Encapsulation materials for a flip-chip package design: Mechanical requirements.

Requirements for the mechanical properties of the encapsulation material in a flip-chip design to prevent the solder and the encapsulation materials from failure are analytically presented on the basis of the developed stress analysis model. The calculations are carried out for the 32100 Micropac package design, for which silicone gel, low expansion epoxy, parylene and polyimide are currently under consideration as candidate encapsulants. Our analysis has shown that low modulus Dow Corning Q1-4939 silicone gel results in the lowest stresses in solder joints and the encapsulation material itself. Parylene should be considered as the second preference, as far as these stresses are concerned. Polyimide is also acceptable. Epoxies, however, could result in significant stresses in solder joints and are not recommended. The final selection of the most feasible encapsulant should be done, of course, with consideration of all the electrical, chemical, and technological requirements.