Development of a low stress silicon-carbon liquid encapsulant

Mismatches in thermal expansion between encapsulant, die and substrate lead to stresses that can cause failures in thermal cycling of electronic parts. These stresses become especially troublesome as die size increases. In order to minimize failures, low stress encapsulants are needed. This paper describes development of a low stress silicon-carbon liquid encapsulant. Development was done on multichip modules (MCM-L) with large die. In order to develop a low stress encapsulant, a designed experiment was run. Variables in the full factorial design included: (1) encapsulant filler level, (2) encapsulant elastomer type, (3) encapsulant elastomer level, (4) board thickness, and (5) cure schedule. The material variables (1-3) gave rise to 8 formulations with widely varying properties, such as CTE and modulus. Air to air thermal cycling (-55°C to +125°C) was performed on MCM-L test boards with encapsulated 7.62 mm×19.05 mm (300 mil×750 mil) die. Parts were tested in situ throughout the cycling up to 2000 cycles. Results of the designed experiment indicated that filler level was the dominant variable, with high filler level providing the most reliability. Several other variables perhaps had minor effects. Continuing work is examining filler content, elastomer content, and dispensability further