Abstract
Flexible electronics have recently attracted very intensive studies because they are thinner, lighter, and more flexible. Similar to other electronic products, flexible electronics products comprise numerous devices and modules. The interconnection technologies of these devices and modules to the flexible substrates are crucial for manufacturing. Two metallic interconnection technologies, Au-Sn bonding and soldering, are frequently used in flexible electronic packaging. Thus, interfacial reactions in the two interconnections with different surface finishes are investigated in this study. Au bumps are formed on the chip side, and the Cu tracks on flexible substrates protected by the Sn surface finish are attached to the Au bumps. Consequently, a three-layer Au/Sn/Cu structure is frequently encountered in flexible electronic products. The reaction progression of the Au/Sn/Cu interfacial reactions was determined. Initially, the reaction path in the Au/Sn/Cu specimen is Au/AuSn/ AuSn2/AuSn4/Sn/Cu6Sn5/Cu3Sn/Cu. At longer reaction time, the (Cu,Au)6Sn5 phase was formed on the Au/Sn side interface as well. The Sn phase is completely consumed with even longer reaction time, the (Cu,Au)6Sn5 phases on the two sides would merge together and the reaction path then becomes Au/AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Cu3Sn/Cu. The AuSn4 and AuSn2 phases disappeared step by step. The reaction path is then Au/Au5Sn/AuSn/(Cu,Au)6Sn5/Cu3Sn/Cu. The reaction path would continuously evolve until the specimen reaches thermodynamic equilibrium, and the final phases can be predicted from the phase diagrams. Low melting-point Pb-free solders, Sn-Zn and Sn-In based alloys, are selected to investigate in this study. Interfacial reactions between the Sn-8wt.%Zn-3wt.%Bi alloy and the Cu, Ag, and Ni substrates are examined. Two different kinds of substrates, bulk plate and electroplating layer, are used, and the reactions are carried out at 250 and 220oC. Although the Zn content is only 8wt.%, gamma-Cu5Zn8 and epsilon-CuZn5 phases are formed in the Sn-Zn-Bi/Cu couples. In the Sn-Zn-Bi/Ag couples, three Zn-Ag compounds are observed. The gamma-Ni5Zn21 phase is formed in the Sn-Zn-Bi/Ni couples. Similar results are found in the couples prepared with an electroplating layer: the reaction phases are the same, but the growth rates are different. Phase equilibria of Sn-Bi-Zn ternary system are determined in this study as well. Addition of Zn in Sn-20wt.%In alloys is effective on reducing undercooling and dissolution rate of Ag and Ni substrates in molten solders. For Sn-In-(Zn)/Ag and Sn-In-(Zn)/Ni interfacial reactions, the Zn dominating reaction products are formed when the Zn concentration is higher. A new kind of Sn-In based Pb-free solders with Zn addition is proposed in this study.