Challenges and resolutions of Pd-Cu wire bond biased HAST failures from additive particle in the green mold compound

In recent years there were numerous reported failures of Cu wire packages after moisture related reliability stress such as biased HAST (highly accelerated stress test) due to corrosion of copper-aluminum intermetallics by trace amounts of chlorine in mold compound. As such mold compound selection and certification are of critical importance to ensure good reliability of packages with copper (Cu) wire. In this paper, we report continuity opens after 192 hours of biased HAST at 130C/85% RH on two devices (Y and P) using existing production green mold compounds. The failure was due to a large additive particle adhering to the Palladium-Copper (Pd-Cu) ball bond. Modifications were made in mold compound raw material milling or crushing process to ensure a more homogenous blend of particles to eliminate the occurrence of such large additive particles in the mold compound. Despite this, large additive particle are still found occasionally as the raw material supplier could not effectively control the maximum size of the particles. Further improvements were then made by using a super homogenizer milling process on blended compound ingredients and a fine sieve to sieve out the large particles from the formulation. This approach was found to work exceptionally well with excellent moldability, reliability and delamination performance. In conclusion, optimum compound formulation and compound fabricating processes are essential factors to ensure reliable performance of Pd-Cu devices in Low Quad Flat Packages (LQFP).