Study On Tin Whisker Growth Behavior And Mechanism Of SnAgCu Lead-Free Microsolder Joint Under Electro-Thermal-Mechanical Coupling

With the continuous development of lead-free electronic packaging and the increasing minimization and densification of interconnection solder joint,the failure problem caused by tin whisker on the surface of lead-free micro-interconnect solder joint has become more and more prominent,which seriously threatens the service performance of electronic equipment and electronic components.In this paper,the growth behavior and mechanism of tin whisker on the surface of Cu/SAC0307/Cu micro-solder joint under the action of electric field,thermal field and stress field were studied systematically by using linear DC stabilized voltage and steady current power supply,electrothermal constant temperature blast drying box and high precision dynamic mechanical analyzer.The micro-solder joint model was designed by molecular dynamics method,and the effects of different electric field strengths,temperatures and external pressure loads on the atomic diffusion and tin whisker growth of micro-solder joint under electro-thermal-mechanical coupling were simulated and calculate.The main research conclusions are as follows:（1）The growth behavior of tin whisker on the surface of micro-solder joint was investigated under different current densities（1×10³,1×10⁴ and 1.5×10⁴ A/cm²）,temperatures（300,333,363,393 and 423 K）and external pressure loads（0.2,0.4 and 0.6 N）.The result shows that with the increase of current density,the length of the whisker increases,and the tin whisker tends to grow at the anode of the micro-solder joint and holes and cracks are more likely to occur in the cathode of micro-solder joint.It is found that the growth trend of tin whisker is more obvious,and the length and growth rate of tin whisker reach the maximum value at 333K.Besides,the growth trend of tin whisker decreases gradually with the increase of temperature from 333 K to 423 K.It is found that the growth direction and shape of tin whisker are diversified and have no certain regularity under the external pressure.（2）The effect of external pressure on the growth of tin whisker is the most significant under the action of electric field,temperature field and stress field.The growth mechanism of tin whisker on the surface of micro-solder joint is the compound mechanism of oxide film rupture mechanism,recrystallization mechanism and compressive stress mechanism,in which compressive stress provides driving force for tin whisker growth,Sn dynamic recrystallization provides the necessary atoms for tin whisker growth,finally the tin whisker is extruded and grown under the action of compressive stress at the weak point of oxide film on the micro-solder joint.（3）The effects of electric-thermal-mechanical coupling on the interfacial atomic diffusion and surface tin whisker growth of micro-solder joint under different electric field strengths（0.002,0.003 and 0.004 V/（?））,temperatures（300,333 and 393 K）and external pressure loads（0.01,0.02 and 0.03 nN）were calculated with molecular dynamics simulations.The interdiffusion of Cu atoms and Sn atoms occurred at the interface of micro-solder joint,and with the increase of electric field strength,temperature and external pressure,the diffusion rate and diffusion distance of Cu and Sn atoms increased.At the same time,the intermetallic compounds Cu₆Sn₅ and Cu₃Sn will successively form at the interface of the micro-solder joint,resulting in compressive stress inside the micro-solder joint,which in turn promotes the growth of tin whisker on the surface of the micro-solder joint.（4）Under the action of electro-thermal-mechanical coupling,the growth rate of tin whisker increases with the increase of electric field strength,temperature and external pressure,in which the external pressure has the greatest influence on the growth of tin whisker,followed by the electric field strength,and finally the temperature,which is consistent with the experimental results.Tin whisker grow freely from the weak oxide film on the surface of micro-solder joints,indicating that the growth of tin whisker on the surface of micro-solder joints is also affected by oxide film rupture and compressive stress under the action of electro-thermal-mechanical coupling.