Effect of thermal ageing on (Sn-Ag, Sn-Ag-Zn)/PtAg, Cu/Al2O3 solder joints

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Abstract

As-fabricated solders of eutectic Sn-Ag and ternary Sn-3.5 wt% Ag-1 wt% Zn alloys are coupled with metallized substrates including PtAg/Al2O3 and Cu/Al2O3 to simulate the solder joint in microelectronics. The growth mechanism of intermetallics and the mechanical properties of solder joints after thermal ageing (150 °C and 200 °C) are evaluated. In this study, a 1206 passive device/solder/metallization/Al2O3 surface mount technology (SMT) assembly is employed, and a Cu stud is attached on the ceramic substrate assembly to evaluate mechanical properties and the fracture morphology by the pull-off test. In addition, microstructure evolution of the interfacial morphology, elemental and phase distribution are probed with the aid of scanning electron microscopy (SEM), electron probe micro-analysis (EPMA) and X-ray diffraction (XRD) techniques. There are two intermetallics (Cu3Sn and Cu6Sn5) formed at the eutectic Sn-Ag solder/Cu metallized layer interface, while only Cu6Sn5 is observed in the Sn-3.5 Ag-1Zn/Cu system. However, in the PtAg metallized substrate, only Ag3Sn is present, regardless of which solders are employed. Cu6Sn5 and Ag3Sn in the Sn-3.5 Ag-1Zn system contain 2–5 at% Zn due to the higher solubility of Zn in both Cu and Ag. The adhesion strength decreases as the time increases for all solder joint systems in the thermal ageing test. The solder joint with eutectic Sn-Ag alloy exhibits higher fracture load than that with Sn-3.5 Ag-1Zn alloy. From the fracture surface analysis, as the ageing time increases, the fracture takes place from the solderconductor interface toward the inside of the IMC (intermetallic compound).

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