Aging Effects on SAC305 Solder Joints: Correlating IMC Evolution, Hardness, and Drop-Shock Performance in ENIG and Immersion Tin Finishes

Abstract The reliability of solder joints is crucial to the performance of electronic devices, particularly with the transition to lead-free alloys, such as Sn-3.0Ag-0.5Cu (SAC305). Solder joints are among the most failure-prone components in electronic assemblies due to mechanical and thermal stresses. Intermetallic compounds (IMCs), such as Ag3Sn and Cu6Sn5, form during soldering and significantly influence the mechanical behavior and reliability of solder joints. However, during thermal aging, these IMCs tend to coarsen and grow, affecting both the microstructure and mechanical integrity of the solder. This study explores how aging affects the drop-shock reliability of SAC305 solder joints. The test vehicle consisted of a chip array ball grid array (CABGA208) component mounted on the PCB, with two surface finishes considered: Immersion Tin (ImSn) and Electroless Nickel Immersion Gold (ENIG). Four isothermal aging conditions were applied at 125 °C with exposure durations of 0, 2, 10, and 60 days. The tests were conducted with a peak acceleration of 1500G and a maximum pulse width of 0.5 ms. Microstructural analysis was performed to investigate the correlation between the growth of the IMC layer and drop performance. The results revealed that aging significantly reduced drop-shock reliability due to the thickening of brittle IMC layers, which promoted crack initiation and propagation. ImSn samples exhibited superior reliability compared to ENIG, attributed to the formation of less brittle nickel-based IMCs. Aging also reduced bulk solder hardness due to the coarsening of Ag3Sn particles.