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32 SMT Magazine • April 2017 by Dr. Arnab Dasgupta, Elaina Zito, and Dr. Ning-Cheng Lee INDIUM CORPORATION Assembly of components with large pads, such as high-brightness LEDs or high-pow- er dies, often is soldered with preforms. Sol- der preforms are used to lower voiding and to lower flux fumes that can be generated by sol- der paste. The use of solder preforms also result in better thermal and electrical conductivity compared to Ag epoxy. This is particularly true when the joints are to be formed within a cav- ity. Voiding in the solder joint is a concern for high-reliability and high-performance devic- es. In this study, voiding behavior of large pad high-power devices was simulated with copper (Cu) coupon to Cu coupon sandwiches. A flux- coated preform was studied in this assembly, with variation in solder alloy type, quantity of solid flux coated on solder preform, Cu cou- pon pre-oxidation extent, reflow temperature, and pressure exerted onto the sandwich dur- ing reflow. Experiment 1. Solder Alloy Type Preforms of three solder alloys were test- ed: 96.5Sn/3Ag/0.5Cu (SAC305), 63Sn/37Pb, and 57Bi/42Sn/1Ag. The preform diameter was 0.906-inch, and 0.006-inch thick. 2. Flux Coating The preform was pre-coated with a solid flux film using liquid flux at 0.5, 1.0, and 2% flux concentrations in isopropyl alcohol upon appli- cation. 3. Cu Coupon Oxidation Pre-treatment The copper coupon (0.906-inch diameter, 0.020-inch thick) was used to simulate both die and substrate. It was pre-cleaned by soaking in a diluted HBF4 aqueous solution, followed by deionized water rinsing, and air drying. These cleaned Cu coupons were then oxidized by placing them on a 230°C hot plate for 0, 30, and 60 seconds, and five minutes prior to the soldering process. Voiding Control at High-Power Die-Attach Preform Soldering FEATURE

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