Research On Effects Of Ultrasonic On Microstructure And Reliability Of Fluxless SAC305Solder Joints

Applying Ultrasonic Vibration （UV） on solidifying metal has been using for along time, which could refine grain size, disperse grain orientation, and improve thealloy performances. Meanwhile, the electromigration damage strongly depends onthe Sn grain orientation in solder joints. When the c axis of Sn grain is aligned withthe current direction, the fast diffusions of Cu and Ni atoms along the c axis result insignificant damage. In order to decrease this kind of damage by shifting the grainorientation and to improve the reliabilities of lead-free solder joints, this papersystematically studied the influence of ultrasonic vibration on the solidificationbehavior, morphology evolution, and mechanical performances of pure Sn andSn-3.0Ag-0.5Cu（wt.%）（SAC305） joints and bulks. The corresponding influencescaused by ultrasound and cooling speed were also carefully studied by comparingair-cooled samples with water-cooled ones. Theoretically, the local undercoolingcaused by ultrasound was also calculated, and the most proper ultrasonic powerdensity was also determined. This research is of great importance for the followingapplication of power ultrasound on improving the solder joint reliability.The grain size could be significantly refined after being treated by ultrasonicvibration and the final diameter could be smaller than15μm for both200W and267W ultrasonic vibration. The cross sectional area was only175.31μm²under267W, which was decreased by99.6%. According to the calculation, after being treatedby267W power ultrasonic, the nucleation rate in SAC305joint was4.05×10¹⁴m^-3s^-1, T localwas248k, and the local pressure reached18GPa. For both solderjoints and bulks, the solidification temperatures increased with the ultrasonic power.But the solidification time decreased for the former, and increased for the latter.Moreover, the shear strength, hardness, and aging behavior were examined. Theshear strength of Sn joints and hardness of Sn bulks increased by35.8%and9%respectively, which was due to the strengthening mechanism of grain sizerefinement and the uniformly distributed Cu₆Sn₅particles. For SAC305joints andbulks, however, only the shear strength was improved by15.0%and the hardnessdecreased by9.2%and20.1%, respectively. The most possible reason for thisreduction was the disuniform microstructure caused by excessive ultrasonic power.The coarsen behavior of IMCs like Ag₃Sn weakened the mechanical performance.After being aged for16days, the interface IMCs grew at the least speed in sampletreated by200W ultrasonic vibration, and the ultrafine grains in the correspondingbulk sample did not change at all. These results mean that the ultrafine grains werestable for application. All in all, by relating the grain size and mechanical performances, the best ultrasonic power should be among133W and200W.