A study on interfacial phase evolution during Cu/Sn/Cu soldering with a micro interconnected height

In this study, the interfacial phase evolution during Cu/Sn/Cu soldering (260 °C, 1 N) with a micro interconnected height of 6 μm was analyzed. During soldering, the Cu6Sn5 precipitated first along Cu/Sn interfaces in a planar shape. After the appearance of Cu6Sn5, the Cu3Sn emerged between Cu and Cu6Sn5 in a planar shape as well. Then, until residual Sn was completely consumed, the Cu6Sn5 layers at opposite sides continued to grow with a change from the planar shape to a scallop-like shape. In the meantime, the Cu3Sn layers continued to grow with a round-trip change from the planar shape to a wave-like shape. After the total consumption of residual Sn, the Cu3Sn grew at the expense of Cu6Sn5 until the formation of full Cu3Sn solder joints at 300 min. Further, concrete reasons for the interesting shape change in both Cu6Sn5 layers and Cu3Sn layers were given. With the soldering time increasing from 10 min to 60 min, the morphology of Cu6Sn5 grains agreed with the shape of Cu6Sn5 layers well. As the growth of Cu6Sn5, a ripening process without the dependence on the morphology of Cu6Sn5 grains occurred. The dependence of mean radius of Cu6Sn5 grains on the soldering time was based on the relation of R = C1tk at different time segments. The morphology of Cu6Sn5 grains affected Cu flux, leading to different growth mechanism at these time segments. From 10 min to 30 min, the constant k was calculated to be 0.53, which was due to the growth of Cu6Sn5 being only supplied by the interfacial reaction flux. From 40 min to 60 min, the constant k was calculated to be 0.35, which was due to the growth of Cu6Sn5 being supplied by both the interfacial reaction flux and the ripening flux. Compared with the growth of Cu6Sn5 grains at 30 min, the growth of Cu6Sn5 grains at 40 min was supplied by an additional Cu flux (the ripening flux), which led to the constant k being 3.82 from 30 min to 40 min. Moreover, the growth process of Cu6Sn5 grains was thought to be the accumulation of a growth period of “formation of small grains on the surface of big grains → growth of small grains along the preferential growth direction of corresponding big grains → mergence of small grains on corresponding big grains → formation of new big grains on previous big grains → mergence between new big grains and previous big grains”.