Transition of Bi embrittlement of SnBi/Cu joint couples with reflow temperature

Abstract

The current study revealed the effects of reflow temperature and the reflow time on the interfacial embrittlement of SnBi/Cu joints. When the reflow temperature is below 220 °C, the joints reflowed for 150 min often fail in brittle mode because the Bi atoms from the SnBi solder easily segregated at the Cu³Sn/Cu interface. In contrast, Bi embrittlement did not occur for joints reflowed at above 260 °C for 150 min because the Bi particles were frozen in the Cu³Sn layer during the formation of intermetallic compounds (IMC) at the initial reflow stage, mainly located at the Cu³Sn grain boundary. It is interesting to note that the Bi embrittlement did occur when the joints were reflowed at above 260 °C for 250 min, which should be attributed to Bi diffusion. It is concluded that the Bi particles are frozen in the Cu³Sn layer with increasing reflow temperature, that cannot eliminate Bi embrittlement, and can only delay the occurrence of Bi embrittlement.