Reliability of Postmolded IC Packages

Abstract

This paper discusses the current reliability issues involved with typical postmolded IC packages. Four major topics are presented, namely, stress, moisture permeation and corrosion-related problems, adhesion, and outgassing impurities. As the trend moves toward higher lead count, with increasingly larger dies which require more power dissipation within smaller package outlines, stress reduction will become a main concern. Methods to quantify package stresses in situ are outlined, together with the advantages and drawbacks of various schemes for decoupling the silicon die from the molding compound. Since the typical molding compound is permeable to moisture, the package collects water during stringent qualification tests (autoclave, T/H, and HAST), and even under normal ambient conditions. Any ionic impurities in the molding compound will combine with the absorbed water to form an electrolytic pool. This increases the likelihood of corrosion in the presence of passivation defects or exposed metal lines. Device structures designed to follow moisture permeation and detect water-induced damage are discussed. The integrity of the interface between the molding compound and the device components is crucial to the long term reliability of the package. Delamination at the die interface produces complex stress profiles that can result in various defects ranging from metal line shift, passivation cracking, corner die chipping, to ball bond fatigue. Poor adhesion along the leads also opens potential paths for moisture ingress. The effects of assembly conditions and various adhesion enhancing schemes on the package integrity are evaluated. Finally, outgassing of halogenated (bromine and antimony oxide) byproducts added for flame retardancy during high temperature storage life testing results in ball bond degradation. The interaction yields typically porous intermetallic structures which grow in size with exposure time, leading ultimately to bond lifting. Possible reaction mechanisms are presented.