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IntroductionThe 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 [ 11.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 M A data, to focus on factors critical to initiating brittle fracture, and to suggest possible root cause in the event of a brittle failure. Statement of the ProblemIndustry packaging demands are accelerating in response to design requirements for higher electrical and thermal performance. Near the: forefront of this trend is the development of a class of thermally enhanced ball grid array packages. These packages are characterized by a variety of package substrate designs and materials, increasing size (footprint) and weight, and pincounts in excess of 500 U 0 [2]. Large cavity-down perimeter array packages improve heat dissipation but present technical challenges for assembly processes and attachment reliability.A previous publication documents the occurrence of early failures ("infant mortality" failures) of large, thermally enhanced, cavity-down packages during reliability testing using accelerated thermal cycling [I]. A significant number of failures occurs at a relatively low number of thermal cycles by brittle crack propagation at the interface between the package metallization and the eutectic Sn-Pb solder ball. This is a catastrophic failure mode, that is completely different than the solder fatigue failure mode experienced normally at higher...

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Mass fraction profiling is a specialized tomographic technique designed to allow one to measure the concentration profiles of the component species of a synthetic composite material. While developed for the express purpose of determining the dopant, density, and porosity profiles of unsintered soot boules, the method hould also be useful in analyzing other types of ceramic such as dried gels. Mass fraction profiling amounts to a three-step process consisting of scanning the object of interest with well-collimated beams of x rays, reconstructing the attenuation coefficient profiles for several x-ray energies, and generating the density and concentration or mass fraction profiles. The method for determining these profiles is outlined, and some measurement results obtained for soot boules are presented.