Calculation of shape and experimental creation of AuSn solder bumps for flip chip applications

Eutectic Au/Sn 80/20 solder is used more and more for flip chip assembly especially of optoelectronic devices because no flux is necessary during soldering. Galvanic deposited AuSn bumps are created on wafer scale and therefore are of interest in terms of costs. The creation of electroplated AuSn bumps comprises the galvanic deposition of Au and Sn in successive steps followed by a reflow in liquid medium which is necessary to form the eutectic solder cap on top of the bumps. Depending on the application the pitch and the height, diameter and shape of the bumps have to be chosen. The shape of the bump (i.e. the height of the total bump and the thickness of the eutectic cap as well as the height of the remaining Au socket beneath the eutectic solder cap) can be adjusted by calculating the heights of the Au and Sn layers that are deposited. The remaining Au socket absorbs emerging stresses during soldering and during thermal cycling. The bumps' shape is calculated prior to the experimental setup. The bumps are created experimentally by means of electroplating and reflow in liquid medium and are analyzed using cross sections and SEM. Different bump diameters and thicknesses of the Sn cap are used. The microstructure of the eutectic solder cap, the shape of both the solder cap and the remaining Au socket as well as the formation of intermetallic phases are investigated and discussed.