Study On Grain Orientation And Microstructure Of Solder Joints Under Aging And Thermal Cycling

The reliability of the products and components has been one of the problems withmost concern in the electronic industry. As the improvement of packing density, thevolume of solders become smaller and smaller and there will be one or several grains ina solder joint. But the grain orientations are different, which will show obviousanisotropy that gives rise to the difference of reliability between the solder joints due tothe different grain numbers and different grain orientations.Sn-3.5Ag, Sn-1.0Ag-0.5Cu solder balls and the practical BGA devices are used asthe experimental materials and tested after thermal cycling and aging. There will be anin situ observation on the changes of grain orientations and microstructure will be donein the solder joints.Through the in situ observation on the Sn-3.5Ag lap joints under thermal cyclingfrom0C to100C, there will be a different degree of recrystallization in the solderjoints which present different grain orientations. While the solder joints which have thesame or similar grain orientation show similar reliability. For solder joints which areunder some particular stress, the changes of grain orientations present certain regularity.After researching on the mechanism of grain orientation evolution and finding thatwhen the grains slip, external force will be disturbed and become a couple which forcesthe slip face turn to the direction which is parallel to tensile axis direction. At the sametime, grains will rotate around the axis of normal in the slip plane and slip to themaximum shear stress direction，which leads to changes in the relative positions amongthe atoms during the deformation and then the crystal orientation changes of solderjoints occur.Though statistically, it is clearly that single crystal solder joints reliability issuperior to polycrystalline solder joints reliability, which is based on the assessment ofthe reliability of a large number of solder joints. There are two main structures inpolycrystalline solder joints, one is the normal polycrystalline structure of the solderjoints whose reliability is relatively poor. While the reliability of the grains interlacedstructure of the solder joints is much higher. There is a large difference betweendifferent crystal orientations of single grain solder joints and the largest difference canbe27.9%.The inside microstructures of in situ observation images of the Sn-3.5Ag and Sn-1.0Ag-0.5Cu under different aging temperature （85C and150C） and differentaging days are discussed in this thesis. We find that, after aging, Ag3Sn particlescoarsening will occur and the higher the temperature, the faster the speed of coarsening.No matter aged at85C or150C, the coarsening speed of Sn-1.0Ag-0.5Cu solder jointsis faster than Sn-3.5Ag solder. The coarsening mechanism of Ag₃Sn particles is thattwo adjacent particles grow up with annexation and when the large particles grow larger,small particles gradually disappear which is caused by atomic diffusion. Through thecompare of grain orientation images with the microstructure images, we find that afteraging, intermetallics compound （IMC） particles in grain boundary are more likely togrow up and they go coarsening serious in the recrystallization region, leading to thedecline of solder joint performance.