Interfacial Reactions Between Sn-Aglead-free Solder And Single Crystal Cu

With the rapid development of high-density electronic packaging technology, the size and stand-off height(SOH) of solder joints decrease constantly. Therefore, interfacial reaction and microstructure plays a more and more important role in the properties of joints and brings a huge challenge for the reliability of electronic products. On the other hand, Sn-Ag solders have been recognized as the most hopeful substitute of Sn-Pb solder. However, there are still some problems on melting point and wettability of Sn-Ag solders. The addition of third component is one way to improve the performance of Sn-Ag solders. Sn-Ag-Cu and Sn-Ag-Zn alloys are the most promising Sn-Ag solders.In this dissertation, polycrystalline and single crystal Cu sheets were used as the substrates and Sn-3.5Ag-0.7Cu, Sn-2Ag-2.5Zn and Sn-2Ag-4Zn solders were used as solder alloy. Interfacial reactions between solders and Cu substrates with different orientations have been studied through isothermal aging. The microstructure and growth of intermetallic compound(IMC) have been also studied. In addition, the effect of SOH on the growth of IMC has been discussed.The study shows that: Cu6Sn5 formed at the interface of Sn-3.5Ag-0.7Cu/Cu solder joint after reflowing. The thickness of Cu6Sn5 formed on single crystal Cu was larger than that of polycrystalline Cu because the dissolution of single crystal Cu was faster than that of polycrystalline Cu. Cu6Sn5 grains displayed in prism shape on(111) Cu substrate, while only scallop-like Cu6Sn5 grains with smaller grain size formed on polycrystalline Cu substrate. With the increase of reflowing temperature, prism-shape Cu6Sn5 transformed to scallop shape. The thickness of IMC increased with the increasing aging time and a Cu3 Sn layer appeared between Cu6Sn5 and Cu substrate. Cu3 Sn formed on single crystal Cu substrate grew faster than that on polycrystalline Cu substrate while the growth rate of Cu6Sn5 formed on single crystal Cu was slower.In Sn-2Ag-2.5Zn/Cu solder joint, a continuous scallop-like Cu6Sn5 layer formed at the interface after reflowing. The size of Cu6Sn5 grains on single crystal Cu was larger than that of polycrystalline Cu. Double layer IMC can be found at the Sn-2Ag-4Zn/Cu solder joint, which are Cu5Zn8 and Cu6Sn5. Cu5Zn8 was unstable and cracked after long-term aging. Sn atoms diffused towards to the Cu substrate to form Cu6Sn5 rapidly.The interfacial reaction at Cu/Sn Ag Cu/Cu was affected by SOH. The thickness of IMC reduced along with the reduction of SOH in reflowing process. After aging at 150℃ for 384 h, the IMC growing from the upper side and from the lower side touched each other in solder joints with 10 μm SOH. We found that Cu3 Sn formed on(111) Cu substrate grew faster, owing to Cu3 Sn columnar growth on(111) Cu and grain boundary diffusion control.