Optimum conditions of molding VFBGA chips with very minimal gold wire damage using the Taguchi methods

This study employed the analysis of mean (ANOM) of the Taguchi methods to determine the optimal conditions for the very fine pitch ball grid array (VFBGA) transfer molding of a microelectronic assembly with the aim of minimizing wire damage. The optimal conditions include: (1) a substrate preheat temperature of 177 °C, (2) two-step mold clamping in which the clamping force in the first-stage is 9 t and the full clamping force in the second-stage is 28 t, (3) a mold temperature of 180 °C, and (4) a total of 20 vacuum holes in the bottom mold. Experiments to confirm the efficacy of the optimal conditions indicated that the average percentage of samples with wire damage was reduced to 0.02%. The percentage contribution of each controllable factor in the current investigation was also determined via ANOVA (analysis of variance) of the Taguchi methods. Among the four controllable factors, the number of vacuum holes in the bottom mold proved to be the one with the greatest influence on wire damage with a percentage contribution of 71.25%. We also describe the mechanism underlying wire damage associated with vacuum holes. Our findings led us to modify the existing VFBGA process, wherein the maximum defect percentage per sub-lot was decreased from 1.31% to 0.12%, and the average defect percentage in 14 sub-lots was decreased from 1.10% to 0.03%.