Molded underfill development for flipstack CSP

The current standard for flipchip encapsulation is a two step process. Following flipchip attach, a liquid underfill material is dispensed along the die edge and allowed to flow, via adhesive capillary force, through the gap between the flipchip die and the PCB. Upon cure of the liquid underfill, complete die encapsulation is achieved with a standard transfer mold process using preformed epoxy pellets. When applied to a FlipStacktrade CSP product, a matrix of die on the substrate are individually underfilled and cured. The transfer molding process then encapsulates all die in a single transfer of the molding compound. Nowadays, the molded underfill process includes the injection of a mold compound into the mold cavity, underfilling the space between the flipchip die and the substrate with mold compound, in order to simplify the underfill system and increase solder joint reliability. Migration to this molded underfill process offers substantial cost reduction through savings in materials, equipment & tooling, floor space, throughput, and other factors associated with the liquid dispensing process. The scope of this evaluation was to qualify the molded underfill process for FlipStack CSP, or an equivalent flipchip CSP application, in order to meet the following requirements: Low cost process and material; Green and low stress molded underfill; Filling capability up to 40 mum gap; Max package height: 1.2 mm Rel target: L3/260'C, TC1000X, HAST & HTS. Through this study, a robust molded underfill material set and process was identified for our applications.