.5%, there is 0.5% in reserve. The advantage of using the higher of the recommended tensions is that a smaller gap can (and must) be used between the screen and the substrate. The high tension causes the screen to peel readily from the substrate even at small gaps. The small deflection for the screen to touch the substrate causes no significant change in screen image size. For the very best image size control, it would seem advisable therefore to use a highly tensioned screen. The disadvantage of using high tension is that the screen is virtually on the point of being overstretched and damaged and the slightest carelessness by the operator will destroy the screen. Conversely of course, a screen stretched with 0.5% in reserve is rather less susceptible to damage but needs to be used with a larger gap between screen and substrate to promote the peeling action. The rule of thumb of gap equals screen width multiplied by 0.004 can be used at 0.5% reserve and it is quite simple to calculate image distortion. (The calculation is essentially the same as that in Figure 2). A typical thick film screen 300 mm wide might be used to print substrates 150 mm square using a squeegee 175 mm wide. The gap will be 1.2 mm. The screen will be stretched from 300 to 300.023 mm so the substrate image will increase to 150.012 mm. This lack of precision hardly justifies the extra risks of high screen tension. It is normal practice in Europe to use 0.5% reserve stainless steel screens although 0.1% reserve is not unknown in the USA.

(The extension used for polyester and nylon meshes are typically 3% and 6% respectively while the rules of thumb for screen gaps are the screen width multiplied by 0.006 and 0.010 respectively).

It is perhaps worth commenting at this stage on the use of “screen gap” rather than the more common “snap off”. Screens should always peel from the substrate rather than suddenly releasing bodily. The latter causes a distinct snapping sound and, more significantly, non-uniform prints. In order not to encourage poor reproducibility therefore, “screen gap” has been used.

2.1.3 Stencils

In classic screen printing, the stencil’s role is to define the shape of the pattern to be printed onto the substrate. However, in thick film applications the stencil also has a secondary role of having a major influence on the print thickness. Not surprisingly the thicker the stencil, the thicker the print is likely to be.

There are three principle types of stencil, these being photo-emulsions, metal masks and what are best described as other plastic sheets. In many ways the latter type is the easiest to understand.

Thin sheets of plastic, mylar or PTFE can have holes cut in them corresponding to the desired pattern. This can be done with a sharp blade or more typically using printed circuit board drills or routers. The patterned sheet is then attached to the mesh of the screen (or in some cases directly to the frame) with a suitable adhesive. This technique finds favour with owners of PCB drills who of course tend to be PCB manufacturers and the principle application of such screens is printing solder paste for surface mounting.

Like plastic sheets, metal masks are generally used for printing solder paste although they are very occasionally used for printing fine line thick film. The metal foil will usually be of copper, brass, beryllium copper, nickel or stainless steel. Typical thicknesses are 75-375 mm depending on the application. In order to image the foil it is coated on both sides with an acid resisting photo-sensitive material. Mirrored pairs of photographic images of the required pattern (or photo-positives) are used during an exposure of the photo-resist to an ultra violet light source. Areas shaded by the opaque parts of the photo-positive remain unexposed.

Some areas of the photo-resist become insoluble, others become soluble when treated with a suitable developing solution and are dissolved. When the metal foil is sprayed with a suitable acidic etching solution, the areas no longer protected by photo-resist dissolve and the foil becomes suitably perforated. As in the case of plastic sheets, the metal foil can be fixed to the mesh (or the frame) with adhesive. Two variations in the etching technique are sometimes used. Because the foil is etched from both sides, different patterns can be etched on each side. Sometimes a relatively large area of metal is dissolved from the top face, thus locally thinning the foil and producing a thinner print in that area, (Figure 5).


Fig 5. Recessed etching

Secondly, it is reasonably self-evident that the standard technique can be used for etching circles but not “O’s”. A mesh-like pattern can be etched on the top surface of the foil to support the centre of the “O”. This technique is also used for the production of the fine line thick film screens mentioned earlier.

Other techniques are used for making metal masks. The most common method uses a laser to cut the apertures. It has the reputation of achieving excellent control of dimensions and so has benefits at printing very small features, such as solder paste lands at 0.5 mm pitch and below. Masks are also manufactured by electro-plating techniques. A plain, flat, polished sheet, normally of stainless steel, is coated with a rather thick layer of resist which is then photo-imaged and developed, thus typically leaving small resist islands on the steel surface. A nickel alloy is electro-plated onto the surface, but not on the resist islands, until the required thickness is achieved, typically 0.1 to 0.15 mm. The resist is dissolved out and the steel is peeled from the nickel sheet, thus producing a stencil, (Figure 6).


Fig 6. Metal addition stencil

This technique also produces stencils having good control of dimensions but both methods are more expensive than chemical etching.

For thick film applications, by far the most common stencil materials are photo-emulsions. These are water soluble polymers generally based on PVA, which when exposed to ultra-violet radiation become insoluble. Hence if a photo-positive is placed on the photo-emulsion and this is exposed to UV, the areas which are exposed will become insoluble while those areas shadowed by the photo-positive’s opaque areas will remain soluble. A wash in warm water will thus create the desired pattern in the stencil. Photo-emulsions come in two different forms, either as a liquid (known as direct emulsion) or as a dry film coated onto a plastic backing sheet (known as indirect emulsion).

In either case the screen mesh should be degreased with a suitable proprietary screen cleaner, thoroughly rinsed in water and blown dry before being coated with emulsion. Coating should take place in reasonably dust free conditions in a room lit only with suitable yellow safe lights.

Direct emulsion is applied to the mesh by one of several techniques. The emulsion can be more or less poured over the lip of a coating trough (Figures 7 and 8) as the trough is slid up the near vertical screen.


