Service Behavior Of Automotive Electronic Solder Alloy

With the development and miniaturization of various electronic products at high-performance,the density of electronic packaging becomes higher and higher.As the size of the solder bump and the pitch are decreasing,the reliability of lead-free solder joints becomes particularly important.Especially with the development of autonomous driving technology,the reliability of automotive electronics has become an important issue for developers.On the other hand,the decrease in the size of the solder bump leads to a series of electromigration problems.Therefore,it is necessary to evaluate the reliability of high reliability solder alloys during high temperature service and the performance of resistance to electrical migration.In this paper,two types of high reliability commercial solder pastes used for automobile electronics,IA and MB,were investigated,using Sn3.8Ag0.7Cu(SAC387)and Sn3.0Ag0.5Cu(SAC305)lead-free solder pastes as comparison.This study mainly focuses on the following aspects:(1)to study the interfacial intermetallic compounds(IMC)of the solder joints after reflow,and the microstructure evolution after high temperature storage,the growth behavior and mechanism of the IMC were also studied.(2)The interfacial reaction between high reliability alloy and copper substrate was studied,then the influence of reflux time on IMC thickness was investigated according to the growth morphology and growth rate of the IMCs,and finally the optimal reflux time was obtained.(3)The resistance of electromigration of soldering joints was studied,and the influence of added elements on the anti-electromigration capability was investigated based on the evolutions of IMC growth,cathode void and cracks.High temperature storage experiments were carried out on WLCSP interconnection solder joints at 125?,150?,175?,focusing on the interfacial compounds growth and kirke'ndall effect.The research result shows that,in IA and MB solder joints the IMCs was(Cu,Ni)6Sn5,and inside the solder joint there are ?-Sn,lath-shaped Ag3Sn and the eutectic phase consisting of ?-Sn and Ag3Sn particles.The Bi elements is distributed in the Sn matrix,and the Sb elements with content of 1.2at%-1.8at%can be detected in(Cu,Ni)6Sn5.However,the interfacial compound is Cu6Sn5 in the SAC387 solder joint,and the solder joint is consisting of p-Sn,lath-shaped Ag3Sn and the eutectic phase of ?-Sn and Ag3Sn particles.The thickness of IMC increased with the aging time,but the the growth rate of IMC in IA and MB is less than that in SAC387.Moreover,Cu3Sn between Cu6Sn5/(Cu,Ni)6Sn5 and Cu substrate was generated,and inside the Cu3Sn,kirkendall void can be observed.The number of Kirkendall voids increase with the aging time.Because the growth rate of IMC in IA and MB is lower than that in SAC387,the number of kirkendall voids is less than those in SAC387.The diffusion activation energy of IMC at three solders joints were calculated according to IMC thickness measurements.The diffusion activation energy of IMC in SAC387 is the smallest,which is 66.8KJ/mol,and the IA have the maximum activation energy(106.8KJ/mol),while the activation energy of MB is 89.26KJ/mol,The larger the diffusion activation energy,the smaller the IMC growth rate.Besides,the grain orientation of IA and MB solder joints is the same,while the grain orientation of SAC387 solder joints is greatly different.The interfacial reaction between I A/SAC305 solder and Cu substrate when reflowed at 250 ? was also studied.The IMC after reflow was composed of(Cu,Ni)6Sns and Cu6Sns respectively.The thickness of IMC in IA soder joint is greater than that in SAC305 solder joint.When the reflux time is 5s,the reaction between IA solder and Cu substrate is not enough,and some Sn remains within the IMC,resulting in less IMC formation,and part of dendrite-like(Cu,Ni)6Sn5 stretch into the inside of the solder.As the reflux time increased,the thickness of the IMCs increased.And the IMC of IA becomes fine and close.The branched IMC will break out and spall into the solder joint,lead to the decrease of IMC thickness.High temperature storage at 150? for different time shows that,the thickness of IMC increased with the extending of aging time.It is worth noticing that the growth of Cu3Sn is obviously inhibited in IA solder joints,since its thickness is obviously lower than that of SAC305 solder joint.The Bi element originally segregated in IA solder joints is dispersed in Sn matrix with prolonging of thermal aging.Finally,we studied the effect of electromigration on IA/MB solder joints,and took SAC387 as a comparison.The results show that the three solder joints exhibit polarity effect during electromigration,that is,the anode compounds are thickened and cathode compounds are consumed.The IMC at the anode of IA and MB solder joints grew faster than that in the SAC387 solder joint,while the cathode consumption rate was lower than that of the SAC387 solder joint.With the electromigration process,the Bi elements in IA and MB solder joints are segregated at the anode interface.In terms of the time of electromigration,the width of the cathode crack and the growth rate of the anode compound,the anti-electromigration performance of the IA and MB solder is worse than the SAC387 solder.