High thermal conductance liquid encapsulants for direct chip attach

Effective power dissipation and thermal management are becoming increasingly critical to today´s small and thinner semiconductor packages. In direct chip attach where flip chip and chip on board packages are employed the polymer encapsulant materials can influence strongly the package heat dissipation. The development of liquid epoxy based encapsulants having enhanced thermal conductance using high conductivity fillers is described. Two epoxy resins and a silicon-carbon liquid system are investigated; the filler materials include aluminum nitride and boron nitride. Due to many processing and performance requirements for the liquid encapsulants, the filler loading factor (i.e., packing fraction) is more restrained than that for the molding compounds. Therefore, the filler surface, particle and shape play important roles in affecting both the thermal and theological properties of the encapsulant. For each type of filler, two particle size distributions and different shapes are compared for their effects on the rheology of the filled matrix. The results, albeit preliminary, indicate that thermal conductance can be significantly increased over conventional liquid encapsulants using these fillers. However, pretreatment of the filler particle surface is needed to reduce the viscosity so that the high thermal conductance encapsulants would not incur a severe penalty on other rheological and mechanical properties