Fig 7. Coating trough

Fig 8. Screen coating

This is repeated a number of times to build the thickness of emulsion to that required. Alternatively, the screen is laid horizontally, strips of thin adhesive tape are attached near the edge of the screen, emulsion is poured onto one end of the screen and a straight edged blade, resting on the tapes, is used to draw the emulsion over the screen (Figure 9). An adaptation of the latter technique can be mechanised relatively simply. Typically a hopper containing emulsion is fixed over the screen which itself rests on a moving carriage. A straight edged blade is set immediately behind the hopper or forms part of it. Its height is set accurately above the screen surface. The carriage is drawn under the hopper which opens, depositing emulsion on the screen and the blade controls the emulsion thickness.


Fig 9. An alternative screen coating method

Coating troughs are generally preferred by the professional screen manufacturer who supplies finished or part finished screens to the thick film industry. In skilled hands they are quick, convenient and give good thickness control of + 1 or 2 mm over the screen area. However, in unskilled hands they can be disastrous.

“Gifted amateurs” often prefer the blade and tape method because the tapes go a very long way towards giving a uniform thickness even if the absolute thickness is proportional to the thickness of the tape(s) rather than be continuously variable. Coating machines are useful for making large numbers of good screens but they are expensive, need setting up for each screen type. Hence they may not give as good control of thickness as can be achieved by a skilled trough coater.

Indirect emulsion can itself be divided into two different types. The emulsion on its backing can be exposed via the photo-positive to UV, washed out with warm water, pressed wet onto the underside of the screen, allowed to dry and the backing peeled away. The emulsion adheres to the screen simply by being deformed around the mesh. Alternatively, (and nearly always preferably) the emulsion on its backing is placed on the bench, the screen is placed on the emulsion and the emulsion is wetted through the screen with water, sensitiser or emulsion of the direct type. The screen is then dried. If emulsion has been used for wetting, it will dry, adhere to the indirect emulsion and encapsulate the mesh. This will give a good, long lasting bond of the emulsion to the mesh in the same way that direct emulsion bonds the mesh. If water or sensitiser has been used, the bond will be by a deformation of the emulsion giving only moderate life. After drying, the plastic backing sheet is peeled away and the emulsion is exposed via the photo-positive to UV radiation. The latter type of film is sometimes known as “capillary”, “direct/indirect” or “combination” film and the process is physically easier than handling large wet sheets of indirect emulsion (although the processing involves one or two extra steps).

Film emulsions have very smooth undersides because they have been coated onto a flat sheet of plastic and so they form a very good seal against the substrate. This gives good line definition. Direct emulsions tend to follow the unevenness of the surface of the mesh, thus allowing paste to spread out into these small cavities. A skilled operator can apply direct emulsion onto a screen with good uniformity and to any normal thickness. Indirect emulsions are available in a range of well controlled thicknesses and require only a little skill in their application. However, if emulsions are available in thicknesses of say, 10, 20 and 30 mm, it is difficult to attain say, 23 mm. In addition, the mesh will sink more or less into the emulsion depending on the mesh type, the degree of wetting, the pressure applied to the mesh and so on. Indirect emulsion is also more costly than direct emulsion.

Thickness control of the emulsion is vitally important as it has a major influence on print thickness. For small features (resistors below 1 mm in length or width, conductor tracks etc.), each additional 1 mm of emulsion thickness will increase the wet print thickness by something approaching 1 mm. For features more than 10 mm wide, there is little or no influence on print thickness except at the very edges of the feature. The mesh near the centre of the feature is easily pressed against the substrate surface, thus the print thickness is controlled only by the mesh thickness. It is a common mistake to try to increase the print thickness of a large area of multi-layer dielectric by increasing the emulsion thickness. While this technique is valid for resistors, the only effect on the dielectric print is to produce a high wall and a thin print, see Figure 10. (In order to print thicker over large areas, it is necessary to change to a mesh with a higher theoretical colour volume).


Fig 10. Effect of emulsion on print thickness over a large area

Even if precise print thickness control is not necessary for a particular application, emulsion thickness has a pronounced effect on print definition. If the emulsion is very thin it is very weak and not self-supporting. It tends to draw back to the nearest adjacent thread of the screen, thus producing a saw-tooth line edge. This problem is made worse because the mesh at the edge of the line is in contact with the substrate surface and paste is unable to flow underneath the wire knuckles. Hence the line edge is defined by the mesh, not by the emulsion. It is necessary to increase the emulsion thickness to something like 10-12 mm in order to obtain straight edged lines. Very fine meshes can give good results with a rather thinner emulsion, very coarse ones require a much greater minimum thickness. If the emulsion layer is built-up excessively however, to above 25 or 30 mm for common meshes, then the area of the walls of the emulsion become comparable with the area of the print surface. The paste tends to stick to the sides of emulsion and so gives a poorly defined print with inferior thickness control (see Figure 11).


Fig 11. Effects of emulsion thickness on print definition

It was quite common in the early days of thick film to try to align mesh wires and printed tracks in order to eliminate saw-tooth edges. If the mesh and tracks are not quite parallel however, stair stepping occurs (Figure 12).


Fig 12. Saw toothing

Most patterns have tracks running in both X and Y axis. While it was possible to align tracks in say, X, it was quite likely that the Y tracks would not be parallel with the mesh because of difficulties in stretching mesh uniformly. It was found better therefore, to align tracks at about 45° to the mesh. In this way there would be no stair stepping and at least any saw-toothing would be fairly small and uniform. As line widths became narrower, it was found that interference fringes occurred between tracks and meshes but these became less significant at 22½° than at 45°. 22½° became fashionable.

