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Sn-3.0mass%Ag-0.5mass%Cu (SAC) solder, which is widely used in Japan, has a relatively low impact reliability. This is because of the solder alloy hardness which induces a high stress concentration at the interface between the solder and the substrate. The impact reliability of the solder joint can be controlled by changing the composition of the under-bump metallurgy (UBM). This study aimed to determine the effect of electroless Co-P plating on the impact reliability of the solder joint using SAC, SAC-0.05Co and SAC-0.2Co solders. The intermetallic compound (IMC) layer formed at the interface between the three types of solder and the electroless Co-P plating was thinner than that of the SAC and electroless Ni-P plating. The hardness of the solders with electroless Co-P plating was lower than that with electroless Ni-P plating.The impact test results show that the solder joints with electroless Co-P plating are better at absorbing impact energy than SAC/Ni-P.

A solder joint is required to have a impact reliability for use in portable electronic products. In general, there is a correlation between the impact reliability and the morphology and thickness of the reaction layer formed at the solder/under bump metallurgy (UBM) interface. The characteristics of the reaction layer depend on the UBM. This study aims to clarify the effect of UBMs on the reaction layer formed at the solder/UBM interface. For this purpose, Sn-3.0 mass%Ag-0.5 mass%Cu was soldered on UBMs such as electroless CoP plating (Co-P) and Ni-CoP plating (Ni-CoP). An electroless NiP plating (Ni-P) substrate was used as a reference substrate. The peak temperature of the preset reflow profile was 523 K for 60 s. Then, some of the specimens were subjected to aging at 423 K for 168-1008 h. After soldering, the spreading area of the solder on the UBMs was measured using an optical microscope (OM). The spreading area of the solder on CoP was larger than that of the solder on Ni-CoP and NiP. Therefore, the solder on CoP had better wetting characteristics than that on Ni-CoP and NiP. Further, the reaction layer at the solder/UBM interface was observed using a scanning electron microscope (SEM). A fine and a large needle-like intermetallic compound (IMC) were formed at the solder/Co-P and the solder/Ni-CoP interfaces, respectively, whereas a layer-like IMC was formed at the solder/Ni-P interface. After aging for 1008 h, the thicknesses of the continuous IMC layers for CoP and Ni-CoP were approximately 10 μm and 5 μm, respectively, whereas the thickness of the continuous IMC layer for NiP was only approximately 3 μm.