Impact of 5% NaCl Salt Spray Pretreatment on the Long-Term Reliability of Wafer-Level Packages with Sn-Pb and Sn-Ag-Cu Solder Interconnects

Understanding the sensitivity of Pb-free solder joint reliability to various environmental conditions, such as corrosive gases, low temperatures, and high-humidity environments, is a critical topic in the deployment of Pb-free products in various markets and applications. The work reported herein concerns the impact of a marine environment on Sn-Pb and Sn-Ag-Cu interconnects. Both Sn-Pb and Sn-Ag-Cu solder alloy wafer-level packages, with and without pretreatment by 5% NaCl salt spray, were thermally cycled to failure. The salt spray test did not reduce the characteristic lifetime of the Sn-Pb solder joints, but it did reduce the lifetime of the Sn-Ag-Cu solder joints by over 43%. Although both materials showed strong resistance to corrosion, the localized nature of the corroded area at critical locations in the solder joint caused significant degradation in the Sn-Ag-Cu solder joints. The mechanisms leading to these results as well as the extent, microstructural evolution, and dependency of the solder alloy degradation are discussed.