Micro-Mechanical Behavior Of Sn-Ag-Cu/Cu Solder Joints Consisting Of Numerable Crystal Grains

Due to the appreciation of high-density assembly of microelectronic devices and microelectronic system, the solder joints have to become miniaturized. The dimensions of the solder joints have been scaled down to the crystal grain size. As a result, the mechanical behaviors of the micro solder joints will become very different from that of large joints. In this paper, the micro-mechanical behavior of solder joints made by Sn3.0Ag0.5Cu solder on Cu pads that consists of only limited number of crystal grains were analyzed thoroughly.The grain number and crystal orientation in micro solder joints was studied and the results showed that the solder joints contained only 1~6 crystal grains and no more than three grains on average. The high thermal storage had little effect on the crystal numbers and a small increase in grain numbers occurred after the multiple reflow. The dynamic recrystallization happened in the solder joints during the thermal cycling which resulted in the increase in the grain number. The orientation analysis showed that the c axis of Sn grains tended to be at a small angle with the solder pads and the preferred 60 twin misorientation was observed.The shear force of solder joints decreased with the decrease in solder joint size and shear strength increased remarkably with the decrease in joint size. The shear strength dropped by 20% after thermal storage or thermal cycling. The small joints showed great ductility and the joint failure occurred primarily inside the ductile solder. For the large joints, however, the failure occurred in the vicinity of the IMC/solder interface. After the thermal storage, the plasticity was improved for the lager joints and was degraded for the small joints. Furthermore, the mechanical properties of individual phases of the solder joints were obtained by nanoindentation tests.The dendrite arm sizes were found decrease with decrease in solder joint size. Plenty of large tubular shape Cu6Sn5 was observed in the as-reflowed small solder joints. With the reduction in joint size, the thickness of interfacial IMC increased and the Ag3Sn morphology changed from coral-like to particle-like. After thermal storage the interfacial IMC consisted of two layer, Cu6Sn5 layer and Cu3Sn layer. The interfacial IMC was found to increase linearly with the square root of aging time. The Cu3Sn growth in the small joints was greatly inhibited. The interfacial IMC orientation analysis showed that Cu6Sn5 (0001) planes and Cu3Sn (100) planes were both parallel to the solder pad surface and Cu6Sn5 (10-10) panes were parallel to the TD-RD planes. The Cu6Sn5 (0001) planes were parallel to the Cu3Sn (100) planes.The effect of solder joint size on the shear deformation behavior was investigated and the deformation mechanism was confirmed by the in-situ TEM tensile test. Results showed that the remarkable dynamic recovery and dynamic recrystallization occurred in the small joints. The Ag3Sn can pin the grain boundary which limited the grain size and can also block the crack propagation. The failure of large joints happened by the linkage of discrete cracks in the vicinity of interface. It was found that the change of Ag3Sn morphology resulted in the distinctly different shear behavior. After thermal storage the shear deformation process of large joints became the same with the small joints.To use the in-situ SEM tensile test, the deformation behavior of solder joints composed of numerable grains was examined in micro scale. The grain orientation had key effect on the joint deformation. The deformation order and degree of solder joints during the test was greatly different due to the different in the grain orientation. In the case of small deformation, the deformation occurred by cooperative slip in the different grains. It was found that the much activity on the (100)[001] and (110)[001] slip system happened and slip on (110)[-111] and (101)[-101] was more different. And the slip on (121)[-101] can't happen. The thermal storage gave rise to degradation of interfacial properties and the solder lifted off from the interface.