Graphic screen printers often have to print narrow lines at many different angles. It is obviously difficult to align the graduations on a clock face at 90, 45 or 22½° to the mesh! Provided that the emulsion thickness is sufficiently great (i.e., above 10-12 mm) and the mesh is sufficiently fine, any angle is satisfactory. However, there are other advantages in having mesh at 45°. If the mesh is at 90° to the frame the squeegee bears on single filaments in succession whereas if it is at 45°, it bears on many at the same time. Stress is reduced therefore and screens with their mesh at 45° retain tension longer than those at 90°.

Because of the relative complexity and the need for specific skills in making coated screens, it is quite common for thick film manufacturers to purchase coated screens from specialist suppliers. They may also have them exposed and washed out ready for use but exposure is quite a simple process. There are advantages in keeping a stock of conductor, resistor and dielectric screens ready to be exposed rather then keeping one spare screen of every design.

Screen exposure requires little equipment or space. The room needs suitable yellow safe lighting, warm or “hand hot” running water and ideally, a compressed air jet. The equipment comprises a UV lamp and a contact frame or vacuum frame. The purpose of the frame is to hold the photo-positive and the screen in intimate emulsion-to-emulsion contact. In the former case, the photo-positive is placed on the screen which in turn is placed on a matt black or dark grey polyurethane foam block. A glass plate is clamped down to hold all in compression (Figure 13).


Fig 13. Contact frame

In the latter case, the positive and screen are placed on the glass plate and a flexible plastic sheet is laid over the screen and sealed to the plate. When evacuated with a small pump, the positive and screen are held firmly together (Figure 14).


Fig 14. Vacuum frame

The assembly is exposed to the UV lamp. The exposure time varies with mesh type, emulsion type, thickness and age and the exposure lamp type and distance. Initially, a series of exposure trials will be necessary to establish the optimum exposure time for each screen type. Since lamp emission decreases with age, it is a wise precaution to change the light source periodically say, every year.  Most emulsions require exposure times of several minutes and have wide exposure latitudes. However, for the best definition it is good practise to expose the screens for the minimum possible time. As soon as the exposure is started, the emulsion starts to polymerise at the surface which, in due course, will be in contact with the substrate. If the emulsion is very underexposed it will not be polymerised at mesh depth and so the thin exposed layer will slide off the screen when the screen is washed out and vanish down the sink. As the exposure continues, polymerisation progresses through the thickness of the emulsion so that the emulsion and mesh key together. However, with over-exposure, scatter in the emulsion and reflection from the mesh surface cause some polymerisation in the areas shadowed by the opaque parts of the positive. This causes poor subsequent washing out of the emulsion, hence poor definition. For general purpose screens, slight over-exposure is to be preferred to gross under-exposure.

When the exposure has been completed the screen should be soaked for a minute or two and then gently sprayed with hand hot water until the unexposed emulsion has been washed away. The water temperature is not critical but cold water dissolves emulsion only slowly while very hot water can cause the emulsion to reticulate. In any case both will be uncomfortable to work with. After washing, the screen should be blown dry with a clean compressed air jet. It is preferable to blow the water out of the screen rather than simply to dry it. In the latter case, anything dissolved in the water, particularly emulsion, can cause some partial blockage in the screen. After drying, any small pinholes in the emulsion should be touched in with suitable filler (proprietary screen stopper or emulsion). Finally, the whole of the inside of the screen should be re-exposed to the UV source to ensure that it is fully polymerised. This is particularly true if the screen originally received only a minimum exposure. An exposure time similar to the initial exposure is suitable but the time is not critical

From the foregoing discussions, it should be reasonably evident that the photo-positive itself should be totally opaque and totally clear with no pinholes or scratches. Grey areas and scratches in clear areas transmit some UV and so cause partial exposure. Pinholes and scratches in otherwise opaque areas will allow UV to expose the emulsion. Photo-positives should be inspected before use and stored carefully when not in use. It is also most important that the photographic emulsion is brought into intimate contact with the screen emulsion during exposure. If this is not the case, then a penumbra effect will cause partial exposure at the edge of the positive. The positive should be what is known as “right-reading”. In other  words, in order to print a letter F onto the substrate the positive should appear as a black letter F when viewed looking at the emulsion side of the film or plate, as in Figure 15.


Fig 15. Right reading positive

A particularly common fault is to try to use the same positive to produce a PCB or thin film metalisation and a solder paste screen. As can be seen from Figure 16, the emulsion side of the positive for etching metalisation must be mirrored when making a matching screen.


Fig 16. Right-reading & mirrored positive for screen and circuits

2.3 Squeegees

Squeegees have three functions to perform:

(a) to press the screen into line contact with the substrate
(b) to push paste down into the stencil and onto the substrate
(c) to cut the paste level with the top of the screen.

2.3.1 Material

Substrates are never completely flat and as layer is built upon layer, so the surface becomes still less flat. A flexible squeegee is necessary in order to conform with the uneven surface. It needs to resist wear as it slides over the screen surface and it must not be attacked by the constituents of the paste. For these reasons, polyurethane is used almost universally as the squeegee material.

Squeegees are available in a wide range of hardness but are normally in the range of 60 to 80 Shore. Very soft squeegees conform well with uneven surfaces but collapse under applied pressure, very hard ones behave conversely. 65 Shore is probably the most commonly used hardness for general thick film printing, harder ones, say 75 Shore being used if high squeegee pressure is required and/or to give thinner prints.

2.3.2 Angle of Attack

In order to push the paste into the stencil and to shear it at the screen’s top surface, an angle of attack at the squeegee tip of around 45° is used. Much steeper angles give insufficient filling of the stencil, much shallower angles give erratic shearing and hence poor thickness control. The tip angle may well be achieved by holding a flexible squeegee at an angle of around 60°, then allowing pressure on the squeegee holder to deflect the tip to 45°.

As the angle of attack becomes more shallow, so hydro-dynamic pressure increases and causes an increase in print thickness. The supposition is that the high pressure in the fluid causes it to flow back past the squeegee, thus increasing print thickness (Figure 17).


