Research On Micro-structure And Creep Of Solder Joint Under Thermomigration

In order to satisfy the more portable,lightweight and functional requirements,electronic products tend to be miniaturization and integration is more and more higher,which will leads to the less and smaller features of the interconnected welds and makes interconnection solder joints at the ends of the heat production more imbalance,causing the solder joints under a higher temperature gradient.This would trigger a more pronounced thermal migration.In electronic package,the materials used in the structure of the interconnect solder joints are different,which leads to a difference in their coefficient of thermal expansion.During the service process of the interconnect solder,the joint will makes joule heat,which will inevitably make interconnection solder joints under thermal stress.Finally,the interconnection solder will creep and fail.Therefore,the study on the thermal migration of micro solder joints and the creep behavior under thermal migration is an important way to study the reliability of electronic packaging interconnection solder joints.With the promotion of lead-free solder in recent years,it is of great significance to study the thermal migration effect of lead-free solder and the creep under thermal migration.In this paper,the thermal migration experiment of solder joints was carried out under the heat transfer condition of the middle value temperature of 125 degrees and the temperature gradient of 1120℃/cm,which was based on the structure of Cu/Sn0.7Cu/Cu,Cu/Sn0.7Cu/Ni and Ni/Sn0.7Cu/Ni,analyzing the growth patterns of the interface and the effects of thermal migration.Different stresses are applied under the same heat transfer conditions to investigate creep behavior of the micro-solder joints under the coupling thermal migration and explore its creep mechanism.It was first carry out that heat transfer experiment of Cu/Sn0.7Cu/Cu and Ni/Sn0.7Cu/Ni solder joints structure,and studied the evolvement law of the tissue after thermal migration200 h,400 h,600 h and 800 h.It is shown that unsymmetric growth occur in both solder joint structure under thermal migration.When Cu is used as substrate material,the difference of IMC at both ends is very significant,and the wastage of hot end substrate is obvious.However,when Ni is the substrate material,the difference of IMC at both ends is relatively small,and the wastage of hot end substrate is not significant.The calculation of heat transfer flux of Cu and Ni atoms indicates that,the heat transfer flux of Cu atom is much larger than Ni atom under the effect of thermal migration,which shows that the effect of thermal migration on Cu atom is greater than Ni atom.Secondly,the thermal migration of Cu/Sn0.7Cu/Ni solder joint structure was studied,and the thermal migration phenomenon of Cu as hot end and Ni as hot side were investigated.It is shown that both end interface IMC become（Cu,Ni）₆Sn₅ in the case of Cu and Ni on both end substrates.The diffusion of Cu atom dominates the growth and change of IMC in the whole interface,and the migration and diffusion of Ni atoms are not obvious.Although the diffusion of Cu atoms is dominant,the Ni atoms in the interface IMC can hinder the diffusion rate of Cu atom.Investigation of four kind of solder joints,Ni as hot side can significantly reduce the influence of thermal migration on solder joint.In the end,the creep experiments of four solder joints,including Cu/Sn0.7Cu/Cu,Cu（hot）/Sn0.7Cu/Ni（cold）,Cu（cold）/Sn0.7Cu/Ni（hot）and Ni/Sn0.7Cu/Ni,were analyzed,and the creep curves,fracture morphology and fracture location of the four solder joints were analyzed.The results show that the relationship between the creep life of solder joints is Cu/Sn0.7Cu/Cu<Cu（hot）/Sn0.7Cu/Ni（cold）<Cu（cold）/Sn0.7Cu/Ni（hot）<Ni/Sn0.7Cu/Ni,but the steady creep rate is the opposite.After analyzing creep mechanism of joints,it can be concluded that under the stress of 2 MPa and 3 MPa,the creep mechanism of joints is diffusion creep and grain boundary slip,and the fracture mechanism is micropore polymerization fracture mechanism.In the stress of 4 MPa,the creep mechanism of joint becomes the dislocation slippage mainly,and the fracture mechanism is the clevided fault.The fracture location of joints can be explained that,under the thermal migration,the IMC/solder interface is the weak area of the whole joint.Even if the substrate material and the loading stress are changed,the fracture zone of the joints will not be changed.