Surface Mount International Conference Proceedings


OPTIMIZING REFLOW PROFILE VIA DEFECT MECHANISMS ANALYSIS

Author: Ning-Cheng Lee
Company: Indium Corporation of America
Date Published: 8/23/1998   Conference: Surface Mount International


Abstract: The reflow profile is engineered to optimize the soldering performance based on defect mechanisms analysis. In general, a slow ramp-up rate is desired in order to minimize hot slump, bridging, tombstoning, skewing, wicking, opens, solder beading, solder balling, and components cracking. A minimized soaking zone reduces voiding, poor wetting, solder balling, and opens. Use of low peak temperature lessens charring, delamination, intermetallics, leaching, dewetting, and voiding. A rapid cooling rate helps reducing intermetallics, charring, leaching, dewetting, and grain size. However, a slow cooling rate reduces solder or pad detachment. The optimized profile favors that the temperature ramps up slowly until reaching about 175C. The temperature is then gradually raised further up to 180C within about 20-30 seconds, then raised rapidly until reaching about 220C. After that, the temperature is brought down with a rapid cooling rate. The conventional profile was developed due to the limitation of past reflow technologies. Implementation of the optimized profile requires the support of a heating-efficient reflow technology with a controllable heating rate. Vapor phase reflow can provide a rapid heating, but has difficulty to control the heating rate. Infrared reflow can regulate the heating rate, but is sensitive to variation in parts features. Emergence of the forced air convection reflow provides controllable heating rate. In addition, it is not sensitive to variation in parts features, thus allows the realization of the optimized profile.

Key words: reflow, profile, defect, flux, solder paste, soldering, convection, infrared, vapor phase, SMT



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