| A complete microelectronic assembly solder joint system consists of solder,pad and interface between the two intermetallic compound layers.The formation of a continuous and uniform IMC between the solder alloy and the pad is the basic guarantee of good metallurgical connection.Soldering plays an extremely important role as one of the most commonly used connection methods.Brazing welding head of pulse type hot-pressing welding can achieve rapid heating and cooling,in connection with field temperature sensitive devices,hot-pressing welding due to the advantages of high efficiency,high joint strength,high temperature and short residence time has been widely used.In this paper,the hot-pressing welding test was adopted.SAC305 lead-free solder was used as the base solder and nano-Ni particles were used as the reinforcing phase.Nano-particles were added on the surface of the brazing sheet to prepare Cu/SAC/Cu and Cu/SAC-nano Ni/Cu micro welds,respectively.First,the effects of peak temperature,welding time and welding pressure on the microstructure and interface IMC morphology and thickness of Cu/SAC305/Cu solder joints were studied,and the optimal welding parameters were obtained.At the same time,the differences between the microstructure and interface IMC layer thickness and mechanical properties under hot-pressing and reflow-welding were compared and analyzed.The results show that:When the optimum parameters range of welding pressure is studied,when the welding pressure is 10 N,the IMC layer of the interface is very thin.When the pressure is 20N-40 N,the microstructure of the solder joints is fine and the thickness of the interface IMC is moderate.When the pressure is 60 N,the liquid filler metal is extruded at the interface because of pressure overload.The residual liquid solder is not enough to interact with the base metal,so that the brazing interface is hard to be fully filled,resulting in the coarse structure of the solder joint and the roughening of the interface compound layer.Therefore,the optimum range of welding pressure is 20N-40 N.After determining the range of welding pressures,the preferred ranges of peak temperature and welding time are discussed.The peak temperature was 240?,it could not form effective solder joints if the welding time was less than 7s and the optimal welding time range was 7s ~ 8s.when the temperature was 260?,the shortest welding time was 5s and the optimal welding time range was 6s ~ 8s.when the temperature was 280?,the welding time was less than 4s without wetting reaction to form effective solder joint and the optimal welding time range was 5s ~ 7s.The optimum welding time decreases with the increase of peak temperature.With the extension of the welding time,the thickness of the IMC layer of Cu/SAC305/Cu solder joint increases continuously,meanwhile the average size of ?-Sn primary crystal phase in the bulk solder increases.The ?-Sn primary crystal phase changes from cellular distribution to dendritic distribution,and finally becomes cellular distribution.The IMC layer of solder joints is thickening with the increase of welding temperature.The depth of groove valley of interface scallop IMC increases,and its morphology changes from flat scallop to steeply scallop.When the welding pressure is raised,the IMC thickness of the solder interface increases first and then decreases slowly.Under the same temperature condition,the thickness of the IMC layer after hot press welding is thinner than the reflow welding spot.When the welding temperature is 260 and 280 IMC,the thickness of the solder joint after hot pressing is only 46% of that of the solder joint after reflow.The hardness of solder joints after hot pressing welding is significantly higher than that of the solder joints after reflow welding.Secondly,the influence of nano Ni content on microstructure,intermetallic intermetallic compound evolution and micro hardness of Cu/SAC-nano Ni/Cu solder joints is analyzed.The results show that:The added nano-Ni reacts with the matrix solder to form micro-scale(Cu,Ni)6Sn5 compounds distributed in the bulk solder,providing more heterogeneous nucleation sites for the solidification of the ?-Sn phase,while refined ?-Sn increased and grain refining effect with the content of nano Ni is more significant.The IMC morphology of the solder joint is gradually transferred from scallop to layer shape.When the content of nano Ni is 1 wt.%,the layer characteristics of IMC are more obvious.The thickness of interface IMC increases with the increase of nano Ni content.When the content of nano Ni particles is 1 wt.%,the thickness of solder joint interface IMC layer is 2.43 ?m,which was 1.08 ?m more than that of IMC without adding nanoparticles.With the increase of the peak temperature,the size of the ?-Sn primary phase in the bulk solder increases and the eutectic structure is uniformly distributed at the ?-Sn phase boundary.The thickness of IMC layer at the solder joint interface slightly thickens with the increase of the peak temperature,and the influence of temperature on the thickness of the IMC layer is insignificant.With the addition of nano Ni particles,the hardness of solder joint is obviously improved.When the content of nano-Ni particles is 0.5wt.%,The hardness of solder joint solder has the maximum value of 190.56 MPa.Compared with the hardness of the solder joint without adding Ni nanoparticles,the hardness of the solder joints with nano-Ni particles increased by 12%-25%. |
PDF Full Text Request