Study Of Interfacial Microstructure Evolution And Nano-Indentation Properties Of Sn-9Zn/Cu Joints

With the development of microelectronic products becoming more portableminiaturization and high performance, the size of solder joints is decreasing, whichcauses the mechanical, electrical and thermal load getting more and more heavier.The series of Sn-Zn lead-free solder is one of the most potential candidates, whichhas low melting point, low cost and non-toxic, etc. In this paper, according to thedevelopment of the electronics industry and requirements on the mechanicalproperties of lead-free solder, the study on the mechanical properties and thereliability of Sn-Zn/Cu joints with different sizes has been done, which is extremelysignificant and useful to the improvement of lead-free solder.In this paper, Sn-9Zn solder was adopted to study the interfacial reactions andthe microstructure changes in Sn-9Zn/Cu joints with three BGA ball size (1000,700,500m) under the aging and multi-reflow conditions. Modulus of elasticity andcreep stress exponent of Sn-9Zn/Cu joint has been measured by nano-indentation.The result of the aging test shows that the solder joints size has a great influenceon the type and thickness of Sn-9Zn/Cu interfacial compounds. With the solder sizedecreasing from1000m to500m, the IMC of the interface changes from Cu5Zn8tothe mixture of Cu5Zn8and Cu-Zn; the IMC morphology varies from flat tointermittent strips; the decrease of the joints size restricts the growth of the IMCthickness.The multi-reflow experiment indicates that even though the type of IMC doesn’tchange when the joints size decreases, the internal structure comes to coarseningobviously after multi-reflow, while the thickness of IMC increases along with theincrease of reflow times.The nano-indentation experiments illustrate that the indentation hardness andelastic modulus of Cu5Zn8are5.031±0.234GPa and104.100±0.954GPa respectively, the indentation hardness and elastic modulus of bulk solder are0.189±0.017GPa and17.287±0.852GPa respectively. In addition, the creep stress exponent of Sn-9Zn/Cubulk solder obtained from curves with the concept of “work of indentation” is5.128.