New Adhesive Materials with Improved Thermal Conductivity

Abstract

There is increasing interest in the use of conductive polymeric adhesives as a replacement for soldering technology in electronic packaging applications. Relatively low cost, lower toxicity, reworkability and ease of processing make them an attractive alternative. To date, however, most conductive adhesives have certain critical limitations that inhibit their utility in many applications. We report on the development of new, low-temperature sintering adhesives that combine ease of processing of polymeric bonding materials with performance characteristics closer to solder. These new formulations incorporate novel fluxing polymeric resins in combination with a blend of metal alloy filler particles. During the cure process, the metal filler particles sinter together and are capable of forming "solder-like" metallurgical connections to a variety of metal surfaces resulting in lower and more stable thermal contact resistance values. These materials have both high electrical and thermal conductivity-closer to solder materials than most polymerbased materials but with the processing advantages of the best polymeric adhesives. IntroductionThe issues associated with the thermal management of electronic devices are complex. New generations of electronic devices are growing smaller but have increased power and performance. This trend towards miniaturization, coupled with higher operating frequencies, places ever-increasing demands on design engineers to remove heat from circuitry. Unfortunately, most of today's polymer-based conductive bonding materials are not suitable for the more demanding applications. While some have excellent bulk thermal properties, they suffer from higher than expected contact resistance, leading to poor thermal performance.As a result of the limitations of current polymeradhesives, many power devices make use of solders such as eutectic gold-tin to attach the device. The problem with the use of solder bonding lies not so much in its reliability or performance, but in the high temperatures at which it must be processed. For example, eutectic AuSn solder has a liquidus temperature of 280°C, hence process temperatures greater than 280°C are required. Additionally, rework of AuSn solder is not an option. In assemblies incorporating multiple devices, this can lead to yield issues since a single bad device can lead to the loss of the entire assembly. In certain applications, such as microwave radar systems, manufacturers circumvent this problem by soldering the device to an intermediate shim and then using a conductive polymer adhesive to bond this assembly to a thermal heatsink. Unfortunately, the use of current polymer adhesives again becomes a limiting factor, contributing to a significant portion of the thermal rise in