Fig 17. High squeegee pressure causing paste to flow back under
squeegee edge, thus increasing print thickness.

Very worn squeegees present very shallow angles of attack (Figure 18) and so can cause problems with thickness control. Their shearing action is poor and print definition suffers as well. For the best definition of fine lines therefore, a sharp edged squeegee is necessary.


Fig 18. Effect of squeegee wear

For good control of print thickness however, a slightly worn squeegee is to be preferred. The rate of wear of the edge is not linear, the rate decreasing as the number of prints increases. It is wise to use a squeegee which has been “run in” when printing resistors so that the change in squeegee edge and therefore in print thickness is small throughout the batch.

Squeegee life depends on many factors, such as squeegee pressure, speed, screen material, paste type etc., but several tens of thousands of prints can normally be expected. Dry cycling, i.e. performing a print cycle with no paste on the screen to lubricate it, causes rapid wear both of the squeegee and the screen and is to be avoided. It is good practice therefore to apply a little paste to the squeegee edge before printing.

2.3.3 Squeegee Dimensions

The squeegee needs to extend beyond the substrate by a minimum of some 10 mm on each side because screen tension tends to lift the ends of the squeegee. Squeegees can be wider still but this will require more squeegee pressure. Very wide squeegees will reduce screen life because the screen will be overstretched (Figure 19).


Fig 19. Effect of squeegee width on mesh stretching

2.3.4 Squeegee Shape

Two shapes are commonly used, the diamond section and the blade or trailing edge section Occasionally a knife edge section is used. (Figure 20).


Fig 20. Squeegee shapes

The trailing edge squeegee is used almost universally in classic screen printing. It is inherently flexible so conforms well with uneven surfaces. It exerts relatively uniform pressure on both the crests and valleys of the surface and so gives the best uniformity of print thickness. It has three limitations however. It cannot normally be used to print in the reverse direction, it tends to collapse under very high squeegee pressure and so cannot be used with pastes of extremely high viscosity and it cannot be used sensibly with a down stop (q.v. paragraph 3.2.5) because the squeegee is inherently flexible.

The diamond squeegee was conceived with the intention of precisely controlling the position of the squeegee edge. Such control would then allow the edge to be moved in a flat plane thus doctoring a flat layer of paste onto the substrate. The printing operation would thus be controlled by a precision printing machine rather than relying on the screen to give good thickness control. The plan (which at the time was a good one) was made when substrates were typically 12 x 12. Screen manufacturing skills were not terribly advanced. However since then, substrate sizes have become substantially greater and multi-layering has increased, both of which increase the effects of the substrate bow and general unevenness (Figure 21).


Fig 21. The effect of width on substrate “height” for a constant
radius of curvature

The inherent rigidity of the diamond squeegee limits its ability to conform with uneven substrate surfaces. However, it is almost essential for screen printing very viscous paste, particularly solder paste and for two directional printing.

3. PRINTING MACHINES

3.1 Machine Requirements

In the foregoing discussion no mention has been made of printing machines. It is quite simple to produce a very good quality screen print without the use of any machinery. Indeed, most fine art screen printing is made by hand. There are many third world manufacturers or resistor networks and other simple thick films who have only the most rudimentary “machinery” comprising little more than a hinged screen and some simple fixed stops to locate the substrate.

The primary purpose of a printing machine is to give print uniformity, the secondary purpose is to save labour.

Print uniformity can be divided into two distinct areas. The print thickness needs to be controlled in order to give uniform resistivity, insulation and other “electrical” properties. Print position on the substrate must be controlled to give layer to layer interconnections where they are needed, and to prevent short circuits where they are not.

3.2 Controlling Print Thickness

Printing engineers have been known to play a party game during the less interesting parts of thick film seminars. The object of the game is to produce the longest list of parameters which affect print thickness. While the list can legitimately contain many tens of variables many have a minor effect. Others cannot readily be controlled, or are out of the control of the printer operator. Table 4 shows those variables which are used by the printer operator to control print thickness on a batch to batch basis. (For completeness, Table 5 lists those which are readily usable for process control but which are not directly controlled at the printing stage. Table 6 lists other major factors which affect print thickness but which would not normally be used as a means of control). The tables indicate what end of the range of the variable is necessary to give a reduced print thickness. From a machine stand point, only Table 4 is of consequence.

Table 4

PRINTER
VARIABLES
FOR A THINNER
PRINT, SET:-
squeegee pressure high
squeegee  speed low
squeegee angle high
squeegee hardness high
screen gap low
down stop low

Table 5

PROCESS
VARIABLES
FOR A THINNER
PRINT, SELECT:-
mesh theor. col. vol low
emulsion thickness low
paste viscosity low

Table 6

OTHER
FACTORS
FOR A THINNER
PRINT, USE:-
room temperature
(hence viscosity)
high
squeegee wear low
screen tension low

3.2.1 Squeegee Pressure

As squeegee pressure is increased the screen mesh is pressed down towards the substrate and so a thinner print is produced as the paste is scooped out.

Pressure is normally applied by a spring or by a pneumatic cylinder. More rarely, hydraulics or dead weights are used. The method is not particularly important but it is essential that the load remains constant as set and is not significantly

affected by a small variation in the substrate thickness. Uniformity across the width of the substrate is achieved either by adjusting the squeegee, making it parallel with the substrate or, better, by having it floating on its mounting so that it sets itself parallel. Pressure uniformity along the length of the substrate is ensured by having the squeegee run along rails set parallel with the substrate surface. Pressure should normally be set to push the screen into contact with the substrate and wipe the paste clear from the screen surface, typically around 0.2 kg per centimetre width of the squeegee. For good thickness uniformity it is helpful to increase this to around 0.3 or 0.4 kg per centimetre width.


