Study Of The Interfacial Reaction And Reliability Of BGA Structure "Cu/Solder-ball/Solder-paste/Cu" Interconnects

With electronic products and systems continuously developing toward miniaturization,multi-function and high reliability,ball grid array?BGA?interconnect technology,which enables high packaging density,has become a popular solution in board-level packaging.Generally,during the board-level BGA assembly process,the hybrid interconnect structure of"substrate?UBM?/solder-ball/solder-paste/substrate?PCB?"is widely used.With the increasing demand for low-cost and low-temperature reflow processes in electronic industries,recently Sn–58Bi?SnBi?solder paste has been explored and used to replace high-cost Sn–3.0Ag–0.5Cu?SAC305?solder paste.Thus,the mixed assembled multi-interface"Cu/SAC305-ball/SAC305-paste/Cu"and"Cu/SAC305-ball/SnBi-paste/Cu"structured joints are gradually adopted in BGA board-level packaging process.However,thus far,relatively little is known about the interfacial reaction and electromigration behavior in these types of joints with multi-interface structure,especially the shear fracture behavior of the single Cu/SAC305-ball/SAC305-paste/Cu joint or single Cu/SAC305-ball/SnBi-paste/Cu joint.To clarify these issues,the single-sided Cu substrate multi-interface Sn-ball/SAC305-paste/Cu joints,and dual-sided Cu substrate multi-interface Cu/SAC305-ball/SAC305-paste/Cu and Cu/SAC305-ball/Sn Bi-paste/Cu joints were designed and prepared in this thesis study.The effect of the critical reflowing temperature and solder joint size on the microstructural evolution,premelting behavior and the interfacial reaction of Sn-ball/SAC305-paste/Cu joints were studied.Then,the effects of the loading rate and isothermal aging treatment on shear deformation behavior of mixed assembled Cu/SAC305-ball/SAC305-paste/Cu and Cu/SAC305-ball/Sn Bi-paste/Cu joints were investigated.Finally,the effect of high-density current stressing?i.e.,electromigration,EM?on the interfacial reaction of Cu/SAC305-ball/Sn Bi-paste/Cu joints was studied.The results show that the Sn ball of the Sn-ball/SAC305-paste/Cu joint exhibits premelting behavior at reflow temperatures below its melting temperature,and the premelting develops from the bottom to the top of the Sn-ball.The premelting of the Sn ball results from two eutectic reactions,i.e.,the reaction between Sn and Cu and that between Sn and Ag.Sn ball of Sn-ball/SAC305-paste/Cu joints reflowed at 217.2^?C does not melt,regardless of change in solder joint size.The microstructure of the mixed part of the Sn ball and SAC305paste in joints reflowed at 227.0^?C varies obviously with the decreasing joint size.The premelting behavior of the Sn-ball has a significant influence on the growth of interfacial IMCs at the SAC305-paste/Cu interface.The thickness of the Cu₆Sn₅ layer at the SAC305-paste/Cu interface increases with decrease in size of joints reflowed at 217.2 and227.0^?C.Growth of the interfacial Cu₆Sn₅ is greatly influenced by Cu concentration distribution near the SAC305-paste/Cu₆Sn₅ interface.Cu concentration in small joints increases faster than that in large ones,and relatively long time influence of high Cu concentration in the small joints leads to the formation of thicker interfacial Cu₆Sn₅ layer with large grain size.The results of the characterization of the IMC growth and morphological evolution as well as shear fracture behavior of mixed assembled Cu/SAC305-ball/SAC305-paste/Cu joints show that the growth of the interfacial IMC layer is mainly controlled by bulk diffusion.The shear strength of the joints decreases with prolonging thermal aging time,while increasing with the loading rate.Thermal aging treatment and variation of loading rate have little influence on the shear fracture location of Cu/SAC305-ball/SAC305-paste/Cu joints.The fracture mainly occurs at the Cu/SAC305-ball interface of the joints with ductile fracture mode,regardless of change in thermal aging time and loading rate.The results of the study on shear strength and fracture behavior of mixed assembled Cu/SAC305-ball/SnBi-paste/Cu joints manifest that with increasing the loading rate from0.01 to 5.00 N/min,there is a shift in shear fracture position from the SnBi-paste/Cu interface to the Cu/SAC305-ball interface of the joints,which is caused by the difference in strain-rate sensitivity between the Sn–58Bi solder and Sn–3.0Ag–0.5Cu solder,and meanwhile the corresponding fracture mode changes from mixed ductile-brittle fracture to ductile fracture.Comparatively,the shear strength of Cu/SAC305-ball/SnBi-paste/Cu joints is lower than that of Cu/SAC305-ball/SAC305-paste/Cu joints at low loading rate,while conversely at high loadingrate.Duringsheardeformationprocessofmixedassembled Cu/SAC305-ball/SnBi-paste/Cu joints at a low loading rate of 0.01 N/min,the crack initiates at the SnBi-paste/Cu interface,and then propagates easily in the Sn-rich phase and along three interfaces,i.e.,the interface between the Bi-rich phase and Sn-rich phase and that between the bulk Ag₃Sn phase and Sn-rich phase as well as between the SnBi-SAC305 mixed solder and interfacial Cu₆Sn₅,resulting in mixed ductile-brittle fracture at the SnBi-paste/Cu interface in joints.The results of interfacial IMC growth and morphological evolution in mixed assembled Cu/SAC305-ball/SnBi-paste/Cu joints during EM process show that the growth of the interfacial IMC exhibits polarity effect,and the diffusion coefficent of Cu atoms at the solder/Cu interface on the anode side is higher than that on the cathode side.In addition,Bi atoms migrate to the anode side under the driving force of electron flow;when the electron flow direction is from the SnBi-paste/Cu interface to the Cu/SAC305-ball interface,Bi atoms migrate to the SnBi-SAC305 mixed solder region on the anode side,resulting in accumulation of Bi atoms in the SnBi-SAC305 mixed solder region;when the electron flow direction is from the Cu/SAC305-ball interface to the Sn Bi-paste/Cu interface,Bi atoms migrate to the SnBi-paste/Cu interface on the anode side,resulting in the formation of the Bi-rich phase layer at the SnBi-paste/Cu interface.Consequently,The difference in migration rate between Bi and Sn atoms leads to the formation of voids near the SnBi-paste/Cu interface,regardless of change in the direction of electron flow.