Peel Stress Analysis Of Lead-free Solder Ball Based On Numerical Simulation

For a long time, the tin-lead solder in the electronic industry as a connecting material has been occupying the dominant position. However, it was found in the long-term use of lead that, the lead element was a serious threat to the human's health. Out of the consideration of protecting environment and human's own health, the electronic industry has comprehensively banned the use of lead containing solders, and in the recent two years, it must achieve leadless production within the electronic packaging and assembly.After the substitution of lead-free solders for Sn-Pb solder, many problems emerged in the surface mount technology of electronic components and reliability of solder joints. The probability of various defects increased as well. The lift-off phenomenon was one of them. Furthermore, the rapid development of electronic packaging and assembly technologies has been making the size of solder joints increasingly smaller. It is very difficult to conduct researches with the testing methods on solder joints. But the finite element simulation has exactly provides a favorable means to study tiny solder joints.This thesis was utilizing the finite element software ANSYS to simulate and analyze the peel stress of lead-free Sn-Ag-Cu solder joints. First, a three dimensional finite element model was established. As the mechanical properties of lead-free solders dependent on temperatures, the peel stress of solder joints at different temperatures in a range of -50℃to 150℃were respectively examined.The results indicated that, with the peel stress increasing, the max. equivalent stress gradually increased, and the max. equivalent stress appeared at the edge of interface between solder joint and Cu pad. At room temperature 25℃, with the peel stress increasing from 33.6MPa to 336MPa, the max. equivalent stress of the solder joint increased from 7.59MPa to 52.2MPa.Under the same peel stress, the max. equivalent stress gradually decreased along with the temperature elevating. When the peel stress was 336MPa, the max. equivalent stress at 25℃was 52.2MPa, while at 150℃merely 25.0MPa.According to the tensile strength of Sn-Ag-Cu, the value of critical peel stress of the solder join was estimated. It indicated that, with temperatures increasing, the values of critical peel stress of the solder joint reduced gradually. At room temperature 25℃, the critical peel stress was about 273MPa. While at 150℃, the critical peel stress value reduced to 108MPa. According to the values of critical peel stress corresponding to the different temperatures, it was found by fitting data that, in the temperature range of -50℃to 150℃, the trends of the critical peel stress varied with temperature was characterized by an exponential decreasing law.