The effects of Sn solder grain size on the morphology and evolution of Cu6Sn5 in the aging process

Three Sn layers of different grain size were electroplated on Cu foils to investigate the effects of Sn solder grain size on the morphology and evolution of Cu6Sn5 during aging. In the early stage, scallop Cu6Sn5 formed in three joints. For the joint with Sn layer of 2 µm grain size, the Cu6Sn5 grain size was uniform and small, and the scallops had many facets. For the joints with Sn layers of 8 µm and 16 µm grain sizes, the Cu6Sn5 forming along the Sn grain boundary was of larger grain size than that forming inside the Sn grain, so it displayed network shape. And the scallop had fewer facets. As the aging time prolonged, the facets of Cu6Sn5 in the joint with Sn layer of 2 µm grain size became less and the scallop-type gradually transformed into rod-like. For the joints with Sn layers of 8 µm and 16 µm grain sizes, the Cu6Sn5 transformed from scallop-type to granular-like in a ripening growth, leading to the disappearing of Cu6Sn5 crystal network. Moreover, the interfacial intermetallic compound layer tended to be thicker in the joint with Sn layer of smaller grain size.