Underfill selection strategy for Pb-free, low-K and fine pitch organic flip chip applications

The role of underfills is expanding from preserving solder joint reliability to also protecting fragile low-k chip dielectric layers. Traditionally, solder joints required stiff and rigid underfills. Today, low-k layers require more compliant underfill properties. Further complexity comes from the migration to Pb-free solders and changes in chip carrier materials. The myriad of candidates prohibits long term reliability testing of module hardware for every available underfill. A sequential three phase selection strategy is used to characterize and systematically eliminate undesirable candidates and to identify the few favorable underfills that have a high probability of successfully meeting module reliability requirements. The process includes use of industry practices as well as internally developed characterization methods. From an initial list of 20, the selection process identified five underfills for package qualification testing.