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

Abstract 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 Cu 6 Sn 5 precipitated first along Cu/Sn interfaces in a planar shape. After the appearance of Cu 6 Sn 5 , the Cu 3 Sn emerged between Cu and Cu 6 Sn 5 in a planar shape as well. Then, until residual Sn was completely consumed, the Cu 6 Sn 5 layers at opposite sides continued to grow with a change from the planar shape to a scallop-like shape. In the meantime, the Cu 3 Sn 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 Cu 3 Sn grew at the expense of Cu 6 Sn 5 until the formation of full Cu 3 Sn solder joints at 300 min. Further, concrete reasons for the interesting shape change in both Cu 6 Sn 5 layers and Cu 3 Sn layers were given. With the soldering time increasing from 10 min to 60 min, the morphology of Cu 6 Sn 5 grains agreed with the shape of Cu 6 Sn 5 layers well. As the growth of Cu 6 Sn 5 , a ripening process without the dependence on the morphology of Cu 6 Sn 5 grains occurred. The dependence of mean radius of Cu 6 Sn 5 grains on the soldering time was based on the relation of R  =  C 1 t k at different time segments. The morphology of Cu 6 Sn 5 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 Cu 6 Sn 5 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 Cu 6 Sn 5 being supplied by both the interfacial reaction flux and the ripening flux. Compared with the growth of Cu 6 Sn 5 grains at 30 min, the growth of Cu 6 Sn 5 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 Cu 6 Sn 5 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”.