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Applying Adhesives Accurately and Efficiently
December 31, 1969 |Estimated reading time: 7 minutes
As modern placement systems begin to process 60 to 80,000 components an hour, it is important to rethink other SMT assembly processes especially the bottleneck of dispensing adhesives.
By Mike O'Hanlon
It is amazing to think that improvements in assembly technology have gotten to the point that the simple matter of putting down adhesive to hold 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 require, makes this an undesirable solution.
Fortunately, existing technology provides an alternative one that is readily available once we rethink the tools that already are available to the SMT manufacturer.
The Screen Printing OptionPrinting 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, resulting in half a million solder pads printed per hour.
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 the board have the same standoff, a simple metal stencil of the appropriate thickness will work well. This is a relatively simple and robust printing process. Printing dots of various heights, however, becomes more complex with a stencil.
The second obstacle is clearance. If the product has SMT components on the side of the board to be printed, or through-hole component leads protruding from the board's underside, 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.
Stencil ModificationAs it turns out, both problems can be addressed by thinking about the stencil differently. Obstacles on the board's printed side can be addressed by machining route pockets into the stencil's underside. However, a much thicker stencil is needed for this. A 1 mm thick plastic stencil typically provides enough material to clear typical SMT components, whereas 3 mm thick plastic stencils would be necessary to accommodate through-hole components. This provides 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 printed circuit board (PCB) that already has been printed with flux or silver epoxy for a flip chip. This solution can be flexible to accommodate various configurations. Plastic stencils as thick as 8 mm have been used to clear tall obstructions such as radio frequency (RF) shields or partly assembled products with the board deep in the assembly (Figure 1).
Figure 1. Machining a 3 mm thick stencil to accommodate component leads allows the stencil to sit flush on the board and apply glue in a controlled fashion.
This leaves the problem of printing glue dots of variable heights with a single thickness stencil.
In the screen printing application, the solder paste release characteristics for a given stencil design can be predicted by ensuring that the ratio of the aperture area to the stencil wall area is 0.7 or greater.
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When using a thick stencil with a small aperture, more paste will stick to the stencil walls, depositing only a small amount on the board. When printing glue, the same rule applies and can be used to one's advantage.
By increasing the aperture diameter in thicker stencils, larger, taller glue dots result. Conversely, reducing the apertures' diameter produces smaller, lower dots. This understanding provides the control needed to accomplish varying dot heights with one pass of a screen printer.
Squeegees vs. Direct ImagingAlthough adhesive printing with squeegees is possible, there are process concerns that must 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, in turn, affects performance. Maintaining humidity control, either on the production floor or within the printer itself, is one solution. The preferred solution, however, is to keep the adhesive contained and protected from the environment until it is placed on the board.
Another issue with squeegees involves the number of prints required before achieving the first quality print. The thicker the stencil, the more prints required to "fill" it. As stencil thickness increases, it becomes more difficult to force glue into apertures with small diameters. With a 1 mm thick stencil, a few passes of the squeegee can pump glue into the apertures with a single squeegee pass, and the stencil stays full. It can, however, take several more passes to fill the small holes in a 3 mm stencil working against the goal of gaining speed in the adhesive dot placement arena.
To address this dilemma, begin by counterboring the top of the smaller apertures. This reduces the length of the narrow section. Typically, all holes with a diameter less than 1 mm must be counterbored, creating a further problem. 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, a squeegee blade profile with a reasonably wide flat at the squeegee tip is needed. This seals the top of the counterbore and forces the glue through the hole.
Figure 2. The transfer head uses an air piston to provide controlled pressure on the material (paste or glue) cassette for an accurate material flow through the stencil apertures; the head is enclosed to keep out contamination.
While the counterboring does work, it does not address the problem of exposing the glue to the air. The best solutions are simple, and the simplest solution to both problems of adhesive contamination and the squeegee action involve putting the print mechanism in an enclosed print head (direct imaging). In a direct imaging system, the internal pressures within the enclosed head control material flow. As shown in Figure 2, instead of a squeegee forcing paste or adhesive through a stencil, an air piston applies from 0.5 to 4.0 bar 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 solves the contamination issue by keeping out the moisture in the same way the dispensing machine does by holding the material in a container until it is printed.
Figure 3. A set of Mylar wipers contain the glue within the head assembly. As the head moves over the stencil, the paste within the transfer head rolls and the wipers guide the adhesive back into the head, leaving no adhesive on the stencil.Keeping the stencil clean and containing the adhesive to reduce waste is accomplished with Mylar wipers that force excess material back into the head (Figure 3). Additionally, there are skis at both ends of the transfer head that act as a dam to keep adhesive from leaking 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 multi-height 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 the line. Beyond the cost saving for the equipment, production floor space also is optimized. Further, operator and maintenance training is not an issue because it is the same equipment used to place solder. Add to that the need to maintain spare parts for only one type of equipment and it becomes clear 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. These systems are screen printers. In the future, should there be less need for high-speed adhesive application, the machinery still plays an important role, applying solder paste.
Mike O'Hanlon, senior applications engineer, may be contacted at DEK, 8 Bartles Corner Rd., Flemington, NJ 08822; (908) 782-4140, ext. 262; E-mail: mohanlon@dek.com; Web site: www.dek.com.