Reliability performance and failure mode of high I/O thermally enhanced ball grid array packages

The results from a previous investigation show that thermally enhanced ball grid array (BGA) packages can be susceptible to early failures during accelerated thermal cycling. Unlike solder fatigue failures, these failures occur at a relatively low number of cycles and are attributed to a brittle fracture mechanism in which cracks propagate rapidly through package metallization or solder interfaces. Early, brittle interfacial failures are not true wear-out failures typical of long term reliability testing. Usually these early or "infant mortality" failures are indicative of assembly quality or package quality defects. An hypothesis based on previous reliability testing suggests that these thermally enhanced packages can meet long term reliability requirements of the telecommunications industry once the package quality issue is corrected [1]. This paper presents the results of assembly and reliability testing of a set of thermally enhanced packages that are found to be immune to early brittle interfacial failures during reliability testing. Failure mode analysis (FMA) of tested components shows that the crack propagation in the improved package is exclusively by a solder fatigue mechanism. The measured higher characteristic life of these packages validates the hypothesis that the long term reliability is acceptable when package defects are eliminated. Since there is no susceptibility to early failures in this set of packages, the inspection and FMA are expanded in order to understand the improvement in package quality. The proposed mechanisms or phenomena contributing to brittle fracture are reviewed and discussed with respect to the known characteristics of these thermally enhanced packages. A phenomenological approach is used to analyze the inspection and FMA data, to focus on factors critical to initiating brittle fracture, and to suggest possible root cause in the event of a brittle failure.