Heel cracking induced by the steep conversion of EMC in large QFP

Bonding wire heel crack observed in large QFP was studied. From the analysis, it was found that this type of open failure was caused by heel crack at stitch bond during molding. The mold simulation was conducted for quantitative analysis. The software considers the polymerization kinetics of EMC since the mold flow depends on thermal excursion. According to simulation, it was proven that the EMC near gate shrinks with very steep gradient of conversion rate. At this moment the chemical shrinkage of EMC results in the fracture of Au wire due to the excessive tension. The mold simulation analysis coincides with experimental results. Both wire bonding and molding parameters were optimized to solve the heel crack problem. The heel crack resistance could be improved by using a capillary which makes large cross-sectional area of stitch bond. The other solution to suppress the heel crack was to use EMC having a smaller chemical shrinkage. Also an additional improvement was obtained by decreasing molding temperature from 175°C to 168°C. Conclusively, the chemical shrinkage of EMC is an important property and needs to be reduced to minimize the transfer molding failures, especially for the large body size packages