Finding the Cure for Epoxy Dispensing Frustrations
Ken Harvill, Multi-Seals, Inc. — ECN MAGAZINE 12/15/2001
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Traditionally, dispensing epoxy onto electromechanical components is one of the more frustrating processes in manufacturing. Needles clog, requiring costly shutdown and cleanup procedures. Epoxy drips from the syringe, dispensing unwanted material onto leads or other critical areas of a component. Time and temperature affect the polymerization of most liquid adhesives, causing variations in viscosity and seal quality. In fact, the level of expertise required to mix, meter and dispense two-part epoxies is often so complex that many have compared the process to black magic.
Dispensing technologies, such as rotating needles, heated outlets and vacuum pullback features, help to minimize clogged needles, unwanted drips, viscosity variations and other dispensing irregularities. A simpler method of avoiding these frustrations is to seal components with preformed epoxy. Epoxy preforms are molded B-staged thermosets that are solid at room temperature but melt, flow and set when heated. Epoxy preforms do not clog or drip from needles because they are not dispensed through a syringe, and pot-life concerns are avoided because preformed epoxy is pre-mixed. However, the best means of assuring trouble-free sealing, whether the adhesive is solid or liquid, is to first design a component that will accommodate a sealant.
Designing a Sealant-Friendly Component
Some component features will interfere with either preformed or liquid epoxy. Generally, this occurs when manufacturers attempt to add a seal to an existing unsealed version of a part. Most of these difficulties can be avoided if sealing requirements are considered early in a component’s design.
When a sealant is added to a connector, switch or other PCB component, a thin layer of material will travel up a contact due to capillary attraction. If the sealant travels too far, it will interfere with solder filleting. Designers usually include a standoff on PCB components to allow sufficient filleting; however, the standoff may not be high enough to accommodate sealant wicking. An epoxy seal will generally rise about 0.015″ up a terminal as the sealant cures. For example, if an epoxy preform is 0.020″ thick, the epoxy will rise 0.035″ above the surface on which the preform is loaded. Therefore, designers should include a standoff height of at least 0.035″ for 0.020″ of epoxy.