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Voiding Control at Preform Soldering
Keywords: Voiding, Preform, Soldering
Assembly of components with large pads such as high brightness LEDs often is soldered with preform, mainly due to a lower voiding and lower flux fume generated when compared with solder paste. This is particularly true when the joints are to be formed within a cavity. Although lower than solder paste, the voiding in the solder joint is still a concern for high reliability and high performance devices. In this study, voiding behavior of large pad devices was simulated with Cu coupon to Cu coupon sandwiches. Its assembly using flux coated preform was studied, with variation in solder alloy type, quantity of flux coated on solder preform, Cu coupon pre-oxidation extent, reflow temperature, and pressure exerted onto the sandwich during reflow. Voiding of Sn63 was the lowest, followed by 57Bi42Sn1Ag, with SAC305 being the highest. Higher oxidation caused a higher voiding. With increasing flux quantity, the voiding increased when reflowed at temperature below activation temperature, but decreased when reflowed at above activation temperature. With increasing reflow temperature, the voiding increased at low temperature range, then decreased due to better wetting, but increased again due to excessive outgassing. Higher pressure resulted in a lower voiding due to constrained mobility of liquid solder.
Ning-Cheng Lee, Vice President, Technology
Indium Corporation
Clinton, NY

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