Diffusion anisotropy induced uneven regional growth of Cu6Sn5 IMC in Cu/SAC305/Cu micro solder joints under temperature gradient

Abstract Distinct uneven regional growth of interfacial Cu6Sn5 intermetallic compound (IMC) in Cu/Sn-3.0Ag-0.5Cu/Cu micro solder joints under temperature gradient (TG) was reported. The results revealed that thick and thin Cu6Sn5 IMC evenly formed at the whole cold end interface in the joints with small and large θ angle (the angle between the c-axis of β-Sn grain and the direction of TG) β-Sn grains, respectively; however, for the joints with medium θ angle β-Sn grains, distinct uneven regional IMC growth with thick Cu6Sn5 IMC region and thin Cu6Sn5 IMC region was detected at each cold end interface. After aging for 600 h, the average IMC thickness in the thick Cu6Sn5 IMC region gradually decreased as the θ angle increased and presented a linear relationship with cos2θ, while that in the thin Cu6Sn5 IMC region always stuck to 4.5 ± 0.5 µm regardless of θ angle. The uneven regional growth of the Cu6Sn5 IMC was attributed to the strong diffusion anisotropy of Cu atoms in β-Sn grains.