Abstract
The interfacial reactions between Sn–0·7 wt-%Cu solder and electroless nickel immersion gold (ENIG) substrate were investigated during multiple reflows and isothermal aging at temperatures between 70 and 150°C for aging times of up to 60 days. After reflowing and aging, the intermetallic compound (IMC) formed at the interface was only (Cu,Ni)6Sn5. In addition, a P rich Ni (Ni3P) layer formed at the interface between the (Cu,Ni)6Sn5 and the Ni–P layer. The thickness of the interfacial (Cu,Ni)6Sn5 IMC layer slightly increased with the multiple reflows and isothermal aging. In contrast, the thickness of the Ni3P layer did not significantly increase. The formation of the (Cu,Ni)6Sn5 IMC layer retarded the consumption of the Ni from the electroless Ni–P substrate. As a result, a very thin Ni3P layer formed at the interface. In the ball shear test performed after aging treatment, the shear strength decreased for all temperatures tested until the samples had been aged for three days, and then remained at a constant value with prolonged aging. In addition, the shear strength could not be significantly related to the aging temperatures used in the present study. All fractures occurred in the bulk solder. The present study shows that the Sn–0·7Cu/ENIG solder joint has superior joint reliability with respect to shear tests.
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