Highly thermally conductive, low stress molding compounds

There has been an ongoing interest in highly thermally conductive molding compounds for many years. One major reason is the desire to find a low cost alternative to heat sinks, heat spreaders, and fans for large plastic packages. Of course, the new compound should exhibit the same reliability and attributes as the lower conductivity compound it is replacing. One feature especially important is low stress, as the IC devices which go into large packages are large themselves. A typical formulation would have thermal conductivity values of around 0.5-1.0 W/m/spl middot/K. To boost a compound's thermal conductivity, various fillers, have been employed, such as alumina, crystalline silica, and, recently, aluminum nitride. Each has advantages and disadvantages, but none can meet the moldability and low stress performance of fused silica to date, which is the standard filler used. Thus, most of the thermally conductive compounds on the market are not low stress, and applications are limited due to the difficulties in transfer molding. The challenge now to develop this compound technology is balancing thermal conductivity with moldability and stress relief characteristics. It is a delicate balance, and technologies are continually evolving, with new fillers and new resins (biphenyls) to explore. In this study, these latest technologies were evaluated. For the first round of evaluations, six thermally conductive, low stress, molding compounds were studied, both in 11-lead isolated TO-220 packages and 160-lead, 28 mm, PQFP packages.