Flexibility in designing new printed circuit assemblies requires the use of both surface mount technology (SMT) and through hole technology (THT). There are many ways to deal with the mixture of the technologies to manufacture boards. Ideally, designers will attempt to keep the soldering technology consistent by going almost exclusively to a surface mount technology as recommended by DFM best practices. In many cases there is a need for THT components on one side of the PWB that must be concurrently processed with SMT components on the opposite side.

In cases where the mass of the component is greater than 4 grams, a heat cured adhesive can be placed on the bottom side components with an automated dispensing system that compliments the subsequent pick and place operation on the SMT side of the PWB. A dancer wave, or secondary wave, designed for SMT components, is typically used in this assembly model to “kiss” the secondary side with enough solder to form a proper fillet.

There are some precautions that must be considered in the dispensing of adhesive under components such as 0603 resistors and similar components. The volume of adhesive that is applied to the board must be enough to secure the component when the pick and place handler sets it on the board, while preventing excess adhesive to bleed under the pads and prevent the formation of an acceptable solder joint. According to IPC-A-610 section 8.1 (Staking Adhesive), “regardless of class, there is to be no adhesive present on solderable surfaces of the termination area, and that adhesive should be centered between the lands.”

The capability to apply adhesive in a precise manner is a critical manufacturing step for electronic assemblies, where the
application of a gasket or the dispensing of an adhesive dot becomes too expensive and inaccurate for hand operations. The complexity of the various designs, along with increasing component density, often necessitates the use of intricate applications of adhesive. Removal of unwanted adhesive due to excess deposits adds additional cost to the assemblies. Quite often, excessive or missing adhesive does not manifest itself until further down the process, where dewetting can occur as a result of extraneous adhesive deposits on pad and fillet areas.

Dispensing Systems

If a pneumatic dispensing unit is used, the adhesive dispenser settings must be experimentally determined to achieve a repeatable glue dot. Some of the variables that affect the dot size are tip diameter, dispensing pressure, actuation time and adhesive viscosity. As the adhesive in the dispenser is used, there will be a greater volume of air, resulting in a longer time to build up pressure. If the dispense time is short, the volume of adhesive dispensed with each pulse will be less. A control process, such as refilling the adhesive after a given number of cycles, will allow the unit to maintain a consistent dispense volume.

Adhesive dispensing can be versatile and cost effective, and may help simplify some of the more complex assembly operations which may require multiple thermal steps. For more information related to this article, please call the Helpline at (610) 362-1320.