Fig 22. The use of a flexible polyester screen to facilitate the
printing of upper layers

On very uneven surfaces, for instance when printing the top conductor of a simple crossover, pressure must be increased above that which is normal so that the screen is brought into contact with the substrate adjacent to the dielectric. (The use of a more flexible polyester screen would be advantageous in this case (Figure 22)).

3.2.2 Squeegee Speed

At low speed it is useful to imagine the squeegee dropping down over the edge of the screen emulsion, pressing the screen mesh onto the substrate and then climbing back over the emulsion edge again, thus producing a thin print. At high speed, the squeegee will tend to float over the surface, thus increasing print thickness. Speed is controlled either by hydraulic resistance in pneumatic systems or by electronics, controlling a motor.

Speeds are very dependant on paste types and the paste manufacturer’s advice should be sought. However, a range of 50-250 mm per second covers most requirements. As a general rule, pastes of high viscosity require the slowest speeds.

3.2.3 Squeegee Angle of Attack

As was explained in paragraph 2.3.2, print thickness decreases as the squeegee becomes more perpendicular to the screen surface. Some machines have an adjustable angle of attack, others do not. The angle between the tip of the squeegee and the screen will always be less than the angle indicated by the squeegee holder because friction between the screen and squeegee and in the case of trailing edge squeegees, squeegee pressure will deflect the tip. High squeegee pressure, high paste viscosity and high squeegee speed or the use of stainless steel rather than polyester mesh will all decrease the angle of attack. However, for any particular set of conditions the angle will remain constant.

3.2.4 Screen Gap (snap off)

As the screen gap increases so more of the available pressure applied to the squeegee is used to deflect the screen and bring it in contact with the substrate. Thus the use of a large screen gap has the same effect on print thickness as the use of low squeegee pressure.

Classically, the screen gap should be set just big enough so that the screen peels away immediately behind the squeegee as it travels over the screen surface. If the gap is too small the screen will remain stuck to the substrate surface. As the gap increases, so the screen will begin to peel some distance behind the squeegee. However, when the squeegee lifts at the end of the print stroke, the screen will begin to peel in the reverse direction. The two advancing fronts will decrease the area of screen in contact until suddenly, the screen’s tension will be able to pull the screen from the substrate. A distinct snapping sound can often be heard as the screen leaves the substrate, hence the name “snap off distance”, “snap off gap” etc. for the distance between the screen and the substrate. If the screen snaps away in this manner, it will tend to spatter paste onto the substrate, leave paste in the screen and give a poor surface finish in that area. Screen snap off should most certainly be avoided and so “screen gap” is a better phrase to use than “snap off distance”.

To prevent screen snap off the screen gap must be increased a little more. A small increase will cause the screen to peel a short distance behind the squeegee such that the peeling action is completed just before the squeegee lifts. This is perhaps just acceptable but any small change in conditions may cause the screen to stick and snap off again. Hence the gap should be increased until the screen peels immediately behind the squeegee.

A gap which is slightly greater than optimum is better than one which is too small, but large gaps should be avoided. Image distortion is increased, and both screen and squeegee life will be reduced, because higher squeegee pressures are necessary to force the screen down onto the substrate.

A good guide for gap settings of stainless steel screens is to multiply the screen width by 0.004. For polyester, multiply the screen width by 0.006. Thus for a typical screen 300 mm wide, the gap would be some 1.2 mm for stainless steel and 1.8 mm for polyester. (For completeness, the figure for nylon is 0.010, giving a gap of 3 mm).

3.2.5 Downstop

One use of a downstop is to prevent damage to screens. Referring back to Figure 1, it will be seen that the substrate is nested in a surround which supports the screen and squeegee. It is cheaper to rest the substrate on a flat surface, locating it against three pins, (Figure 23).


Fig 23. Simple three point registration of a substrate

However, in this arrangement the squeegee will coin (permanently deform) the screen against the substrate edge. To overcome this problem, (other than by nesting), a mechanical stop or “downstop”, can be used. This prevents the squeegee from dropping too far below the level of the top of the substrate. Classically, the downstop should be adjusted so that it is not quite supporting the squeegee when the latter is resting on the substrate. However, by raising the downstop the squeegee is also raised, thus increasing the print thickness. The downstop setting can have a large effect on print thickness so it is very useful for correcting major errors in the choice of a screen. Conversely the setting can be very critical, so errors can easily be introduced. Because the squeegee will tend to move parallel with the guide rails on which it runs, any lack of parallelism between the rails and the substrate surface will be reflected in changes in print thickness. For example, it is easy to calculate that the print thickness would increase by some 90 µm over the length of a 2" substrate if the machine is 0.1° out of parallel.

If a downstop is used, it is essential to use a diamond section squeegee. A flexible, trailing edge squeegee will bend when its normal pressure is applied to the substrate surface. It will be straighter both before and after the substrate and so it will still coin the screen. However, the converse does not apply. It is not always essential to use a downstop with a diamond section squeegee. A nested substrate is a perfectly acceptable system without the need of a downstop.

3.2.6 Squeegee Hardness

Print thickness is related to squeegee hardness principally because a hard squeegee deforms less under pressure than does a soft one. The angle of attack of a hard squeegee is greater therefore than of a soft one and so print thickness is decreased.

By far the most common squeegee hardness used for thick film printing is about 65 Shore which optimises lack of distortion with flexibility. Slightly harder squeegees of around 75 Shore are used to cheat the process and give a reduction in print thickness when all else fails, while softer ones are recommended for giving the best pinhole free characteristics of large continuous layers of, say, dielectric. Conversely, hard squeegees cause less paste spreading and so are to be preferred for precise, fine line printing. Very hard squeegees are also chosen when printing materials such as solder paste through open metal masks. Soft ones are too easily deflected into the open apertures, thus tending to scoop the paste out again and reducing the printed thickness.

