NanoFlux — doping of solder pastes

Abstract

The reliability of automotive electronics depends a great deal on solder joints. After the WEEE and RoHS came into effect the new focus became lead-free solder pastes such as SnAg or SnAgCu (SAC). One approach to improve the thermal fatigue properties of these alloys is to add a third or fourth alloying element SAC + X. Until now, this was always done by metallurgical alloying, requiring special apparatuses, conditions and, exceptional processing expertise. Hence, it is even more difficult and more expensive to produce a powder out of these alloys.The project "nanoFlux", funded by the German federal ministry of research and education, is a new approach to alloy special elements into a solder joint during a conventional reflow process used in SMD technology. A standard powder is mixed with a flux containing Al, Co, Fe, or mixtures of these elements. NanoFlux should not significantly influence the melting point or the pasty range to be used in established lead-free processes. This could drastically lower the costs for the manufacturing and storage of special alloys. Recent results already show successful alloying for Co and Fe. The thermal properties were monitored during an insitu reflow differential scanning calorimeter (DSC) experiment. To verify a successful doping, reflowed, cleaned, and dissolved solder balls were analysed by inductively coupled plasma optical emission spectroscopy (ICP OES). Parallel to those experiments, a test board was set up and a thermal cycle test has been conducted. The higher remaining shear strength was linked to the microstructural characteristics of the different solders, through scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. It was possible to successfully dope a solder joint with a fourth element by mixing a Co or Fe containing flux with the solder powder.