Development of void free, high reliability underfill encapsulated fine pitch system on flex packages

In system on flex (SOF) packages with high density I/O's, a number of factors induces concentrated stress field around the bonded areas (bumps & traces) during assembly process. For example, the choice of underfill / encapsulant and the associated processes could affect the package integrity. Poor control of the underfill process could potentially result in delamination on various interfaces. It also increases the chances of moisture absorption thus potentially results in electro-chemical corrosion of the stressed area which affects the package reliability. In this paper, we present the development of processes targeted at ensuring good underfill reliability. It is revealed that delamination is driven by either the adhesion failure within the substrate and underfill or by the stresses induced in the interfacial sites. We developed an extra cleaning process to remove seed-layer Cr+ to avoid adhesion failure due to contamination of flex. Filler type underfill material is found to give more strength than non-filler materials but at a cost of flow rate and capturing voids. Effect of peak temperature during assembly process was studied; this shows that through controlling the reflow temperature, the underfill integrity may be maintained which potentially allows more complex assembly processes. However, at the same time, poor control of the reflow temperature may significantly affect the underfill material causing it to deteriorate at a rapid rate. Further development into assembly processing at lower temperature would be beneficial for high reliability of SOF packages.