Effects of additional elements (Fe, Co, Al) on SnAgCu solder joints

The electronics industry has successfully transitioned to lead free soldering for computer and consumer products while in the automotive industry due to the very high reliability requirements this step is still in an ongoing process. In order to ensure or even to enhance the reliability of electronics the improvement of lead free solder joints based on Sn-Ag-Cu (SAC) solder has been the focus of many research projects already. One approach to improve the thermal fatigue properties of these alloys is to add a further alloying element SAC + X. Until now, further elements have mainly been added by metallurgical alloying, requiring special equipment and conditions. The powder production is even more difficult. In this paper a new way how to add minor amounts around 0.1 wt.-% of a forth element to SAC solder joints is presented. One possibility is to do the doping during the reflow process adding the additional elements via the flux by incorporating Co-, Fe-, Al- or other metal compounds into the flux. Experiments have shown that Co and Fe were successfully added while for certain other elements like for example Al this was not possible until now. In comparison to standard SAC solder the Co containing solder shows a refined microstructure and Co is found in the intermetallic phase Cu
₆
Sn
₅
. Fe which has been found at grain boundaries is not only attractive as alloying element but as metallization layer, too. It is possible to deposit Fe on printed circuit boards using electroplating. During soldering the slowly growing FeSn
₂
phase forms between the solder and the Fe layer.