Influence Mechanism Of Porous Cu On Microstructure And Properties Of SnBi-xAg Solder Joints

With the rapid development of electronic packaging technology has made chips tend to miniaturization and densification,which demands solder joints of higher reliability.The micro solder joints under high temperature for a long time will occur bonding failure.Therefore,improving the thermal property of solder joints plays an important role in enhancing the reliability of solder joints.Due to its good wettability,creep resistance,tensile properties and low melting point,Sn58Bi(SnBi)solder alloy is widely used in low temperature packaging industry.However,brittleness and bad thermal property by bismuth has limited the application of Sn58Bi solder in electronics packages.In this paper,SnBi-x Ag(x=0,0.4,1)solder was used as the based solder,110 and 500 ppi(pores per inch)porous Cu(P-Cu)with two types of pore diameters were used as the reinforcing phase to fabricate SnBi-x Ag@P-Cu/Cu composite solder joints.The effect of SnBi/Cu solder joints with Ag content and pore sizes of porous Cu on mechanical properties,microstructure and thermal properties were investigated through a series of experiments.The results indicated that the microstructure of the composite alloy exists some small areas enclosed by the porous Cu after the addition of 110 ppi porous Cu.The?-Sn and Bi phases in such areas was significantly refined.The microstructure of the solder outside the region was similar to that of the pure solder joint.By contrast,porous Cu is densely and uniformly distributed in the SnBi@500P-Cu/Cu solder joint.Therefore,the solder matrix showed fine morphology in every area.The grains size of couple phases was between the area blocked by porous Cu and pure solder.It is not obvious that the microstructure of solder had a significant transformation after adding 0.4 wt%Ag into the solder.On the contrary,the increasing concentration of Ag enlarges the?-Sn grains and large amount of coarsed Bi-rich dendrites transformed into Bi particles when the Ag content reached 1 wt%.The element mapping analysis of SnBi-1Ag/Cu solder indicated that Ag atoms aggerated to form Ag₃Sn IMCs.The thickness and grain size of interfacial IMC layers in solder@P-Cu/Cu solder joints with the porous Cu addition.The thickness of IMC layer has a maximum value of 0.72?m in SnBi-1Ag@P-Cu/Cu solder joint.By contrast,the other two solder joints showed similar thickness and grain size of IMC.The Cu skeletons will consume more Sn to form Cu₆Sn₅ IMC in the soldering process due to the lower porosity in 500 ppi porous Cu.Therefore,the thickness of IMC at the SnBi@500P-Cu/Cu solder joint interface is slightly lower than that at SnBi@110P-Cu/Cu solder joint.The addition of 110 ppi porous Cu contributes to the grain refinement of the microstructure in the solder alloy,and thus increases the hardness of the solder bulk.In contrast,the hardness of outside area was similar to pure solder.The hardness of SnBi-1Ag@P-Cu/Cu solder joint is the maximum of 372.2 MPa among them.The result was attributed to the distribution of Bi particles and Ag₃Sn IMCs in solder bulk,which lead to the hardness enhances.On the other hand,the SnBi@500P-Cu/Cu solder joint showed the hardness between enclosed area and pure solders.The shear property of composite solder joint was significantly improved with the addition of porous Cu,but has limited effects on the fracture position and failure mode.The shear strength of three composite solder joints were enhanced about 10 MPa after adding 110 ppi porous Cu into solder.The mechanical properties of solder joints had a rising trend with the increase of Ag content.However,the Ag₃Sn IMCs of solder bulk led to the degradation of strength when Ag reached 0.8 wt%.Thus,SnBi-0.4Ag@P-Cu/Cu solder joint has a maximum shear strength value among them.On the other hand,due to the uniform distribution of 500 ppi porous Cu in the solder joint,lots of Cu skeletons can hind the crack propagation,which leads to the improvement of mechanical properties.The added porous Cu act as highly conductive cores and promote the heat transfer process of SnBi solder layer,which improved the thermal conductivity of the solder alloy.Among them,the improvement of thermal conductivity of SnBi solder bulk increased significantly with 500 ppi porous Cu addition,and its value of thermal conductivity was two times that of pure solder alloy with average thermal conductivity of 41.32 W/m·K.