Effect of reaction time and P content on mechanical strength of the interface formed between eutectic Sn–Ag solder and Au/electroless Ni(P)/Cu bond pad

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Abstract
In this work, shear strengths of the solder joints for Sn–Ag eutectic alloy with the Au/electroless Ni(P)/Cu bond pad were measured for three different electroless Ni(P) layers. Sn–Ag eutectic solder alloy was kept in molten condition (240 °C) on the Au/electroless Ni(P)/Cu bond pad for different time periods ranging from 0.5 min to 180 min to render the ultimate interfacial reaction and the consecutive shear strength. After the shear test, fracture surfaces were investigated by scanning electron microscopy equipped with energy dispersed x ray. Cross-sectional studies of the interfaces were also conducted to correlate with the fracture surfaces. It was found that formation of crystalline phosphorous-rich Ni layer at the solder interface of Au/electroless Ni(P)/Cu bond pad with Sn–Ag eutectic alloy deteriorates the mechanical strength of the joints significantly. It was also noticed that such weak P-rich Ni layer appears quickly for high-P content electroless Ni(P) layer. However, when this P-rich Ni layer disappears from a prolonged reaction, the shear strength increases again.

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