Study On Plastic Flow Behavior And Homogenization Welding Of Column Interconnect Obtained By Friction Plunge Welding

Ceramic Column Grid Array(CCGA)packaging has the advantages of high reliability,good electrical performance and good heat dissipation performance,which is widely used in the electronic packaging industry.In recent years,in order to meet the needs of lead-free,Copper Column Grid Array(Cu CGA)has gradually become a lead-free substitute for CCGA.Our research group proposed a method of column interconnect obtained by friction plunge welding in Cu CGA solder joints without a mode free,which overcomed the problems of poor mold universality,high cost and complex process existing in the traditional welding column method.In this paper,the viscoplastic flow behavior of solder in column interconnect obtained by friction plunge welding was studied by designing tracer experiment.Based on the current situation of dissimilar material connection between Cu column and Sn based solder in Cu CGA,a homogeneous interface of friction plunge welding was designed,and its microstructure was analyzed and mechanical properties were tested.SAC305 and Sn58Bi solder by layer by layer welding were superimposed on the copper pad to form a solder ball with the diameter of 3 mm.The solder column with the diameter of 0.8 mm was rotated at a high speed and pressed down into the solder ball with a depth of 2 mm to form a solder joint.The flow behavior of the two kinds of solder along the axial direction of the solder column was observed by using the difference of microstructure of different kinds of solder.The axial flow behavior of solder at different heights of solder joints was studied by adjusting the thickness ratio of Sn58Bi solder layer and SAC305 solder layer.The results indicated that the viscoplastic solder on different cross sections of the solder joint had an upward flow trend along the interface in welding column process.The maximum upward migration distances of SAC305 solder on cross sections at 0.6 mm,1.2 mm and 1.8mm from the top of the solder joint were 480?m,300?m,297?m,respectivelySAC305 solder balls with the diameter of 3 mm were reflowed on the Cu pad.A blind hole with the diameter of 0.8 mm and the depth of 2 mm at the top of the solder ball was drilled.The blind hole was filled with Pb90Sn solder.Then the solder column with the diameter of 0.8 mm was rotated at a high speed and pressed down into the filled blind hole to form a solder joint.The migration range of Pb90Sn solder in the blind hole along the radial direction of the welding column was observed by using the difference of microstructure of different kinds of solder.The results indicated that the maximum areas of Pb90Sn solder on the cross section at 0.6 mm,1.2 mm and 1.8 mm from the top of the solder joint to the outside along the radial direction of the welding column are 21551?m~2,35484?m~2,20169?m~2,respectively.In the process of column interconnect obtained by friction plunge welding,the solder at the interface between the welding column and the solder joint was thermally coupled to produce viscoplastic flow.The solder at the bottom of the welding column flowed radially outward along the welding column,and flowed upward along the friction interface between the side of the welding column and the welding ball at the corner of the welding column.At the same time,it also drived the solder at the side of the welding column to flow upward.The viscoplastic solder at the interface flowed upward rapidly at the initial stage of welding column,which made up for the weak connection defect caused by insufficient heat input at the beginning of contact to a certain extent.However,with the increase of embedding depth,the holding force of solder ball to column continueds to increased,and the fluidity of solder in the axial and radial direction of column decreased.The flow behavior of viscoplastic solder along the axial direction of the welding column at 3000 r/min,5000 r/min and 7000 r/min was analyzed.The results indicated that with the increase of the rotation speed of the welding column,and the welding heat input increased.The flow of viscoplastic solder along the axial direction of the welding column was better,and the weak connection defect area at the top of the welding joint decreased.But,the interface was affected by the equipment vibration when the rotation speed was too high.The location of weak connection defect was easy to crack.In order to realize the homogeneous welding of dissimilar materials in Cu CGA solder joints,the pure copper solder column was immersed in liquid tin at 260°C for60s to obtain a hot-dip tin solder column.The tin immersion layer included copper substrate and 3?m thick Cu₆Sn₅ interface IMC layer and 20?m thick pure tin layer.The hardness of the pure tin layer was low.The mutual solubility of the pure tin layer with the tin based solder was good,which made the friction impedance of the interface larger and the heat production increased during column planting.The viscoplastic solder at the interface was easier to flow upward along the welding column and flowed out of the solder joint.The recrystallization area caused by stirring friction in the solder joint was narrowed,and the softening area was reduced,and the weak connection defect area at the top of the solder joint was also reduced.The viscoplastic flow upward along the interface was produced by the pure tin layer of the solder column and the tin based solder in the solder ball.The pure tin layer and the tin based solder were fully mixed.The pure tin layer absorbed part of the stress caused by the solder ball clamping force during plastic deformation,which protected the Cu₆Sn₅IMC layer between the pure tin layer and the pure copper solder column.A good interconnection structure was formed between solder and pure tin layer and between pure tin layer and pure copper welding column.The pull-off load of the solder joint was increased by 38.75%.