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Development, Evaluation, and Implementation of Thermally Conductive Component Attach Adhesives in Inline Curing SMT Assembly Processes

1998; Elsevier BV; Volume: 3582; Linguagem: Inglês

1879-2391

Leonard R. Enlow, Dale W. Swanson,

Electronic Packaging and Soldering Technologies

Thermally conductive adhesives are extensively used for component attachment in Surface Mount Technology (SMT) assembly operations. Heat sinking adhesives are required in high-power dissipating devices to maintain θ JA and θ JC for packaged parts. Adhesive curing during inline Infrared (IR) soldering or vapor-phase mass soldering operations can result in misaligned or lifted parts due to shift and swell mechanisms. Shift and swell of parts during inline operations result from solvent in formulations and possibly catalysts that decompose in soldering processes. Volatilization of solvents or decomposition of catalysts induces voids. Misalignment, excessive rework, and degraded thermal conductivity of the cured material result. Process modifications that may be required limit single-process initiative and throughput in rate-manufacturing operations with IR and vapor-phase soldering. Excessive rework or scrap rates for assembly, failure to meet visual inspection, or variable thermal dissipation performance are other consequences of improper adhesive selection or application. Finally, adhesive materials must meet the NASA outgassing standards and be reworkable at or below 80°C. A series of evaluations on off-the-shelf materials that are 100-percent solids materials or solvent-containing adhesives was performed. The commercial materials evaluated were not suitable for a specific, mature program and required rework under excessive conditions or off-line vacuum bake cycles that reduced production throughput. As a result of this initial investigation, a custom formulation was developed by Aptek Laboratories (Valencia, Calif.) and qualified at Boeing/Anaheim. The new formulation, an 100-percent solids epoxy, was evaluated for inline cure characteristics, dispensability through a Camalot dispenser over work life, reworkability, thermal conductivity, outgassing per the NASA test, and insulation resistance at ambient and after MIL-STD-202 humidity cycling. Other process evaluations for qualification included resistance to cleaning solvents, cracking of glass-body diodes, and adhesion to nickel plated (electrolytic/electroless) and gold-plated surfaces. The material developed has been qualified to an existing specification and implemented in low-to-medium volume, high-mix prototype operations at ES&MD-Anaheim and rate-manufacturing operations at the El Paso manufacturing facility.