Effect of Aging on Component Reliability in Harsh Thermal Cycling

Reliability of solder joints has been a serious concern since tin-lead solders were replaced with SnAgCu based solder alloys, especially in harsh thermal cycling environments. Aging has been adding to the detrimental effects of harsh environments on solder joints. Aging has been found to alter the mechanical and structural properties of solder joints, which in turn deteriorates its fatigue life. Several new elements such as Bi (bismuth), Ni (nickel), Sb (antimony) have been micro-alloyed to mitigate the aging effects and thus improve the reliability of solder joints. In this paper, we investigate the effects of aging with different factors such as solder paste, solder sphere alloy and surface finish that constitute a solder joint. Two Bi-based pastes and one non-Bi based paste are considered for this study. Popular solder sphere alloys, namely, SAC105 and SAC305 are considered. The surface finishes are ENIG (Electroless Nickel Immersion Gold) and ImAg (Immersion Silver). The test vehicle used in the study consists of 15mm and 6mm Ball Grid Array (BGA) components, 5mm Quad Flat No-lead (QFN) packages and 2512 Surface Mount Resistors (SMR). One set of boards are isothermally aged at 125°C for 12 months and subjected to thermal cycling with a temperature range of -40°C to + 125°C. Another set of boards were thermally cycled after assembly. After the test, it was observed that most of the CABGA208s (15mm) failed while there were few failures for the other components. So, only the larger BGAs (15mm) are considered in the statistical analyses. Two-parameter Weibull analysis is done to quantify the performance of the different combinations. DOE analysis is also done to determine the most influential factors affecting the reliability. The results showed that aging affects the fatigue life of solder joints and that addition of elements such as Bi, Sb, Ni, Co has helped to mitigate the aging effects.