A reliability and failure mode analysis of no flow underfill materials for low cost flip chip assembly

Development of new material systems will increase flip chip market growth provided they reduce manufacturing time and enhance reliability. Among these new materials are no flow underfills. Properly formulated, these underfills can significantly decrease manufacturing cost by eliminating the fluxing process, the underfill flow process, and the underfill cure process. This work evaluates the reliability of no-flow underfill materials and performs a critical failure mode analysis of flip chip structures using no flow underfills. Six test vehicles and four reliability tests were used to evaluate and analyze the reliability performance of several commercial no-flow underfill materials. Different test vehicles were used to evaluate the effect of varying chip size, interconnect density, pad surface finish metallization, and soldermask opening design. Accelerated reliability tests performed included liquid/liquid thermal shock (LLTS), air/air thermal cycling (AATC), moisture sensitivity preconditioning, and temperature humidity aging (TH). Materials tested in this work demonstrated the ability to survive 1000 cycles in LLTS and AATC without failure, 1000 hours of TH and level three preconditioning. A number of unique failure modes are identified including bulk underfill cracking, fillet cracking, solder interconnect fatigue cracking and underfill interfacial delamination