3.3 Controlling Print Position

Almost invariably, substrates are positioned against three fixed stops in order to give repeatable location. Figure 24 shows how three points give a unique location. Straight edges, or lay stops as they are known in graphic printing, give no certain repeatability unless the edges of the substrate are perfectly straight.


Fig 24. Three point location gives unique position, unlike straight
edge location

Sometimes the operator is required to locate the substrate against the fixed stops, but for reliability, a centring mechanism is often incorporated in the substrate holder. In either case it is normal to have vacuum to hold the substrate in position during printing. Vacuum also ensures that the substrate does not adhere to the screen.

In a fully automatic system both the screen and the workholder will remain in fixed positions with respect to one another. In a hand loaded system however, the workholder will generally be driven into a fixed location and held there by a slipping clutch drive, a pneumatic cylinder or some similar mechanism. Clearly the whole system must be built solidly so that positioning is repeatable to within a few microns (typically not more than 25 mm).

3.4 Automation

3.4.1 The Need for Automation

Automation of substrate handling is generally used as a means of increasing production speed. While this often occurs, automation also improves the yield of good parts. Thick film printing is a serial operation. As layer upon layer is added, so the effects of poor yield multiply. A yield of 90% good prints per layer yields only 38% good substrates after a typical build of two conductors, two dielectrics, five resistors, one coverglaze and one component adhesive (or solder) print. Major causes of poor yield include dust and similar contamination, accidental damage to prints, caused by the operators hand or tweezers and simple operator inattention. With automation the “operator” stands still and so generates little dust, he does not touch the substrate and he becomes an inspector. Yield is thus improved together with greater throughput.

3.4.2 Auto Handling Systems

3.4.2.1 Substrate Dispensing

The simplest and cheapest method of dispensing is to push substrates out of the bottom of a vertical stack. The weight of the substrates above in the stack makes it quite probable that prints already on the pushed substrate will be damaged as it slides from under the stack.

A better solution is to use vacuum to pick substrates from the top of the stack. There is some risk of contamination of substrates by the underside of the one above it and also from the vacuum pick-up, but the risk is small. Naturally such systems are more expensive because they are more complex.

For the greatest protection (and cost and complexity), substrates should be held in and withdrawn from individual compartments of cassettes. Thus substrates do not touch each other. They are generally withdrawn, supported on their underside by belts or walking beams and so no contamination of the top surface occurs.

3.4.2.2 Feed Through the Print Position

In simple systems substrates will push each other along between guide rails. Sliding therefore takes place which may damage the underside of the substrate. The substrates can also ride over each other, particularly if they are thin, bowed or large in area.

Better systems incorporate belts or walking beams to carry substrates. There is little to choose between these but belt systems are mechanically simpler (and so perhaps more reliable). They are also more gentle so that the risk of substrate breakage in the event of a misfeed is minimised. On the other hand the sequencing of operations with a belt system is not inherently fixed by the mechanism and so more complex electronic control is necessary, incorporating a number of substrate position sensors.

3.4.2.3 The Print Position

Two options are generally available. In either case the substrate will be pushed into register against fixed stops and it will almost certainly be held down by vacuum. However, in the simplest option, the substrate will be resting on a flat surface. In a more reliable option a surround would be provided completely supporting the squeegee all round the substrate’s periphery thus eliminating screen damage (see paragraph 3.2.5).

3.4.2.4 Feed to the Drying System

It is normal to have an in-line drying system whose width is much greater than that of one substrate. Hence substrates must be fed onto the dryer belt in rows. This is done by a collocator. Since the prints are wet at this stage, it is clear that top face contact must be avoided. Two techniques are commonly used. Most simply, sensors count the number of substrates passing and when one belt width has been collected, they are pushed onto the belt of the drying oven. Sliding occurs and so the substrate undersides are vulnerable. Alternatively a belt width of substrates can be picked up by their edges and carried onto the dryer. Thus the integrity of the underside of the substrate is preserved.

3.4.2.5 Feed from the Drying System

Once again two systems are commonly employed. Most simply, vacuum suckers are used to lift the line of substrates from the dryer, the top surface of the substrates being exposed to the normal risks of damage. Alternatively, substrates can be carried from the dryer on take-off belts running at the same speed as the main dryer belt.

3.4.2.6 Reloading Magazines or Cassettes

If the substrates are to be reloaded into stack magazines, vacuum pick-up is almost universally used. If they are to be reloaded into separate sections of cassettes, they are pushed into the slots. In either case the system indexes down by one substrate position to make room for the next substrates to be fed.

3.4.2.7 Print Parameter Storage

The most advance systems incorporate microprocessor control of print parameters. These parameters include squeegee pressure, speed and stroke length, screen gap and screen position. The operator is required to key in the appropriate menu number, fit the screen and leave the machine to perform its own set up. This takes most of the control away from the operator and so can go a very long way to eliminating operator mistakes. In some systems, the operator can have access to a limited number of parameters and so can make minor hour-to-hour changes without reference to a higher authority.

3.4.2.8 Vision Systems

Microprocessor controlled machines can incorporate vision systems enabling automatic screen/substrate alignment to be achieved. After calibration, cameras inspect alignment marks on the substrate, compare these with known positions of similar marks on the screen and thus can move the screen (or substrate) to bring both into register. This is rarely necessary for each individual substrate since the normal substrate registration mechanisms come into play after the initial alignment. However, in those cases where there is no reliable edge location, vision alignment systems are invaluable. Such cases include “co-fired ceramic” substrates, broken snapstrates and the addition of thick film to a thin film substrate.

