Pulse plating of gold-tin alloys for microelectronic & optoelectronic applications

摘要

Gold-tin eutectic solders (30 at% Sn) are used for packaging microelectronic and optoelectronic devices, because of their excellent thermal and mechanical properties and relatively low melting or reflow temperature (280℃; 536°F). Electroplating is a cost effective alternative to current commercial solder deposition processes, such as solder preforms and evaporation. A simultaneous electroplating process for depositing Au-Sn alloys, from a slightly acidic, chloride-based solution using pulsed currents, onto metallized ceramic and semiconductor substrates has been developed. Two separate Au-Sn compositions, containing 15 at% Sn and 50 at% Sn, can be deposited under appropriate plating conditions, i.e., current density, pulse on-off time and pulse duration. These compositions, according to the Au-Sn phase diagram, correspond to Au{sub}5Sn and AuSn, respectively. By using multiple current pulses and varying their duration, it is possible to deposit a composite solder structure with an overall composition ranging from 15 to 50 at% Sn, including the important eutectic composition. Plating results for both unpatterned and patterned (with photoresist) substrates are presented. All substrates were metallized with Ti-Au or Ti-Mo-Au, to provide a gold seed layer for electroplating.