Reliability of Lead-Free Electronic Package Interconnections under Harsh Environment

Abstract

Lead-free solder alloy has evolved and applied in industries for many years. The reliability is of concern for lead-free solder joints of assembled circuitry, especially those exposed to high temperature in a long period of time. In order to provide a better understanding of lead-free solder joint behavior in harsh environments, the research investigates isothermal aging of Sn-Ag-Cu (SAC) alloy in elevated temperature and studied its thermal-mechanical properties by examining the microstructure. The purpose of the study is to discover the effect of isothermal aging on the reliability of Sn-Ag-Cu (SAC) assemblies. After studied different surface finishes, we employed Immersion Ag (ImAg), Immersion Sn (ImSn), Electroless Ni/Immersion Au (ENIG) and Electroless Ni/Electroless Pd/Immersion Au (ENEPIG) which have potential for higher reliability and better performance and received increased attention for both packaging and subtracted applications. A full experiment matrix with varying aging temperatures and solder alloys was considered. Package sizes ranged from 19mm, 0.8mm pitch ball grid arrays (BGAs) to 5mm, 0.4mm pitch μBGAs and in additional, 0.65mm MLF and 2512 resistors be particularly tested. Storage condition are temperatures leveling up from 25°C, 55°C, 85°C, 100°C and 125 °C with aging over time periods of 0, 21 days, 6 months, 12 months and 24 months. Afterwards, the specimens all subjected to accelerated thermally cycled from -40°C to 125°C with 15 min dwell times at the high and low peak temperature. The paper presents the experimental data to justify the investigation of the degradation on the characteristic lifetime of SAC alloy on ImAg and ImSn surface finish in elevated temperature environments. A significant degradation in reliability has been observed for both SAC105 and SAC305 in 10mm packages on ImAg during elevated temperature isothermal aging. There were 8.9% and 49.4% reductions in characteristic lifetime for SAC105 and SAC305 after 24 months 85oC aging, respectively. After 2 years aging at 125oC, the package lifetime decreased by 61% for SAC105 and nearly 60% for SAC305. For passive 2512 resistors, the reliability performance was reduced 24.8% and 35.1% after 24 months of aging at 85oC and 125oC respectively. Long-term thermal aging results in significant reliability degradation for Sn-37Pb, SAC105, and SAC305 solder alloys on ENIG and ENEPIG. The reduction of reliability is observed throughout the aging period. There was ~ 25% reduction in characteristic lifetime for SAC105 and SAC305 on ENIG during 12 month/85oC aging and ~ 35% degradation on ENEPIG. After a 12 months/125oC aging, the package lifetime decreased ~ 40% for SAC105 and SAC305. Traditional Sn-37Pb solder outperformed the SAC alloys over long-term aging. Failure analysis showed dramatic intermetallic binary Cu-Sn and ternary Ni-Cu-Sn film growth at the bottom of the solder joint interfaces for ImAg and ENIG/ENEPIG. For 125oC aged samples, the cracks appeared at the corners of both package and board sides of the solder ball and propagated along (near) the IMC location. For the case of aged fine-pitch packages, the failures tended to start at the component side solder ball corner and then propagate along an angled line downwards into the bulk solder.