Material system for high performance DRAM-CSP

The /spl mu/BGA/sup (R)/ which Tessera Inc. developed is one of the most suitable chip scale packages (CSP) for high speed DRAMs. However, the larger the chip size, the harder it is to achieve high levels of package reliability. Based on the thermal stress analysis, we have determined the threshold values of elastic modulus and coefficient of thermal expansion (CTE) of both the adhesive film and the liquid encapsulant to achieve higher thermal cyclic properties. Through this study, we have developed new polymer alloys composed of a low modulus polymer and an epoxy resin. CSPs fabricated with the newly developed materials showed high reliability including thermal cyclic properties and solder reflow resistance at high temperature under Pb-free requirements.