| Low temperature soldering can reduce the thermal damage which is caused by thermal expansion mismatch among various materials in an electronic package.Sn-Bi based solder alloys are one of a promising low temperature Pb-free solder due to their lower melting temperature and higher wettability.The related properties of Sn-58Bi-xZn solder samples were systematically investigated in this study.They included the massive spalling of CuZn intermetallic compound?IMC?,tensile and creep properties of solder joint after reflow soldering and liquid-state aging,and coupling effects of electromigration?EM?and thermal aging on the mechanical properties of solder joint.The massive spalling of CuZn IMC was mainly affected by the Zn concentration.Only when the Zn concentration was about 0.67 wt.% in the solder sample after a long time of liquid-state aging,this behavior could occur.Comparing with the Sn-58Bi-0.7Zn bulk solder sample,the massive spalling phenomenon was more likely to appear in small solder ball sample.The related microstructural evolution mechanism of solder sample was analyzed using a Cu-Sn-Zn isotherm with the diffusion path concept.Moreover,when the aging temperature was higher than the liquidus temperature of the Sn-58Bi-0.7Zn bulk solder,the Sn concentration would increase greatly between the CuZn and Cu6?Sn,Zn?5 IMC layer.Then,the massive spalling of CuZn IMC also occurred.Thermodynamic analysis showed that this kind of spalling behavior was partly attributed to the reduction of free energy at the Cu6?Sn,Zn?5/CuZn interface due to the change of Sn concentration.With the addition of 0.7 wt.% Zn,ultimate tensile strengths?UTSs?of the Sn-58 Bi solder slabs were respectively increased by 6.05% and 5.50% after reflow soldering and liquid-state aging,and the Cu/Sn-58Bi solder joints were also increased by 21.51% and 29.27%.The increase in strengthening effect of Cu/Sn-58Bi-x Zn solder joints could be attributed to the fracture surface which was changed from the Cu/IMC interface to the IMC/solder interface due to the Bi grain finer.The creep behavior of Sn-58Bi-0.7Zn rectangular solder improved significantly as compared to that of the Sn-58Bi rectangular solder after reflow soldering and liquid-state aging.During tensile tests,the UTS of EM and thermal aging coupled solder joint was ascribed to the weakening of the anode interface between IMCs and Bi layer.The growth of continuous Bi layer accelerated the formation of voids and cracks,and then the tensile strength of solder joint decreased by the further crack propagation.Without current stressing,the UTS of aged solder joint was significantly influenced by the Bi segregation which led to the formation of vacancies and stress concentration at the Cu-Sn-Zn/Cu-Zn interface.Current stressing could affect the void nucleation in the second stage during coupled stressing.Therefore,the UTS of EM and thermal aging coupled solder joint was lower than that of the aged solder joint.Finite element?FE?modeling revealed that the Bi layer could play a role of load transfer for coupled sample,which decreased the penetration depth at the solder joint interface.The creep resistance of the coupled solder joint was thus higher than that of the aged solder joint. |
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