Zero die delamination on die overhang & upset packages

Some discrete power packages require very stringent die attach requirements such as overhang die on upset leadframe. This type of packages shows physical weak point at the epoxy layer whereby reliability stress easily propagate separation near the junction of overhang die and leadframe upset curvature. Normal solution of implementing treated leadframe was not feasible because there was very little mold adhesion on the small flag. By working closely with epoxy supplier, certain epoxy formulation did show improvement in reliability. The main change was on the resin content, promoting adhesion strength. However, production performance started to deteriorate over short period of time. The major issues included fast hardening effect, causing a lot of rejects with insufficient epoxy coverage. Further investigations revealed other severe quality issues, including channel voids, low die shear strength, and also die delamination. Multiple discussions and evaluations were conducted with epoxy supplier, but all failed to identify any assignable or significant root cause. Therefore, a Six Sigma DMAIC (Define, Measure, analyze, Improve, Control) project was executed by performing a series of evaluations to explore effect of all possible root causes, and finding optimum settings. The study covered wide range of factors, from material, machine, until development of whole process window. From evaluations of 10 major factors, the most optimum combination was established. When evaluation results confirmed with larger sample size, the confident level became high to achieve consistent and stable production performance. More verification was performed with most stringent reliability tests, and all units passed without any doubt. All the engineering works and optimum process window were then qualified for production. Finally, production was running with excellent performance in obtaining zero channel voids, high die shear strength and zero die delamination. On top of that, change of epoxy type also generated additional cost saving, together with all the quality advantages. The study of this stringent package became benchmark for all other epoxy packages.