A Study on Mechanical Properties of Bi-Ag high Temperature Solder Joints

• Main Authors: Zong-Mou Wu, 吳宗謀
• Other Authors: Jenn-Ming Song
• Format: Others
• Language: zh-TW
• Published: 2007
• Online Access: http://ndltd.ncl.edu.tw/handle/989wfp

碩士 === 國立東華大學 === 材料科學與工程學系 === 95 === The study investigated and clarified the relationship between microstructure and mechanical properties of Bi-Ag high temperature Pb-free solders, in the form of bulk material or joints with Cu and Ni. Several subjects were studied systematically, including the effect of Ag content and strain rate on the tensile properties of bulk Bi-Ag alloys, interfacial reaction between Bi-Ag solders and the substrates, the nanoindentation analysis of the reaction layers thus formed, as well as the influence of thermal aging on the tensile fracture behavior of solder joints. As for the bulk materials, the microstructure of Bi-Ag alloys exhibited a non-equilibrium eutectic solidification feature showing that together with the primary Ag, Bi-Ag eutectics and the pro-eutectic Bi co-existed in the solidified structure. Experimental results show that the tensile strength increased with a raised Ag content and the elongation in the decreasing order was Bi-11Ag, pure Bi and Bi-2.5Ag. It could be deduced that crack might propagate through the fine, oriented eutectic Ag particles easily and consequently the Bi-2.5Ag alloy possessed poor ductility. In contrast, the massive primary Ag which has the ability to retard strain localization and stunt crack growth is probably responsible for the superior ductility of the Bi-11Ag sample at high deformation rates. Without forming intermetallic compounds (IMC), the molten solder grooved and further penetrated along grain boundaries (GBs) of the Cu substrate. It was also demonstrated that the grooved grain boundaries played the role in interfacial bonding for the Bi-Ag/Cu joints and provided a comparable joint strength to that of the Pb-5Sn/Cu. An increase in Ag content was able to enhance GB grooving and thus a concentrated joint tensile strength could be obtained. The interface between Bi-Ag and Cu became smooth and the GB grooves was suppressed to a certain extent when suffering thermal aging. However, unlike the degraded strength of Pb-5Sn/Cu joints with the thickened Cu3Sn and accompanying Kirkendall voids, an acceptable joint strength still remained for thermal aged Bi-Ag/Cu joints. NiBi and NiBi3 could be observed at the Bi-Ag/Ni interface, however, in the as-jointed state the NiBi located between NiBi3 and the Ni substrate was almost undetectable. According to the nanoindentation results, the hardness and elastic modulus of NiBi are substantially greater than that of NiBi3. During thermal aging, the NiBi3 and NiBi became thickened through the interdiffusion between Ni and Bi and remarkably the thickness of NiBi3 reached 80