The Creep Behaviors Of Lead-free Micro-solder Joints And The Growth Mechanism Of Tin Whisker On Coatings In Electronic Packaging

With the increasing trend moving toward lead-free,multi-functional and minimization of electronic products,the electrical load and thermal load borne by the micro-solder joints and tin coatings of electronic packaging interconnects are becoming more and more serious,resulting in reliability issues such as creep and tin whisker.These problems have become the focus of researchers.In this paper,the effects of Sn-0.3Ag-0.7Cu?SAC0307?lead-free solder joints with different trace alloying elements?Bi,Ni?on the tensile creep and fracture behavior of were systematically studied.Meanwhile,the elements diffusion,growth evolution of intermetallic compound?IMC?and creep fracture behavior of the solder joints under thermal-electro?TE?coupling were further studied.On the basis of clarifying the influence of the tin coating thickness and external loading on tin whisker growth behavior,the composite growth mechanism of tin whisker is proposed for the first time under external loading.The effects of Bi content?1.0³.0%?and Ni content?0.02⁰.10%?on the tensile creep deformation and fracture behavior of SAC0307 solder joints were studied under the stress of 8¹5 MPa at 80¹25?.The creep constitutive equation was established,consequently.The results show that:?1?The creep activation energy?Q?and creep stress index?n?of micro-solder joints were increased by the trace elements?Bi,Ni?,and the micro-solder joints has the strongest creep resistance when the Bi and Ni contents are3.0%and 0.05%,respectively.?2?Bi and Ni have similar effects on Q and n of the micro-solder joints,and their values are between 82.44⁹4.41 KJ/mol and 4.35⁴.75respectively.The main reason is that the creep deformation is controlled by dislocation climbing.?3?Bi and Ni have significant effects on the fracture mode of the micro-solder joints.Bi is dispersed in the SAC0307 micro-solder joints,which acts to hinder dislocation movement and increase grain boundary slip.The creep fracture of the micro-solder joints occurs in the filler metals,and the fracture mode is dominated by ductile fracture.SAC0307 micro-solder joints containing Ni element,a relatively small(Cu_1-xNi_x)₆Sn₅ grains are formed at the interfaces and inside the filler metals,and the formation of grains not only increases the number of dislocations,but also hinders the intergranular slip.The fracture position of the micro-solder joints is evenly distributed at the filler metal/IMC interfaces,and the fracture mode is dominated by ductile-brittle mixed fracture.The effects of TE coupling on the elements diffusion and growth evolution of IMC of SAC0307-3.0Bi solder joints were studied and the influences of TE pretreatment on creep deformation and fracture of solder joints were also analyzed.The creep constitutive equation of micro-solder joints under TE coupling was constructed.The results are shown as follows:?1?The growth of IMC increases parabolically with TE coupling time at the anode,and the cathode IMC thickness decreases at the same time.Meanwhile the IMC morphology of the cathode changes significantly.In the initial stage of TE coupling experiment,the element diffusion is mainly caused by high concentration gradient in solder joints.When the concentration gradient decreases,the electronic-wind force plays a major role to atomic diffusion.The preprocess of TE coupling has less effect on creep deformation curve characteristics of micro-solder joints.However,the creep rate increases and the creep life decreases significantly along with loading time.Therefore,the TE coupling preprocess declines creep resistance and the creep fracture location and mode of SAC0307-3.0Bi solder joints were changed.The location of creep fracture changes from the solder metal to the cathode interface,and the fracture mode converts ductile rupture into brittle rupture.The cracks and voids are founded at the section of creep fracture.However,the creep deformation of pretreated solder joints is still controlled by dislocation climbing.The effect of thermal-mechanical?TM?coupling on the growth density and rate of tin whiskers on tin coatings was investigated.The results show that:?1?The growth rate and density of tin whiskers increase along with TM time.After the TM coupling load at50?for 24h,the average growth rate and the average length of tin whiskers are the largest,which are 194 roots/mm² and 3.43?m,respectively.The number of tin whiskers under tension stress is obviously decreased compared with compression stress.Further research also shows that the growth mechanism of tin whiskers under external loading is a composite growth mechanism of dynamic recrystallization and internal stress induced by intermetallic compound?IMC?growth.?2?The maximum length and diameter of tin whiskers at the anode are obviously larger than that of the cathode under current stressing.At the same current density,the circular cavity appears firstly at the cathode.The holes accumulate and form cracks gradually along with loading time at the cathode.The width of crack is increased when the current density grows.