Study On The Mechanism Of Recrystallization In The Lead Free Solder Joints During Thermal Fatigue

With the rapid development of the information age and artificial intelligence,and the requirement of people for the new generation of products,the electronic packaging is developing towards to miniaturization,lightening and multifunction.As an important mechanical connection and signal transmission structure,solder joints play a significant role in the reliability of electronic packaging and even equipment.However,under the service,the Joule heat gradually increases with the size of solder joints decreasing.Then,with the frequently switching of equipment and the instability of service environment,the solder joints will be served under the thermal fatigue condition for a long time.The microstructure of solder joints at the recrystallization area is weakened by the continuous recrystallization mechanism of subgrain rotation during thermal fatigue.The cracks initiate and expand between recrystallized grains and even pass through the solder joints,resulting in the failure of solder joints.However,until now,the study of recrystallization in lead-free solder joints during thermal fatigue does not point out the relationship between the initiation and propagation of cracks and the evolution of crystal orientation,and yet the manner of subgrain rotation is not pointed out during thermal fatigue in the lead-free solder joint.For the mechanism of recrystallization in the lead-free solder joint during thermal fatigue,the subgrains formation and the reason of recrystallization grains refinement are not clearly stated.Base on above problems,Sn-3.0Ag-0.5Cu solder joints as the most widely used solder materials in industry is selected as the subject with the ceramic tap ball grid array structure.The mechanism of recrystallization in Sn-based lead-free solder joints is studied during thermal fatigue.Firstly,the relationship between the crack initiation and propagation and the evolution of grain orientation is discussed by the quasi in-situ SEM and EBSD.Secondly,the subgrain rotation behavior of lead-free solder joints at the early stage of recrystallization is observed by quasi in-situ EBSD during thermal fatigue,and established the relationship between the subgrain rotation behavior and the slip systems.Finally,the mechanism of recrystallization in lead-free solder joint during thermal fatigue is systematically stated by means of HRTEM,which lays a theoretical foundation for further improving the thermal fatigue reliability of solder joints.In this work,it is found that the plastic deformation and recrystallization is easily occurred at the chip-side of the solder joint where is the main source of the crack initiation and propagation during thermal fatigue,because the rigidity of the chip is much larger than that of PCB.The crack could be initiated and propagated into the solder joint,when the as-reflowed grain orientation changed a small angle(less than 10°)during thermal shock.In addition,the grain orientation evolution is not terminated,though the crack initiated in the solder joint,which demonstrated the crack propagation and grain orientation evolution occurred simultaneously.At the early stage of recrystallization,the mechanism of subgrain rotation was illustrated.During thermal fatigue,the same subgrain had the same rotation mode due to the same activated slip system,while different subgrains rotated with different rotation modes due to the different mechanical environment.In this work,a method related to determine the activated slip systems in the solder joint during thermal shock was proposed.Firstly,several slip system groups are determined by the subgrain rotation axes.Secondly,according to the slip traces analysis method,slip systems that maybe activated during thermal shock were determined.Finally,the intersection of the results obtained above was the activated slip systems.In addition,the relationship between the subgrain rotation behavior and the slip systems was established.The dislocation morphology in lead-free solder joints was observed by HRTEM,which was accumulated in a dot-shaped defect.The dislocation distribution at recrystallization area,recovery area and non-recrystallization area was compared by the HRTEM and kernel average misorientation maps.The results show that recovery area had the largest dislocation density.Moreover,low-angle boundaries with the ordered cyclic structure were found in the recovery area of the solder joint,which demonstrated that the formation of new low-angle boundaries is closely related to the dislocation piling-up.In addition,low-angle boundaries appeared in the recrystallization area,which indicated the process of recrystallized grains refinement was that the low-angle boundaries divided the old recrystallized grains into two parts.Comprehensively analyzing the results above,the mechanism of recrystallization in lead-free solder joint during thermal shock was illustrated in detail.During thermal shock,at the early stage of recrystallization,cyclic thermal stress was generated in the Sn-based lead-free solder joint due to the mismatched CTE,leading to the results of dislocation propagation and movement.The dislocations were piled-up to form the new low-angle boundaries.With the increasing of thermal shock cycles,the number and of low-angle boundaries and the misorientation angle were increased.Finally,large number of subgrains were generated in the solder joint,then the subgrains could be rotated into recrystallized grans by means of the mechanism of subgrain rotation.In addition,the process of recrystallized grains refinement was that the low-angle boundaries divided the old recrystallized grains or subgrains into several newly fine recrystallized grains or subgrains.