Perhaps a more obvious use of vision systems is for automatic inspection of the printed parts. Such systems may not be fast enough to thoroughly inspect all of each substrate (unless the substrates are very small or printed with simple patterns). Some sampling system may be used therefore, inspecting a different area of each substrate or inspecting all of an occasional substrate.

 

4. A TYPICAL PRINT SET UP

The following represents the steps typically necessary to produce one printed layer on a batch of substrates. The details will vary from machine to machine and some actions may not be necessary. The machine’s handbook or manufacturer should be consulted in any case of doubt. For simplicity however, it is assumed that the machine is semi-automatic, that is, hand loaded.

  1. Fit the appropriate screen to the machine, having checked drawing issue numbers and that the screen is in good condition.
  2. Fit the appropriate workholder to the machine and place the substrate in it.
    NOTE. If the machine’s squeegee is fixed, not floating, it is more appropriate first to fit the workholder and substrate, then to temporarily attach the squeegee and ensure that its edge is parallel with the substrate before fitting the screen.
  3. Bring the screen and workholder into the printing position, set the screen gap close to zero and looking through the screen, align screen and substrate.
  4. Set the screen gap appropriately (see paragraph 3.2.4).
  5. Check that the substrate and screen are parallel.
    NOTE A good check that everything is parallel is to make test prints while reducing the squeegee pressure. The print should cover the entire substrate area to begin with. As the pressure is reduced, so the area of the print should reduce towards the centre of the substrate. If it reduces towards one corner or one edge, it implies that the substrate, squeegee and/or screen are closer together in that area than elsewhere.
  6. Check that the squeegee edge is straight and in good condition.
  7. Fit the squeegee and ensure that during the flood stroke it will not touch the screen.
  8. If using a downstop, lower the squeegee to its printing position on the substrate and set the downstop so that it is just not supporting the squeegee mechanism.
  9. Fit the floodblade and ensure that it is no more than just touching the screen.
  10. Ensure that the paste is of the correct type and batch number, that it is of the correct viscosity, that there are not settling solids in the bottom and that it is at room temperature. It is traditional to stir the paste but this is unnecessary with modern pastes.
  11. Add an appropriate quantity of paste to the screen (a few tens of grams for a small machine, 100 grams or so for a reasonably large one), and ensure that the edge of the squeegee is wetted with paste. (Cycling a dry and unlubricated squeegee on a screen is very damaging to both).
  12. Set the squeegee speed and pressure as appropriate (see paragraph 3.2.2 and 3.2.1).
  13. Many operators cover the substrate with sellotape, or similar, at this stage so that test prints can be made and easily wiped off. Critical users many then insist that this substrate be discarded because of possible contamination by the adhesive on the tape.
  14. Make several test prints and discard them or wipe them off. Adjust the screen/substrate position as necessary to give the correct alignment and registration. Check that the print has been made over the entire substrate area. If necessary adjust the squeegee pressure (or perhaps parallelism and/or downstop) to achieve this. Check that the screen peels from the substrate immediately behind the squeegee. If it does not, increase the screen gap or decrease the squeegee speed.
  15. After at least 5 or 10 prints, dry one or two substrates in accordance with the paste manufacturer’s recommendations and check the print thickness with suitable equipment. If the thickness is incorrect, adjust the squeegee pressure or perhaps the downstop and repeat the test. (See paragraph 3.2.1 and 3.2.5). Again, several prints will be necessary to establish a uniform paste flow.
  16. When all is correct (and has been checked by a second person), proceed to print the batch. Check perhaps one substrate in ten visually and one or two in three or five hundred for thickness. Once the print run has been established, there will be two probable causes of problems.  Dust, fibres and other contamination on the substrate may transfer to the underside of the screen thus causing blockage (open circuits and pinholes). Generally, a gentle wipe with a dry dust free cloth is sufficient to remove the blockage. Solvents should be avoided if possible because they will at least temporarily dilute the paste and give poorly defined thin prints. Certain solvents, particularly alcohols and ketones, soften PVA based emulsions and so may promote rapid screen thinning, especially if rubbed enthusiastically with an abrasive tissue. After any wiping, two or three prints should be made onto scrap substrates in order to re-establish the paste flow.
    The second problem will be a progressive change in the print thickness, normally becoming thinner as the batch progresses. This is caused by screen wear, changes in paste viscosity, squeegee wear, room temperature changes etc. and compensation will be occasionally necessary.
  17. On completion of the batch, dismantle the flood blade, squeegee and screen.
  18. Remove the remaining paste and return it either to the jar from which it came or perhaps better, to a jar for used paste. By adopting the second option, truly fresh known paste can be used the next time around while the jar of used paste can be requalified, reblended etc. for later use.
  19. When removing paste, the screen should be supported on a flat, clean surface to reduce the risk of puncturing or tearing it. While many operators use a spatula to remove paste it will be found that a hand-held piece of squeegee will remove the paste more completely and also virtually eliminates the risk of damage.
  20. The screen, squeegee and floodblade should be washed with an appropriate solvent to remove all traces of paste. Much argument occurs over the choice of solvent. Check with the paste supplier for recommendations. Alcohols and ketones are reasonably safe but may soften emulsion and so can cause rapid screen wear. Always use a suitable vapour extraction system, wear gloves and dispose of the used solvent safely. Many solvents affect conductors and resistors during firing if their vapours enter into the furnace atmosphere in concentrations of only a very few parts per million. Ensure that the furnace airflow system is set so that contaminated room air will not enter the furnace..
  21. Whichever solvent is used a spray will clean most thoroughly and with no damage. Failing this, a tray of solvent and a soft brush is an acceptable alternative. Abrasive tissues moistened with solvent should be avoided unless the operator is very keen to provide his screen maker with high volumes of repeat business.
  22. Inspect the screen for damage, blockage, loss of tension etc. and return to safe storage.
  23. Complete the appropriate paperwork and take a short, well earned rest.

