In this study, SAC305 and SAC305-0.3Ni solder balls were soldered onto Cu, high temperature treated Cu (H-Cu) and graphene coated Cu (G-Cu) substrates, respectively. The microstructure, the interfacial reaction, and the hardness of the solder joints were investigated. The interfacial intermetallic compound (IMC) is Cu6Sn5 in the solder joints of SAC305/Cu, SAC305/H-Cu, and SAC305/G-Cu. With the addition of 0.3 wt% Ni in the SAC305 solder, the interfacial IMC on Cu, H-Cu, and G-Cu transforms from Cu6Sn5 into (Cu, Ni)6Sn5. The thickness of Cu6Sn5 and (Cu, Ni)6Sn5 is the lowest on G-Cu substrate. Meanwhile, smooth (Cu, Ni)6Sn5 interfacial IMC layers are obtained in SAC305-0.3Ni/H-Cu and SAC305-0.3Ni/G-Cu solder joints. Both the SAC305 and the SAC305-0.3Ni solder bulks have the highest β-Sn content and the lowest concentration of eutectic phases on G-Cu substrate. Consequently, the hardness of the solder bulks on G-Cu is lower than that on the other two kinds of substrates.@en

This study investigated the interfacial reaction, the microstructure, and the hardness of the SAC305-xNi solder on both Cu and graphene-coated Cu (G–Cu) substrates. The experimental results indicate that the increase of Ni content in the solder leads to the roughness of the (Cu, Ni)6Sn5 IMC layer on Cu. In contrast, the growth of the (Cu, Ni)6Sn5 interfacial IMC, which results from increasing Ni addition, is significantly suppressed on G–Cu substrates. As the concentration of Ni ranges from 0 to 0.2 wt%, the microstructure of the solder bulks on Cu substrates shows slight changes. The hardness of the solder bulks in SAC305-Ni/Cu is similar to that in the SAC305/Cu solder joint. The amount of β-Sn rises and the eutectic area shrinks due to increasing Ni addition in the solder bulks on G–Cu substrates. Therefore, the solder bulks in the SAC305-Ni/G–Cu show lower hardness than that in the SAC305/G–Cu solder joint.@en