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Nanoindentation Characterization of Lead-free Solders and Intermetallic Compounds Under Thermal Aging
Keywords: Lead-free solder, Nanoindentation, Strain rate
The enforcement of environmental legislation to replace lead based solders in electronic products has resulted in a lot of studies on lead-free solders. Many lead-free solder alloys have been proposed as alternatives to the conventional eutectic Sn-37wt%Pb (SnPb). The leading candidates include binary and ternary alloys based on the combinations of Sn, Ag, and Cu. This includes a class of near-eutectic SnAgCu (SAC) alloys that provide acceptable electrical and mechanical properties. The increasing applications of lead-free solders in microelectronics have called for the needs of deeper understanding in the mechanical properties of solder joints. The present study investigated the mechanical properties of three kinds of SAC lead-free solders, namely, Sn3.8wt%Ag0.7wt%Cu (SAC387), Sn3.0wt%Ag0.5wt%Cu (SAC305), Sn1.0wt%Ag0.5wt%Cu (SAC105), and their intermetallic compounds (IMCs) with organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG) pad finishes, respectively. Nanoindentation was used to measure the Young's modulus and hardness of the solder matrix and intermetallic phases. In addition, thermal aging was applied to the samples for 500 hours and 1000 hours at 125C. It was found that for both Sn-rich phase and eutectic phase, the mechanical properties exhibited observable dependence on the aging time and indentation strain rate. The hardness of SAC solders and IMCs increased when the indentation strain rate became higher, but decreased when the thermal aging time became longer. The isothermal aging condition had little effect on the Young's modulus of IMC. The aging softening effect occurred in both lead-free solders and IMCs. SEM inspections revealed that the grain size of SAC solders and the thickness of IMCs increased with respect to the thermal aging time. The results of this study will be used in a subsequent computational modeling for the board level lead-free solder joint reliability evaluation under mechanical drop tests.
Shi-Wei Ricky Lee, Professor
Kowloon, HKSAR,
Hong Kong

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