 

5. SPECIAL TECHNIQUES

By far the majority of deposits made in thick film technology are simply printed layer upon layer, e.g. first conductor, first crossover, second crossover, second conductor, first resistor, second resistor, nth resistor and coverglaze. However, there are a number of occasions when slightly more complex techniques are necessary, especially when dense, multi-layered structures are required. Some of these will now be briefly described.

5.1 Complimentary Printing

This technique goes back to the early years of multilayering, the object being to maintain a relatively flat surface as layer upon layer of conductor was built up. It was achieved by printing the conductor pattern (with conductor paste, of course), followed by the inverse of the conductor pattern using dielectric paste (the complement of the conductor). Thus the conductor layer was made more level, ready for the first layer of true dielectric to be printed on top (Figure 25).


Fig 25. Complementary printing

There were some difficulties involved in this operation. A photo-positive and screen had to be produced, followed by one print and one firing operation for each complementary print. The additional cost was not popular with Production Managers. More seriously, it was quite a difficult technique to implement. Small errors in dimensions, caused by photography, screen making, screen stretch, screen-to-print misalignment and paste spread could cause the complementary print to just overlap the conductor. This added to the step rather than decreased it. Alternatively it could give a narrow gap between the complement and the conductor which was difficult to bridge.

The development of thin, inner layer conductors has greatly removed the need for such a technique and it is now rarely practised.

5.2 Via Fills

This technique can be regarded as the inverse of complementary printing. When printing complex multi-layer patterns it is normal practice to print a full layer of dielectric over the underlying conductor, leaving small holes or vias, typically 0.2 to 0.5 mm in dimension where connection to the underlayer is necessary.

Conductor paste is printed into these vias to form the connection. Sometimes the via fill and the upper conductor are one and the same print. However the via holes are relatively small and deep and it is often necessary to work at higher than normal squeegee pressure, or to use two passes of the squeegee to force the conductor to the bottom of the via. Both of these cause degradation of the conductor track’s line definition, so very frequently, the conductor tracks and via fills are printed as separate operations. (Figure  26).


Fig 26. Via fills

Occasionally, the sequence of printing is reversed. The via fill is printed first (when it is often referred to as a via post) and the dielectric is printed around it. A disadvantage of this sequence is that the post may tend to hold the dielectric screen away from the surface around the post, causing poorer dielectric definition.

5.3 The Green Tape or Co-fired Ceramic Process

In this process only the conductors of the multi-layer structure are printed. The dielectric layers are formed from thin sheets of unfired (or “green”) ceramic which are cut to size and have via holes punched in the appropriate positions. Tooling or alignment holes are punched simultaneously so that the individual layers can be printed in the correct positions and subsequently registered with respect to one another. After punching, conductor fills are printed into the vias, and conductors are printed onto the surface. The individual layers of printed ceramic tape are stacked, aligned using the tooling holes, pressed in a hydraulic press and then fired together (hence “co-fired ceramic”) to form the multi-layered structure. The traditional version of this process uses refractory metal pastes which are fired at high temperatures in a reducing atmosphere. More recently the paste manufacturers have introduced low temperature versions. These are based on modifications of ordinary thick film materials which fire at normal thick film temperatures (850°C) in normal oxidising atmospheres. The firing time is somewhat increased, however, from around half or one hour to about four hours , much of which is to allow the organic binders to burn out. However, this is probably less than the total time that a conventional hybrid would spend in the furnace when firing layers individually. Other advantages of these processes include always printing onto a flat surface no matter how many layers will eventually be built up, and the ability to select only perfect prints for subsequent assembly.

From a screen printing point of view there are only three unusual features of the process. The first is that the via fill pastes are of high viscosity and so are normally printed through a punched plastic stencil (see paragraph 2.1.3). The second is that the flexible tape is supported on a porous structure in the workholder. This prevents the vacuum necessary for holding the tape in position from distorting the tape into the vacuum holes in a normal workholder. Finally, the tape is place over registration pins fitting the location holes punched in the tape, rather than edge-locating against three fixed stops.

5.4 Through-hole printing

Increased circuit density has lead to the need to print onto both sides of substrates with interconnections from the front to the back. Conductor tracks can be run to the edge of the substrate and the connection taken round the side by soldered-on wires, clips etc. This technique is very wasteful of space and will probably require the printing of an extra conductor level, plus dielectrics, on each side. It is better to have a hole through the substrate at each required interconnection site. The holes are frequently laser drilled but they may be punched when the substrate itself is punched out if the number of substrates makes tooling economic. There appears to be little standardisation on hole diameters, ranging from around 0.1 to 1 mm although 0.3 to 0.6 mm is relatively popular. Sometimes slots are used instead of round holes.

The basic technique requires screen printing over the holes and applying vacuum to the underside of the substrate to pull the paste through. However, there are nearly as many variations on this technique as there are manufacturers. Some print and apply vacuum simultaneously, others print first and suck later. Some insist on having conductor only on the walls of the holes, leaving a clear centre, others are content to have the holes blocked. Some control the paste flow through the holes so that it penetrates all the way through from both sides, others pull only a little over half way through. There is a wide choice of methods of maintaining the degree of paste penetration. These include controlling one or more of the following parameters. In order to increase penetration through the hole, the operator should:- decrease paste viscosity, squeegee speed, squeegee angle and/or screen gap and increase squeegee pressure, vacuum pressure, vacuum duration and air/or flow rate through the holes.

Other factors which should be considered include special hole preparation, particularly of laser drilled holes which tend to have a very glassy surface, the choice of paste and a decision on whether to print through hole connections and conductor tracks simultaneously or sequentially. Until there is more consensus in the industry, the reader must be left to establish his own preferred technique.