Novel Solder-Magnetic Particle Composites and Their Reflow Using AC Magnetic Fields

Localized heating of solder interconnects in electronic packaging can help alleviate undesirable thermal stresses during the conventional reflow process, where the package is subjected to high temperatures. Localized heating is possible for conductors by electromagnetic induction of eddy currents which results in Joule heating, when subjected to AC magnetic fields. However, for typical solder pastes with fine solder powders dispersed in a flux medium, the eddy current losses, which have strong size dependence, become too small to cause significant heating at reasonable field amplitudes and frequencies. Magnetic materials exhibit losses from hysteretic and relaxation processes, in AC magnetic fields. We investigated the feasibility of solder paste reflow by localized heating of novel solder magnetic particle composites in AC magnetic fields. A solder paste magnetic nanoparticle (MNP) composite was prepared by mechanical mixing of FeCo MNPs with Type III Sn96.5/Ag3.0/Cu0.5 (SAC305) Pb-free solder paste. The pristine solder paste show an insignificant temperature rise when subjected to AC magnetic field of 500 Oe at 280 kHz, whereas the solder-MNP composite samples with particle concentration of 2 wt% or more were able to reflow due to magnetic heating. Here we report first demonstration of a new approach for solder melting in AC magnetic fields using MNPs.