Development Of Lead Free Solder And Its Interfacial Issues In Electronics Packaging

Under the pressure of the public voice, environment consciousness, legislation, as well as the requirement of market, the research of lead free in electronic packaging has become an important front problem in both industry and academia. With the development of electronic industry and the research of lead free solder, the requirement of high quality and low price lead free solder is urgent and the soldering reaction as well as the reliability of solder joints arouses researchers' attention.Due to the unique properties of rare earth elements (RE), this dissertation investigates the effect of trace RE on the microstructure, melting point, wettability and mechanical properties of Sn-3.5Ag, Sn-9Zn and Sn-3.5Ag-0.7C. In addition, the effect mechanism of RE addition is discussed.In order to improve the properties of Sn-9Zn solder, with the addition of Cu, new lead free solder of Sn-Zn-Cu is developed and the research on the microstructure, melting point, wettability and mechanical properties is performed. The compositions and morphologies of intermetallic compounds of as soldered Sn-Zn-Cu/Cu joints are also investigated.The properties of several Sn-Ag-Cu solders are summarized. Based on that, the morphologies and growth of IMC of Sn-Ag-Cu/Cu during soldering reaction are investigated. Then the formation of nano-Ag3Snparticles on the IMC and the adsorption phenomenon is analyzed. The effect of trace Au on the microstructure and mechanical properties of Sn-3.5Ag-0.7Cu solder is studied.The growth behavior of IMC during aging is the function of temperature and time. Because dipping method can control the soldering time and temperature, this thesis uses this method to get the solder joints. The growth of IMC and the microstructure evolution of the joints during aging process are studied. The research contains the IMC growth of Sn-Pb/Cu and Sn-Ag/Cu in which the interfacial microstructure consists of solder/Cu-Sn/Cu; the evolution of Sn-9Zn-3Bi/Cu interfacial microstructure which consists of solder/Cu-Zn /Cu after soldering reaction.Based on the above research work, the main conclusions as follows:(1) Due to the adsorption effect, trace RE elements can refine the microstructure of the solder alloy and the effect is significant with the increasing of precipitate phase. As active materials, RE can lower the surface tension of the liquid solder and improve the wettability. The mechanical properties are improved resulted from the refined microstructures.(2) With the addition of Cu, the surface tension of liquid solder is lowered and the wettability of SZ-xCu is improved which resulted from the decreasing of the activity of the Zn atoms. During soldering, the IMC formed at the interface ofSZ-xCu/Cu changed from layer-ship Cu5Zn8 to scalloped Cu6Sn5 with the increasing of Cu content in the solder. At the same time, the shear strength of the solder joint is increased according to the evolution of the IMC.(3) The microstructure of Sn-Ag-Cu alloy consists of β-Sn, Ag3Sn and Cu6Sn5. With the increasing of Ag and Cu, coarse intermetallic compounds emerge. The IMC layer of Sn-3.5Ag/Cu, Sn-3.5Ag-0.7Cu/Cu, Sn-3.5Ag-l.7Cu/Cu thickens with soldering time and the higher the Cu content, the thicker the IMC is. The IMC layer of Sn-0.5Ag-4Cu solder is thinner under short time while it turns thicken quite soon with increasing of soldering time due to the precipitation effect of Cu6Sn5. The IMC roughness of Sn-3.5Ag-l.7Cu and Sn-3.5Ag-0.7Cu is bigger than that of Sn-3.5Ag and Sn-4Cu-0.5Ag. The IMC dissolution leads to the smaller roughness of Sn-3.5Ag/Cu. For Sn-4Cu-0.5Ag, the higher Cu content causes the higher interfacial energy between IMC and liquid solder which gets a bigger angle between two neighbor Cu6Sn5 grains under equilibrium conditions.(4) When Sn-3.5Ag, Sn-3.5Ag-0.7Cu soldered with Cu or Ni substrate, nano-Ag3Sn particles are found on the surface of Cu-Sn, Ni-Sn and (CuNi)-Sn IMC. During solidification process, the precipitated Ag3Sn particles from liquid solder are absorbed by the IMC.(5) Minor Au element...