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60 SMT007 MAGAZINE I AUGUST 2024 Discussion Although 3,000 cold-biased thermal shock cycles were performed, relatively few fail- ures were observed in the package daisy- chain resistance data. Comparing the CTFs of CTBGA228 under cold-biased cycles, they failed at much higher than CTFs under hot- biased conditions. A Norris and Landzberg relationship 13 , even though not applicable for cold-biased thermal cycled condition, it proj- ects an AF ~2.9 times, which somewhat pre- dicts the increase in the CTFs. Joint failures in the CTBGA228 package appear to be highest in the second row of solder joints, presumably due to being near the high-stress region associated with the edge of the silicon die shadow. e observed failure mode for solder joints in the cold- biased test is cracking along the interme- tallic compound (IMC) boundary with the bulk solder. is failure mode is in common with previous results for hot-biased thermal cycling of the same package. However, closer examination of representative solder joints from the cold-biased and hot-biased test groups does reveal key differences. Specifi- cally, due to the high temperatures experi- enced during thermal cycling, solder joints from the hot-biased group exhibit more grain coarsening. Also, crack initiation can be seen within the bulk solder in the near-intermetal- lic compound region in some joints. Solder joint graininess and crack initiation of this type is not seen in the joints subjected to cold-biased thermal cycling. Although not the dominant mode, this secondary failure mode, which relies on thermally-driven grain growth, appears exclusive to hot-biased thermal cycling. Conclusions is paper presented details on the effect of cold extreme tempera- ture cycle (-105°C to +40°C) when compared to the inverse hot extreme (-40°C to +105°C) for several advanced FPBGAs with SnPb solder assemblies. Cold-biased thermal shock cycle was representative of deep-space missions; however, in highly accelerated rapid rate and low dwell time conditions. Key find- ings are summarized below. • Cycles-to-failures under cold-biased thermal cycles were about twice longer than those under hot-biased conditions. CABGA208, which previously showed failure under hot-biased TC, did not fail under cold-biased TC; therefore, CTFs for CTBGA228 were compared for CTFs. • e 3D X-ray could not clearly identify the location of failures. Micro-sectional evaluation, however, indicates that fail- ures under both cold- and hot-biased con- ditions are in solder joint at the package side with hot-biased showing higher grain growth. Failures were from the edge of the die for both cases as well. • Even though it is known that Norris Lan- dzberg relationship for solder joint is not applicable for cold-biased thermal cycle conditions, nevertheless it projects an AF of about three times of (cold-biased)/(hot- biased) CTFs whereas the limited test data shows an AF of about two times. e lessons learned and qualification guide- lines presented allow testing under highly Figure 9: A typical shear failure for SbPb CABGA208 showing fine, straight crack path at the package side.