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A variable frequency microwave (VFM) was used for soldering the lead-free solders. Melting the Sn-3.5Ag, Sn-3.8Ag-0.7Cu and Sn46Pb solder pastes and their wetting on the metal pads were achieved by heating the organic flux vehicle through a radio frequency microwave. The lead-free solder interconnection between the component chips and the metal pads through VFM was first demonstrated. The microstructures of the lead-free solders/Cu and Sn surface joints formed by a conventional thermal reflow process and VFM were analyzed. From the intermetallic compound (IMC) formation study, it was found that more heat energy could be employed into the solder paste by the VFM heating process than by the thermal reflow process at the peak temperatures used in this study, because VFM provided more uniform heat than the reflow oven that may lead a temperature gradient in the test boards. It is believed that by VFM the soldering process at a lower peak temperature could be feasible than by a conventional reflow process.