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Molding Recipe Study for MUF Solder Crack Improvement
Keywords: solder crack, molded underfill, mold parameters
In recent years, flip chip technology becomes more and more important with benefits of thin package profile, reduction of package outline, excellent electrical and thermal performance by connection of copper pillar bumps (CuP) or C4 solder bumps. In order to fill the die gap to prevent voids problem, two encapsulated solutions could be applied: Capillary Underfill (CUF) and Molded Underfill (MUF). In general comparison, CUF means to dispense underfill first to fill in die gap then proceed over-molding afterward; and MUF is directly fill under and above die by mold compound. The advantages of MUF solution are low cost and high throughput, however, it will suffer other assembly issues such as solder extrusion and solder crack, and might resulted in potential function failure. To form these kinds of defects, we suspected that solder will plastic deform under thermal stress treatment, which comes from unbalance mold transfer pressure and material expansion stress during thermal process. In this article, we have tried to investigate the mechanism of solder crack through molding recipe DOE (Design of Experiment ) and mold flow simulation. The test vehicle is 12x12mm2 FCCSP, with 5.89x4.99mm2 die size. The bump type is copper pillar bump and pitch/size are 126um and 35x60um2, respectively. The molding recipe have been evaluated by cross section, and it revealed that molding transfer time and molding temperature are directions toward improvement of solder crack issue.
Michael Shy, Sr.Engineer
ASE Group Chung-Li
Taoyuan, Taiwan

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