Effect Of Polycrystalline Cu Microstructures On Sn/Cu Soldering Interfacial Reaction

In electronic packaging,the under bump metallurgy?UBM?,which is in direct contact with the solder,plays an important role in the interfacial reaction during soldering process.At present,the studies of UBM mainly focus on the effects of substrate composition,grain boundary and preferred orientation on the morphologies,textures and growth kinetics of intermetallic compounds?IMCs?.In electronic packaging industry,the UBM of polycrystalline copper is widely used.Therefore,pure copper substrates with fiber,equiaxed crystal and columnar crystal respectively were selected to react with pure Sn,Sn-0.5Cu,Sn-0.7Cu,Sn-1.0Cu and Sn-1.3Cu?wt.%?at different temperature for a certain time in this paper.The morphologies of IMCs and copper grains were observed in situ.The similarity and difference of IMC growth behavior on polycrystalline and single crystal copper substrate of same orientation were analyzed.And the effect of polycrystalline Cu microstructures on IMC growth kinetic was investigated.The main conclusions of this study are as follows:?1?The wettability of lead-free solder is controlled by two factors:non-reactive wetting and chemical reaction.With the increasing of reflow temperature or time,the spreading areas on the substrates increase.At pure Sn/Cu interface,non-reactive wetting plays a dominant role at 250?,while chemical reaction plays a leading role at 300?.When the copper concentration is 0.5-1.3 wt.%,the non-reactive wetting determines the wettability.With the increasing of Cu concentration,the wettability of solders changes in a wave manner.?2?When multiple IMC grains can be accommodated in a single grain area of polycrystalline copper substrate,the growth behavior of IMCs on some grains of the substrate is similar to that on single crystal substrate with the same orientation.IMCs with different morphologies appear alternately.And the IMCs distribution is stripy on substrates with fiber structure and blocky on equiaxed substrates.The IMCs distribution was similar in different reflow soldering process.With increasing of reflow temperature or time,IMCs with different morphologies still follow their own growth behavior.The scallop-type Cu₆Sn₅ grains gradually change to faceted or prismatic,and the scallops grow bigger but fewer.While the array of prism-type IMCs is invariable,the sizes increase and the edges are sharper.The proportion of scallop-type?or faceted or prismatic?IMCs on fiber structure and equiaxed crystal substrates increase with the increasing of copper concentration.Conversely,the proportion of prism-type Cu₆Sn₅ decreases.?3?In same soldering process,the relationship between the average thickness of IMC layers on three substrates is:columnar crystal substrate>fiber structure substrate>equiaxed crystal substrate.The grain growth rate of IMCs on columnar crystal is higher than that on the other two substrates.With the increasing of copper concentration,IMC thicknesses gradually increase.However,the number,width and aspect ratio of IMC grains fluctuate up and down on the same substrate.