| With the development of electronic industry, electronic packaging has grown up rapidly as an independent new high-tech calling. Among all the technology options, surface mounted technology (SMT) is the fastest expanding one which was thought as"A Revolution of the Electronic Packaging". SMT has some advantages, but it has also many fatal weaknesses, such as the limited fatigue life and its less reliability of solder joints.However, the problems about the reliability of the solder joint are important and urging to solve in the electronic package. The reliability of the solder joint decides the quality and development of electronic production. In fact, heat action is the main reason for the failure of electronic package. So the reliability study of the solder joint is very important and significant under the condition of thermal recycle. Because of the fine-small solder joint dimension, existing experiment measures can't be used to monitor the stress and strain of solder inner, when thermal fatigue experiment is processing. So, the finite element model analysis is the most feasible means.In recent years, the problem of Lead-Pollution is getting more noticeable increasingly, with the development of electronic industry. The voice of forbidding use of lead and its compound all over the world is getting more powerful. Therefore, the development of lead-free electronic packaging is becoming a irreversible trend.In this paper, the lead-free solder Sn3.5Ag0.75Cu was chosen as a material of solder joint in the simulation of PBGA. And a 3-D slice model of PBGA was also built using the soft ANSYS with the Anand's constitutive model and the process of thermal cycle under the temperature condition from -40℃to 125℃. It's realized that the Mechanical Properties of solder joints are analyzed, which are based on 3-D model. At the same time we gain the Mechanical Parameters of mainly disabled solder joints, and find out the position of the most dangerous solder joint and the dangerous region of the solder joint in PBGA. It is shown that the distributed characteristics of stress and strain in lead-free solder joint(Sn3.5Ag0.75Cu)are given at every time of thermal cycle, and the variable rules of stress and strain with thermal cycle are acquired, which are under alternate thermal cycle. The research finds: when the thermal cycle comes -40℃, the stress of solder joints are much higher, and the viscoplastic distortion in solder joints are great, which conduce to enhance the strain as a result of the thermal expansion coefficient unbalance and upper flexile modulus of solder on the phase of low temperature. At the end of -40℃heat preservation, the stress and strain in solder joint is reduced slightly owing to viscoplastic distortion in the phase of heat preservation. However, when the thermal cycle comes +125℃, the value of stress in solder joints are much lower compare with the phase of low temperature, and the viscoplastic distortion in solder joints are much smaller because of lower flexile modulus and the phenomena of stress looseness in solder at higher temperature. Similarly at the end of +125℃heat preservation, the value of stress and strain in solder joints are cut down appreciably as the phenomena of stress looseness which was caused by viscoplastic distortion.Finally, the thermal fatigue life of the solder joint was predicted using the typical Darveaux's equation, and was compared with the 63Sn37Pb solder's thermal fatigue life. The results show that the lead-free solder (Sn3.5Ag0.75Cu) improves the thermal fatigue life of the solder joint, and also supply better reliability. |
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