As part of a major project to develop a computer-based model to support the product and process design activity for reflow soldered assemblies, it has been found necessary to carry out fundamental investigations of the processes occurring as solder pastes approach and pass the reflow temperature. This paper presents the results and our interpretation of the observed physical phenomena arising from recent high-speed video microscopy studies of the reflow of bulk deposits of solder paste and also of the formation of solder joints during infrared reflow soldering. These studies have provided insights into the physical phenomenology of reflowing solder paste, the interpretation of wetting curves and the mechanisms behind the formation of manufacturing driven defects.

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