The Influences Of Sn0.3Ag0.7Cu Lead-Free Solder Joints Interfacial Reaction On Reliability

Sn0.3Ag0.7Cu solder alloy has been used as a new type of low silver solder, especially in the wave soldering of electronic assembly applications, because it can avoid the emergence of coarsening Ag3Sn and it is very cheap. However, as its high tin content in the solder and high reflow temperature, which make the intermetallic compounds (IMCs) between the solder and pads excessively growing during reflows, this will cause the thermal fracture and lead holes in solder joints and the whiskerlap short circuit between solder joints, leading to a series reliability problems.On the other hand, during the electronic manufacturing and service process, solder joints must bear temperature changes, vibration loads, mechanical stress, thermal stress of shock and the overgrowth of IMCs in solder joints, and soldering defects often result in the failure of solder joints, So it can be seen that the integrity of the PCBA solder joints that play an important role in electronic products reliability.Based on these issues, this dissertation designed the reliability experiments for Sn0.3Ag0.7Cu solder reflows with four kinds of coating PCB (HASL, OSP, Electrolytic Ni/Au and ENIG), and Evaluated the reliability of the solder joints from several aspects such as the reflow number teat, high and low temperature aging tests and temperature shock tests. And then, the relationships between the thickness of intermetallic compounds and the shear strength of solder joints were researched.The results showed that the morphology of IMC (η-Cu6Sn5) turned from scallop-shaped into lath-shaped and theηphase dissolved into the solder joints could be observed with the reflow temperature and reflow times increasing. The role of the IMC growth accorded with exponential growth pattern. The effects of reflow numbers on the shear strength of solder joints were increased at first, and then decreased, and fracture mechanism changed from pure shear fracture to mipor-gathering fracture, finally presented partial brittle failure.It was also indicated that the interfacial IMC between the OSP pads and the solder transformed from scallop-shaped to plane -shaped with aging time increasing, and Cu3Sn was observed with aging for 200 h at 125℃. During the aging for the ENIG finish pads, the block-like (Cu, Ni) 6Sn5 and needle-like (Ni, Cu)3Sn4 were formed, and the IMCs thickness with the square root of aging time followed the linear relationship. For temperature shock test, the interfacial IMC thickness for the four coating pads increased a little. It was the thickest for OSP finish pads, and then followed by Electrolytic Ni/Au, the thinnest was ENIG pads. With temperature shock numbers increasing, the Shear strength of solder joints continuedly decreased. HASL decreased at the fastest rate, and then was OSP, ENIG was the least. There were no Kirkendall holes in HASL and OSP interface during the process of aging and temperature shock, but a large number of Kirkendall holes were found in ENIG and Ni / Au interface.