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Reliability studies of plastic ball grid array assemblies reflowed in nitrogen ambient

In this work, the effect of solder joint geometry, and diameter ratio of solder ball to pad (Ds/Dp), on the reliability of plastic ball grid array assemblies (PBGA) reflow soldered in a nitrogen ambient with oxygen content down to 50 ppm (BTU VIP-70N) is studied. Environmental stress screening such as high-low temperature and humidity cycling (TABAI, ESPEC PL-2FP), and thermal shock (air-to-air, TABAI, ESPEC TSA-70L) are deployed. The uniformity data of solder ball joints, in terms of fatigue life and mechanical shear strength (INSTRON mini44), has yielded useful results on the reproducibility of PBGA similar to typical manufacturing process conditions. All the results show that the reliability of PBGA assemblies reflowed in a nitrogen ambient are better than in a compressed air ambient. The mechanical properties of the PBGA assemblies are found to be best when Ds/Dp is close to 1.2. Theoretical models are developed to explain the experimental data. This work throws new light on major reliability issues of PBGA soldered by nitrogen reflow.