The Effects Of Soldering Temperature And Cooling Rate On Interfacial Microstructure In Lead-free Soldering

In recent years, the rapid development of electronic packaging industry and continuous improvement of consumer demand prompt the development of portable, miniaturized and precise electronic products, and then higher request for the reliabilities of electronic packaging is put forward. The reliability has a lot to do with the microstructure of solder joints, and interfacial intermetallic compound (IMC), usually referring to Cu6Sn5in soldering with Cu substrate, is one of the core problems. For the solder alloy with a high content of Ag, Ag3Sn is formed after soldering reaction, whose morphology and size affect the mechanical properties of solder joints. Therefore, the study on the growth behavior of interfacial IMC is a basic but very important topic.In this study, Sn, Sn-0.7Cu, Sn-3.5Ag, Sn-4.0Ag, Sn-4.5Ag, Sn-3.0Ag-0.5Cu and Sn-3.7Ag-0.7Cu solder alloys were selected to react with polycrystalline Cu substrate at250°C,275°C,300°C, respectively for1Omin and then cooled in liquid nitrogen, water, air and furnace. The morphology and size of interfacial IMC (Cu6Sn5and Ag3Sn) were analyzed to study the effects of soldering temperature and cooling rate on the microstructure of interfacial IMC at the solder joints. Meanwhile, the dynamic growth behavior of IMC was in-situ studied by the synchrotron radiation real-time imaging technology, providing direct proofs to the studies on IMC. The conclusions were summarized as follows:(1) Faceted CU6Sn5grains were formed at higher soldering temperature. There was a critical soldering temperature, at which Cu6Sns became faceted, and in this paper the temperature was between250°C and275°C. The average Cu6Sn5IMC layer thickness and grains diameter were getting bigger with higher soldering temperature. G was used to characterize the three dimensional morphology of Cu6Sn5grains. The higher the value was, the thinner and longer the Cu6Sn5grain was. The value of G at Sn/Cu solder joint was the smallest, indicating that Cu6Sn5was inclined to became long and thin when Ag or Cu was added into solder alloys, and it was largest at275°C.(2) The morphology of Cu6Sn5grains was affected by the cooling rate, because of its secondary growth during solidification stage. Cellular Cu6Sn5grains were formed under water cooling condition and air cooling condition, and hexagonal Cu6Sn5grains were formed under furnace cooling condition at250°C. Facet Cu6Sn5grains were formed under water cooling condition, Cu6Sn5whiskers were formed under air cooling condition and hexagonal CU6Sn5 grains were formed under furnace cooling condition at275oC and300oC. Cu6Sn5IMC layer thickness was related to the cooling rate. Lower cooling rate led to thicker Cu6Sn5IMC layer. With the same soldering temperature and cooling rate, Cu6Sn5IMC layer thickness for Sn-3.5Ag/Cu solder joint was the thinnest. This was because the secondary of Cu6Sn5was hindered by the nucleation and growth of Ag3Sn during solidification process.(3) Soldering temperature had little effect on Ag3Sn. Cooling rate had big effect on Ag3Sn. With the decrease of cooling rate, the morphology of Ag3Sn grew from granular type to linear shape and plate-like shape, and the size grew bigger.(4) The morphology of Cu6Sn5grains had influence on the nucleation position of Ag3Sn. On the surface of cellular Cu6S5grains, granular Ag3Sn arranged according to the spiral dislocation to form a concentric circles structure. Granular Ag3Sn gathered to form a linear shape, and was horizontally or vertically distributed on the surface of faceted Cu6Sn5grains. Linear was Ag3Sn horizontally distributed on the surface of faceted Cu6Sn5grains. And the angle between Ag3Sn and growth direction of Cu6Sns=5facet was90o. Large plate-like Ag3Sn was formed in virtue of interfacial Cu6Sn5grains and rod-like Cu6Sn5penetrated it to form overlapping phenomenon. The angle between them was about60o, and the angle between large Ag3Sn themselves was60°or120°.(5) The synchrotron radiation experimental results indicated that, the formation of Ag3Sn occurred at a short time during solidification process, the angle between large Ag3Sn themselves was60°or120°, the secondary growth of Cu6Sn5was prior to the growth of Ag3Sn with an angle of60°, competitive growth phenomenon of Ag3Sn was observed and the formation of Ag3Sn was prior to β-Sn because of different nucleation way.