Effect of electroplating layer structure on shear property and microstructure of multilayer electroplated Sn–3.5Ag solder bumps

In this paper, Sn–3.5Ag solder bumps were prepared using multilayer electroplating method. The effect of electroplating layer structure (Sn/Ag, Ag/Sn, Ag/Sn/Ag) on shear property and microstructure of solder bumps was investigated.Shear test revealed that Sn/Ag bumps displayed the highest shear strength among different electroplating layer structures, which was 89.3% higher than that of pure Sn bumps, and 43.5% higher than that of Ag/Sn bumps. The interfacial microstructure evolution during reflowing process was also observed. It was found that Cu6Sn5 intermetallic compound formed at the interface between solder and Cu and its grain size increased as reflow time increased for all three electroplating structures. When there was Ag electroplated between Sn and Cu (Ag/Sn/Ag and Ag/Sn structures), the morphology of Cu6Sn5 was thin, hexagonal and scattered, which could be attributed to the adsorption of Ag3Sn on the surface of Cu6Sn5. In contrast, when Sn was electroplated on Cu directly (Sn/Ag structure), the morphology of Cu6Sn5 was thick, rod-like and continuous, which displayed high shear strength.

► Multilayer electroplating, as a novel method, is proposed to form alloy bumps.
► The comparison between different electroplating layer structures is discussed.
► Sn/Ag structure shows the highest shear strength with smooth Cu6Sn5 IMC layer.
► The morphology of IMC layer at the interface is influenced by electroplating orders.