Microstructure Evolution And Failure Mechanism Of Sn-based Solder Joints In Extreme Temperature Environment

During deep space exploration,electronic assemblies without thermal protection have to be subjected to extreme temperature environments with large diurnal temperature variations.Solder joints offer both mechanical and electrical support to electronic assemblies,the integrity of solder joints is crucial to the overall functioning of electronic assemblies.For deep-space applications,it is therefore crucial to understand the mechanical properties of both solder alloys and solder joints,the influence of cryogenic temperature storage on the interfacial microstructure and mechanical properties of solder joints,the microstructure evolution and failure mechanism of solder joints under extreme temperature environments.Firstly,the mechanical properties of Sn-3.0-Ag-Cu（SAC305）and Sn-37Pb solder alloys and joints in the temperature range from-196oC to 25oC were studied using tensile test and shear test at low temperatures.With decreasing temperature,the yield strength of SAC305 and Sn-37Pb solder alloys increased,while the ultimate tensile strength firstly increased and then decreased,and have a maximum value near-150oC.The ultimate tensile strength of SAC305 of solder alloys was always higher than that of Sn-37Pb solder alloys at the same temperature.As the temperature declined to-150oC,the fracture mechanism of SAC305 solder alloy changed from ductile fracture to brittle fracture,and the fracture mechanism of Sn-37Pb solder alloy changed to mixed ductile-brittle fracture.Sn-37Pb solder alloy fractured in a brittle mode at-196oC.β-Sn was the main phase of SAC305 and Sn-37Pb solder alloys.The P-N stress and yield strength ofβ-Sn significantly increased with temperature.The increase of P-N stress inhibited the motion of dislocations which was responsible for the plastic deformation ofβ-Sn,and thusβ-Sn fractured in a brittle mode at cryogenic temperatures.β-Sn exhibited high yield strength at cryogenic temperatures.Therefore the tensile stresses inβ-Sn could increase to overcome the cohesive forces between atoms under external stress at cryogenic temperatures,which led to the cleavage fracture ofβ-Sn.The increment of P-N stress and yield strength ofβ-Sn with decreasing temperature caused the brittle fracture of SAC305 and Sn-Pb solder alloys at cryogenic temperatures.As the temperature decreased,the tensile strength of SAC305/Cu and Sn-37Pb/Cu solder joints increased first and then decreased,the fracture location transformed from inside the solder bulk to inside the interfacial intermetallic compound（IMC）layer or along the solder/IMC layer interface.With declining temperature,the shear strength of SAC305/Cu and Sn-37Pb/Cu single shear lap solder joints increased first and then decreased,the fracture location transformed to inside the IMC layer or along the solder/IMC layer interface.The influence of cryogenic temperature storage on the interfacial microstructure and mechanical properties of SAC305/Cu and Sn-37Pb/Cu（Ball Grid Array Package）BGA solder joints were investigated by using cryogenic temperature storage test.For the SAC305/Cu and Sn-37Pb/Cu joints stored at-196oC and-100oC,the interfacial IMC thickness gradually increased with storage time.The interfacial IMCs in SAC305/Cu solder joints transformed from particle type to column type,while the interfacial IMCs in Sn-37Pb/Cu solder joints transformed from scallop type to column type.The growth rate of interfacial IMCs in the solder joints stored at-196oC was faster than that stored at-100oC.The growth of interfacial IMCs during cryogenic temperature storage was mainly caused by the stress gradient.The increase of interfacial IMC thickness of solder joints after cryogenic temperature storage led to the decline in the shear force of solder joints.The evolution of interfacial microstructure and its effects on the mechanical properties of SAC305 and Sn-37Pb solder joints of quad flat packages（QFPs）under extreme temperature thermal shock between-196oC and 150oC were investigated.During extreme temperature thermal shock,the formation and growth of Cu₃Sn at the Cu lead/solder interface in the SAC305 and Sn-37Pb solder joints were suppressed,and（Ni,Cu）₃Sn₄ IMCs at the solder/Ni（P）pad interface in the SAC305 joints transformed to（Cu,Ni）₆Sn₅ IMCs,due to the inter-diffusion of atoms from the Cu lead and the Ni（P）pad.The growths of IMCs at the Cu lead/solder interface and the solder/Ni（P）pad interface under extreme temperature thermal shock were controlled by bulk diffusion mechanism and grain boundary mechanism,respectively.The thermal mismatch between the solder and the IMC layer,the large temperature variation（ΔT=346oC）and the fast growth of interfacial IMCs caused the early formation of cracks at the solder/IMC interface.With increasing thermal shock cycles,the pull strength of the joints declined,and the fracture location shifted towards the IMC layer and the solder/IMC layer interface.The failure mechanism of SAC305 and Sn-37Pb solder joints of Plastic Ball Grid Array Package（PBGA）assemblies under extreme temperature thermal shock from-196oC to 150oC was analyzed.Double layers of（Ni,Cu）₃Sn₂ and（Cu,Ni）₆Sn₅ IMCs were formed at the SAC305 solder/Ni pad interface at the component side after reflow.During extreme temperature thermal shock,（Cu,Ni）₆Sn₅ IMCs were found to completely transform into（Ni,Cu）₃Sn₂ IMCs,cracks initiated and propagated inside the（Ni,Cu）₃Sn₂ IMC layer under high plastic strain energy density and inelastic strain,finally resulting in the brittle fracture of SAC305 joints.During extreme temperature thermal shock,a continuous layer of Au_0.5Ni_0.5Sn₄ was formed between the Ni₃Sn₄IMC layer and the Sn-37Pb solder at the component side of the Sn-37Pb solder joints.The global thermal mismatch between the component and the substrate,the partial thermal mismatch between Au_0.5Ni_0.5Sn₄ and Ni₃Sn₄,as well as the fast growth of interfacial IMCs caused significant stress concentration at the Ni₃Sn₄/Au_0.5Ni_0.5Sn₄interface.Cracks initiated and propagated along the interface between Au_0.5Ni_0.5Sn₄and Ni₃Sn₄,resulting in the brittle fracture of Sn-37Pb solder joints.The number of cycles to the first failure and the characteristic lifetime of SAC305 solder joints were both highter than those of Sn-37Pb solder joints under extreme temperature thermal shock.