Effect of a barrier layer on the liquid state duration of Gold-Tin solder

Gold-Tin solder has been used in the fiber-optic telecommunications industry for several decades. Recent applications within this industry require the attachment of more than one device sequentially to the top surface of a "heat-sink" substrate with pre-deposited Au-Sn solder. The sequential assembly leads to the need for longer liquidus times than that of current sputter deposited Au-Sn alloys.;Solder alloy containing 80--20 at% of Au-Sn stayed in liquid state for roughly 5--10 seconds at a process temperature of 320°C. The desired liquid state duration is approximately 30 seconds or longer, which is the primary objective of this project. Different barrier layer materials can be introduced between the solder and substrate, which would increase the liquid state duration of the solder.;Different barrier layer materials analyzed here are pure Platinum (Pt), electroless Nickel (NiV) and pure Nickel (Ni). The isothermal solidification of these different samples were done at 320°C. It was found that the sample with 70--30 at% Au-Sn, as solder, and pure nickel or pure platinum, as barrier layer, stayed in the liquid state for more than 300 seconds. Cross sectional analysis indicated that for the pure-platinum sample the barrier layer was completely depleted after long soldering duration, thereby exposing the substrate. The depletion rate was significantly lower when the barrier layer was pure-nickel. The diffusion mechanism at the solder/IMC and barrier layer/IMC interfaces at 320°C were theoretically analyzed using finite difference method. The modeling results are then compared with the data from the experiments.