Study On The Interfacial Reactions In Cu/Sn-xZn/Ni Micro Solder Joints

Microelectronic packaging technology is continuously developing towards miniaturization and high density.The reduction of packaging size requires corresponding decrease in the size of micro solder joints.Therefore,the interfacial reactions in the micro solder joints are more sensitive to the reflow process and the heat generated by the chips in service.Cu and Ni have been widely used as the under bump metallizations?UBMs?and Sn-Zn solders own a good value to research due to the advantages of low cost and good mechanical performance.In this study,interfacial reactions in Cu/Sn-xZn/Ni micro solder joints were investigated during reflow and aging under isothermal and temperature gradient conditions.The effects of Zn content in the solder,Cu-Ni cross-interaction and temperature gradient on the growth and transformation of interfacial intermetallic compounds?IMCs?were analyzed emphatically.The main conclusions can be drawn as follows:1.During the isothermal reflow process,the Cu₅Zn₈ IMC layer at both interfaces grew thicker and became more stable with the increase of Zn content in the solders.With a thicker Cu₅Zn₈ IMC layer,the occurrence of Cu-Ni cross-interaction on both Cu and Ni side was remarkably delayed because of a stronger blocking action of Cu atomic diffusion.That is,the presence of Zn element could inhibit the onset of Cu-Ni cross-interaction.Affected by the temperature gradient,the interfacial reactions of the micro solder joints could be strengthened or slowered by enhancing or restraining the diffusion rate of Cu atoms,respectively.Thus,it can be concluded that the diffusion of Cu atoms dominated the liquid-solid interfacial reactions of Cu/Sn-x Zn/Ni micro solder joints.2.During the reflow process,the formation and transformation of the IMCs at the Cu side could be judged by Sn-Zn-Cu ternary phase diagram.That is,as the Zn content decreased in the Sn-Zn solders,the IMC evolution followed Cu₅Zn₈ type?CuZn+Cu₆Sn₅ type?Cu₆Sn₅ type.For the Ni side,the formation and transformation of IMCs are more complicated.When the Zn was sufficient in the solder and the Cu diffusion was restrained,the IMC evolution could be judged by Sn-Zn-Ni ternary phase diagram.After the laminar Cu₅Zn₈ IMC spalled from the Cu substrate,the content of Cu atoms at the Ni side increased obviously while the Zn content decreased.In this case,the IMC evolution could be judged by Sn-Ni-Cu ternary phase diagram.In a word,the IMC evolution at the Ni side followed Ni₅Zn₂₁ type?Ni₃Sn₄ type?Cu₆Sn₅ type along with the interfacial reaction progress.3.During the isothermal aging process of Cu/Sn-x Zn/Ni micro solder joints,the Cu and Zn atoms dominated the interfacial reactions.The Cu atoms dominated the rate of interfacial reaction and the Zn atoms dominated the growth or dissolution of Cu₅Zn₈ IMC.This rule remained working under temperature gradient.With the Ni substrate as the hot-end,the diffusion of Cu atoms were restrained which led to a slower reaction rate of the whole micro solder joints.With the Cu substrate as the hot-end,the diffusion of Cu atoms was enhanced.However,there existed a positive chemical potential gradient of Zn atoms from Ni side to Cu side,driving Zn atoms diffusing towards Cu side even against the temperature gradient.As a result,laminar Cu₅Zn₈ IMC predominated the interfacial reaction at the Cu side.Therefore,the laminar Cu₅Zn₈ IMC could effectively restrain the diffusion of Cu and slow down the interfacial reaction rate when Zn was sufficient in the solder.While the Zn was insufficient,the dynamic balance between growth and dissolution of Cu₅Zn₈ IMC would be broken.Thus,the Cu₅Zn₈ IMC tended to dissolve and became loose,and then Cu₆Sn₅-type IMCs formed in the solder.