Purpose – The purpose of this paper is to visualise the activities of three solders; Sn37Pb, Sn9Zn and Sn3.5Ag on Cu substrates during reflow near their melting points and to relate them with reflow reactions between solder and substrate. Design/methodology/approach Melting activities of three solders near their melting points on copper substrates are visualised in an infrared reflow furnace. Findings Solder balls demonstrate different ways of melting and reflowing behaviours in dissimilar times and temperature intervals. Melting of Sn9Zn solder balls is initiated simultaneously at the surface and joint between solder balls. This is followed by the melting at the joint between solder balls and the Cu substrate. During melting, solder balls are first merged into each other and then reflow on the substrate from top to bottom. Opposite to Sn9Zn, Sn3.5Ag solder balls start to melt at the surface and the joint between the solder and substrate, simultaneously. Balls are first reflowed from top to bottom and, in the process, liquid solder is merged. Unlike Sn9Zn and Sn3.5 Ag, melting of Sn37Pb solder balls is initially commenced at the surface only. This is followed by simultaneous melting at both joints. Variation in melting activities of these solders is found to be closely related to the coalescence mechanism of solder balls and the reflow reactions between the solders and the Cu substrate. Originality/value The elementary melting activities of different solders on Cu substrates is related with their reflow behaviours. This provides better understanding of solder behaviour and selection of good leadfree solder for applications in the electronic industry.
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