TIM1 reliability of polymer solder

Abstract

Next-generation microprocessors require an advanced first-level thermal interface material (TIM1) to provide the lowest possible thermal resistance while improving reliability, ease-of-use and, most importantly, lowering cost. A potential solution is a newly developed polymer solder hybrid paste which consists of novel liquid epoxy and a mixture of two types of solder powders: tin-bismuth and indium (melting points 140°C and 157°C, respectively). Upon thermal cure, the TIM forms an interpenetrating network (IPN) of Low Melt Alloy (LMA) in a soft gel epoxy matrix (LMA gel). Using the thermal test vehicle which consists of a flip-chip thermal test die mounted on an organic laminate, changes in TIM thermal resistance after solder reflow, and moisture conditioning, high temperature bake, and thermal cycling were monitored. Results indicated that TIM reliability was strongly influenced by the elastic modulus of the epoxy-based gel. An optimum range of elastic modulus was found that showed no significant thermal degradation after all reliability tests. For example, the thermal resistance was significantly increased after 500 cycles that ranged from −55 to 125°C when the modulus of the matrix resin was less than 50 kPa. Failure analysis showed this was caused by TIM being “pumped-out”. A similar increase in thermal resistance was also observed for modulus value exceeding 1000 kPa due to interfacial delaminations. The thermal resistance values for the LMA gel is slightly higher as compared to Indium performance, but the main benefits are ease-of-use and significantly lower cost, as the process does not require additional metallization of the die and heat sink.