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Product category: PCB Assembly Equipment and Tools
News Release from: DEK | Subject: Screen printing adhesives
Edited by the Electronicstalk Editorial Team on 16 December 2002

Applying adhesives accurately at speed

As placement systems begin to process up to 80,000 components an hour, it's important to rethink other SMT assembly processes - especially the bottleneck of dispensing adhesives, says Mike O'Hanlon

It is amazing to think that improvements in assembly technology have got to the point that the simple matter of putting down adhesive to hold the components in place could be the limiting factor in assembly line throughput. Yet it is true. Traditional adhesive dispensing is a serial process, and while an individual machine can be sped up, ultimately the sensible solution is to add more dispensers to the line.

In competitive markets the high capital outlay for additional dispensers, along with the additional floor space they required, makes this an undesirable solution.

Fortunately, existing technology provides an alternative - one that is readily available once we rethink the tools that are already available to the SMT manufacturer.

In particular, we can benefit by looking at another, assembly line process - screen printing.

Printing machines are used almost universally for applying solder paste to surface-mount assemblies.

They typically print thousands of pads per board at a line beat rate of two or three boards per minute.

That results in half a million solder pads printed per hour.

Imagine a single dispensing machine capable of these numbers.

So why not use these machines to print adhesive?

There are two obstacles to consider when printing glue.

First is the required dot height.

If all the components on a board have the same standoff then a simple metal stencil of the appropriate thickness will usually do nicely.

This is a relatively simple and robust printing process.

The issue that can arise is when you need to print dots of varying heights.

How can this be done with a stencil?

The second obstacle is clearance.

Should the product have SMT components on the side of the board to be printed, or through hole component leads protruding up from the underside of the board, a typical stencil cannot be used.

Until now, the advantage of a dispensing machine is its ability to place dots on a board regardless of these obstructions.

Given all these difficulties, is this approach practical?

Well, in fact both problems have solutions that are fairly straightforward.

As it turns out, both problems are addressed in part by thinking about the stencil a bit differently.

Obstacles on the printed side of the board can be addressed by machining route pockets into the underside of the stencil.

Of course a much thicker stencil is needed for this.

A 1mm thick plastic stencil normally provides enough material to clear typical SMT components, whereas a 3mm thick plastic stencils would be needed to accommodate through-hole components.

This will provide adequate clearance for protruding leads much in the same way that the underside of a metal mask is etched for printing solder paste onto a PWB that has already been printed with flux or silver epoxy for a flip chip.

This solution can be flexible to accommodate a variety of configurations.

Plastic stencils as thick as 8mm have been used to clear tall obstructions such as RF shields or partly assembled products with the board deep in the assembly.

This leaves the problem of printing glue dots of variable heights with a single thickness stencil.

How can the stencil solve this problem?

In the screen-printing application the solder paste release characteristics for a given stencil design can be predicted by ensuring the ratio of the aperture area to the stencil wall area is 0.7 or greater.

In other words, if you have a thick stencil with a small aperture, more paste will stick to the walls of the stencil and only a small amount will be deposited on the board.

When printing glue the same rule applies, and we can take advantage of it.

By increasing the aperture diameter in our thicker stencils we get larger, tall dots of glue.

Conversely, reducing the aperture's diameter produces smaller, low dots.

This simple understanding gives us the control we need to accomplish varying dot heights with one pass of the screen printer.

Although adhesive printing with squeegees is possible, there are process concerns that need to be understood before beginning.

The nature of the glue and how it reacts when left exposed on the stencil is one concern.

When exposed to the environment, adhesives absorb moisture and can lose solvent through evaporation, which can affect its performance.

Maintaining control of the humidity, either on the production floor or within the printer itself, is one solution.

However, the preferred solution is to keep the adhesive contained and protected from the environment until it is placed on the board.

Another issue with squeegees is the number of prints required before that first quality print is achieved.

The thicker the stencil is, the more prints required to 'fill' the stencil.

As the thickness of the stencil increases it becomes more difficult to force glue into apertures with a very small diameter.

With a 1mm thick stencil once you've made a few passes of the squeegee you can pump glue into the apertures with a single squeegee pass - the stencil stays full.

However, it can take several more passes to fill the small holes in a 3mm stencil.

This works against our hope of gaining speed in the adhesive dot placement arena.

So what to do?

First, counterbore the top of the smaller apertures.

This reduces the length of the narrow section.

Typically you'll find that you have to counterbore all holes with a diameter less than 1mm.

That in itself is no problem, but unfortunately this fix creates a problem of its own.

Although the squeegee forces glue into the counterbore with some ease, as the squeegee tip crosses the counterbore, an increasingly large hole opens up behind the squeegee, causing the glue to re-emerge through the top of the stencil rather than being forced down into the small hole.

To make the squeegee work properly you need to use a squeegee blade profile with a reasonably wide flat at the squeegee tip.

This seals the top of the counterbore and forces the glue through the hole.

The counterboring does work, but does nothing to address the problem of exposing the glue to the air.

The best solutions are simple, and the simplest solution to both problems of contamination of the adhesive and the action of the squeegee involve putting the print mechanism in an enclosed print head (direct imaging).

In a direct imaging system the internal pressures within the enclosed head itself control material flow.

As in the system shown for this article, Instead of a squeegee forcing paste or adhesive through a stencil, an air piston applies from 0.5 to 4.0bar of pressure to the top of a paste (or adhesive) cassette, or to the bladder of a rechargeable material cartridge.

This system generates paste pressure independently of print speed, allowing for consistent application.

The constant pressure system eliminates the problem the squeegees had with the counterbore, as well as the need for multiple print passes.

The enclosed transfer head takes care of the contamination problem by simply keeping the moisture out in the same way the dispensing machine does, by holding the material in a container until it is printed.

Keeping the stencil clean and containing the adhesive to reduce waste is taken care of by Mylar wipers that force excess material back into the head.

There are also skis at both ends of the transfer head that act as a dam to keep adhesive from sneaking out in those directions.

Containing the adhesive in this fashion results in fewer work stoppages for cleaning or replenishing material.

Also, because the material is isolated from the air, material left over in the head from one shift can be used on the next shift, or even a few days later.

In the end, it is possible to deposit multiheight adhesive dots at speeds three or four times faster than traditional dispensers work, and with better overall process control.

Using a screen printer to resolve the adhesive deposition bottleneck comes with a lower price tag than the alternative of adding another dispenser to a line.

Beyond the cost saving for the equipment, production floor space is also optimised.

Further, operator and maintenance training is not an issue because this is the same equipment used to place solder.

Add to that the benefit of only having to maintain spare parts for one type of equipment and we start to see that the benefits are more than just improving line throughput.

A final benefit of this approach is that it helps SMT manufacturers maintain production flexibility.

After all, these systems are screen printers.

If for some reason there was less of a need for high-speed adhesive application in the future, the machinery still plays an important role applying solder paste.

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