The Reliability Study Of Lead-free Solder Joint Under Board Level Drop/Vibration Impact

The direction of electronic packaging technology development mostly focus on the green environment protection and high level integration.For solder alloy,soldering paste was demanded halogen-free based on lead-free foundation.And the package was demanded smaller size and higher density.Although the halogen-free soldering paste has been applied in the international market with small pieces area,low halogen content soldering paste in the demand of standard was mainly useded.Especially,domestic applicability and substitutability halogen-free and lead-free soldering paste research is very few,because the character of soldering paste flux is chemical diversity and composition complex.Also it have high commercial confidentiality due to the benefit reason.As the major development tendency,the halogen-free and lead free solder paste flux formula need deeply research.For solder joint reliability,it need a lot of work in the bond of market requirement becasuse mostly work focus on the theory foundation study.For example,it need take the functionality of electronic system into account when we study the failure process of solder joint,and it need consider the real work environment and experiment cost in design of loading conditon.Especially,it needs more convenience data analysis method in the work life analysis which can satisfy the different reliability demand.In this research,the halogen-free soldering paste flux for Sn-Ag-Cu（SAC）system was designed,and adipic acid was taken as one of alternative halogen active agent.The formula was:36%solvent、6%active agent、5%surface active agent、35%rosin、9%humectant、and 9%other additives.The test performance indicated that the solder ball test and wettability evaluation was first level and anti-collapse test met the industry standards demand.In the part of solder joint reliability evaluation,firstly,the solder joint failure standard was proposed.The initiation and propagation of cracks in solder joint led to the increase of resistance momently（<1ms）,which result in the decrease of power dynamic output.And the standard defined that the solder joint failure when power dynamic output decrease to 0.Secondly,according to the character of solder joint life under impact condition,it was assumed that impact life distribution or impact life standard deviationσhave exponential relationship with impact level:=₁²,σ=(6⁽（7）.So a conditional probability density distribution surface for the analysis and prediction of solder joint impact life can be obtained.Thirdly,the failure mode of solder joint under drop impact conditions was studied,and the major failure location was the intermetallic compound（IMC）layer of the solder joints in corner,but the failure mechanism was different.（1）Under 300g drop imact condition,the printed circuit board（PCB）easily occured plastic deformation after over10000 drop times,and which would increase the bending of PCB along to the deformation direction,also the pull/push stress would increse correspondingly.So the solder joint would generate cracks in IMC layer easily in the continues drop imapct.（2）Under sequence thermal cycle+drop imapct loading,alternating stress which was induced by first order thermal cycle would lead the cracks initiation in the marix near the IMC layer,but the cracks propagated also through the IMC layer during the second order drop impact,which prove that the preliminary crack location didn’t effect propagation direction,and the pull/push stress was induced by drop impact was the major effect factor.Under sequence aging+drop impact loading,Kirkendall void which was induced by aging effected anti-drop performance deterioration which would accelerate the cracks propagation velocity and decrease drop life.Fourthly,the faiure mechanism under different vibration was researched.（1）Under single vibration impact loading,the major failure laocation was IMC layer was induced by the plastic deformation of PCB,which was similar with the failure moder of solder joint under 300g drop impact.（2）Under sequence thermal cycle+vibration imapct loading,the cracks initiated in the marix near the IMC layer during the firstly order.and the cracks propagated faster along martix during second order vibration impact.（3）Under sequence aging+vibration imapct loading,Kirkendall void would weaken interfacial strength,and the cracks would initiate from Kirkendall void,which result in the failure of solder joint.So Kirkendall void weaken anti-vibration performance as well.