CREEP CORROSION OVER PLASTIC ENCAPSULATED MICROCIRCUIT PACKAGES WITH NOBLE METAL PRE-PLATED LEADFRAMES

Abstract

Field failures were observed to be caused by the bridging of corrosion products across lead fingers. This phenomenon was identified as creep corrosion and was the motivation for this work.

This dissertation advances the state of knowledge on the creep corrosion process and the strategies for mitigation. A range of plastic encapsulated packages with noble metal pre-plated leadframes, from different vendors, with different package attributes, were used in this study. Creep corrosion on the mold compound surface was reproduced in an accelerated manner using mixed flowing gas (MFG) testing in laboratory conditions. Of the three most widely-used industry-standard MFG testing conditions, Telcordia Outdoor was found to be the most effective environment to induce and promote creep corrosion over the mold compound; Battelle Class III environment can also induce the similar effects of creep corrosion on mold compound, but in a limited rate on selective packages; Telcordia Indoor was found to induce no creep corrosion on the mold compound over a 30 day test time. In both Telcordia Outdoor and Battelle Class III environments, packages attributes and applied pre-conditionings were found to have no significant influence on the creep corrosion effect.

Creep corrosion over the mold compound showed a dendritic formation. The thickness of the corrosion product layer on the mold compound tended to increase with increased exposure time. Creep corrosion products were electrically conductive and were able to bridge the adjacent leads, thereby causing electrical shorts. The corrosion products were found to consist primarily of copper oxides, copper chlorides, and copper sulfides. No distinct differences in corrosion products composition was observed on the lead versus the mold compound surfaces. Conformal coating was identified as an effective mitigation strategy to eliminate creep corrosion on noble metal pre-plated leadframe packages.