Formation And Evolution Of The Microstructures Of The Sn-Ag-Zn Solders And The Interfaces With Cu Pad

Lead containing productions have been banned to manufacture in China for almost three years, but lots of lead-free patents are protected by foreign countries. In order to develop the lead-free solder with good properties and self-dominated intellectual property right, the Sn-3.7Ag-0.9Zn eutectic alloys were selected. Firstly, effects of different cooling speed, variation of content of ingredient, micro-alloying, introduction of reinforcing particles on the microstructure of the eutectic Sn-Ag-Zn alloy were studied. Furthermore, the evolution of the microstructure of the solders working under environment temperature was investigated by aging treatment. Then, the hardening theory and failure mechanism were discussed based on the experimental results of microhaedness and tensile strength. At last, the interfacial structure between Sn-Ag-Zn alloys and Cu pad were observed and the formation of the intermetallic compounds (IMCs) layer was discussed. More details were given as follows.The microstructure of eutectic Sn-3.7Ag-0.9Zn (weight percent, hereafter) alloy transformed fç–¤?eutectic microstructure (β-Sn, Ag₃Sn andζ-AgZn) to pseudoeutectic microstructure (β-Sn, Ag₃Sn and??AgZn) and toβ-Sn dendrites along with the increasing of cooling rate. And during aging, the equilibrium eutectic Sn-Ag-Zn alloy is rather stable that the microstructure hardly changed. But for the water-cooled and rapidly-solidified eutectic Sn-Ag-Zn alloy, IMC particles andβ-Sn dendrites congregated and grew up to minimize the energy of the system. The IMC changed from Zn-rich phase to Ag-rich phase (ε-AgZn₃γ-Ag₅Zn₈ζ-AgZn Ag₃Sn) in Sn- xAg-0.9Zn alloy with x increasing from 0.3 to 3.7, and from Ag-rich phase to Zn-rich phase (Ag₃Snζ-AgZnγ-Ag₅Zn₈) in Sn-3.7Ag-x Zn alloy with x increasing from 0 to 3.0 on the contrary. The addition of In into the explored Sn-Ag-Zn alloy promotes the formation of rod-like IMCs for the reason that the growth competition of the Ag3Sn and AgZn IMCs was destroyed by the selective adsorption of In atoms on a certain preferable crystalline planes of the separated IMCs. And the addition of Al suppressed the formation of Ag3Sn and AgZn, but IMC Ag2Al separated out instead.The addition of reinforcing particles (μm-SiC andμm-Cu) into the Sn-3.7Ag -0.9Zn alloy melt prompts the formation of the primaryβ-Sn phase in the solidified structure. For the particles serve as additional nucleation sites for the formation of the primaryβ-Sn phase, the sizes of both theβ-Sn dendrites and the IMCs decrease gradually with increasing the amount of the particle. Differed from SiC particles, Cu particles reacted with the alloy and formed Sn-Cu IMCs wrapping the Cu particle inside. During aging, Ag-Zn IMCs formed on the edge of the Sn-Cu IMCs and prevented the coarsening of the Sn-Cu IMCs. So the interfacial microstructure between the matrix and reinforcing Cu particles could be controlled by controlling the size of Cu particles.Because of the refined microstructure, the microhardness of the eutectic Sn-Ag-Zn alloy increased with increasing the cooling rate. And after aging, the coarsening of the microstructure brought a recognizable softening effect in the explored alloys. The addition of In not only increased the microstructure and tensile strength of the eutectic Sn-Ag-Zn solder, but also decreased melting point. Though the addition of Al could strengthen the alloy, it reduced ductility dramatically and the failure mechanism turned from ductile fracture to brittle fracture with Al content increasing. Moreover, the increase tendency of the measured microhardness of <ç–¤?composite solder alloys could be attributed to effects of grain refinement, dispersion strengthening an??condary strengthening mechanisms.The IMC layer structure of the Sn-3.7Ag-0.9Zn/Cu interface soldered at 250℃with different soldering periods (1, 5 and 10 min) transformed from double layers (Cu₅Zn₈ + Cu₆Sn₅) to a single layer (Cu₆Sn₅). From other results of alloys with Cu pad soldered at 250℃for 1 min, it could be concluded that: with Ag content increasing the thickness of IMC layer grew thinner. With Zn content increasing the thickness of IMC layer grew thicker, and the interfacial structure transformed from one Cu₆Sn₅ layer to double layers (Cu₆Sn₅ + Cu₅Zn₈) and to single Cu₅Zn₈ layer. The addition of Al prompted the formation of the Al-Cu IMCs and when the concentration of Al reaches 1.0 wt.%, the crack failure was observed near the soldered interface.