Board Level Characterization of Pb-free Sn-3.5Ag Versus Eutectic Pb-Sn BGA Solder Joint Reliability

The need to transition to the use of Pb-free solders in the microelectronics industry has resulted in a vast amount of research and development work to identify suitable replacements for eutectic Pb-Sn. Although the near-eutectic ternary Sn-Ag-Cu (SAC) alloys have gained widespread acceptance, they have been found to exhibit poor component and board-level performance in certain applications, such as prolonged high temperature exposure, impact testing, and temperature cycling. Another Pb-free alternative, eutectic Sn-Ag (Sn-3.5Ag), is gaining increasing recognition as an alloy which provides superior component and board-level performance, while not showing the metallurgical deficiencies observed in the SAC materials. As a follow-up to previous work which addressed the comparative component-level performance of the Pb-Sn versus SAC versus Sn-Ag alloys [4,5], this study characterized the board-level temperature cycling performance of the eutectic Sn-Ag (Sn-3.5Ag) solder versus eutectic Pb-Sn (63Sn-37Pb). Test boards were fabricated to analyze the performance of both electroless Ni/immersion Au (ENIG) and copper organic solderability preservative (Cu-OSP) pad surface finishes. Samples were characterized for thermal, electrical, and mechanical response to the test conditions, including both static and dynamic Moire interferometry, and metallographic cross sectioning at several read points. It was found that the Pb-free Sn-3.5Ag BGA alloy exhibited superior performance throughout the entire range of testing.