Fatigue Life Of BGA Composite Solder Joint Under Thermal Cycle Loading

The Finite Element Method (FEM) and Accelerated Thermal Cycling Test (ATC) are the effective ways to predict the thermal fatigue life of BGA solder joints under thermal loads. In this paper, the thermal fatigue behaviors of BGA joints which were joined with 10Sn90Pb and 63Sn37Pb solders have been studied with FEM and ATC. The fatigue lives of different solder joints have been predicted, the failure location and its trend have been summarized, and the geometric and loading factors which influence the life value have been analyzed at the same time.Firstly, the forming process of BGA composite solder joint has been simulated by Surface Evolver software which is based on energy minimization principle. During the simulation, the geometric shape of solder joint can be changed until the energy of the whole BGA packaged devices changed to be the minimum. And these parameters and reasonable simplifying have been extracted to build the FE model.Secondly, the nine parameters for the Anand constitutive equation of Tin-lead have been calculated. Through the help of Table which is belonged to the FE software named MSC.MARC, the Anand constitutive equation has been introduced to the FE Calculation. And a constant strain rate of 0.5% tensile test has been designed to inspect and verify the reasonable of choosing Anand constitutive equation to describe the thermal and physical properties of tin-lead solder.Thirdly, the stress and strain characteristics of the solder joints have been simulated under thermal cycle loading, and the risk region for the initiation of fatigue cracks has been analyzed particularly. The thermal fatigue lives of different composites have been predicted based on the model of modified C-M equation through the value of creep shear strain and plastic shear strain. The results show that the outer BGA solder joints have the minimum fatigue life compared with the others generally. And the failure locations are determined by the amount of eutectic solder paste, the fatigue cracks often start in the region of the solder paste and the ball binding each other when the amount of solder paste is less and the fatigue life is very low. However, when the amount of solder paste is more, the fatigue cracks would change to the binding area of the solder paste and cooper pad, the fatigue life becomes relatively high as while. Synthetically, the three factors have the capability to influence the thermal fatigue life and it can be extended by increasing the Maximum size of solder ball, decreasing the heating and cooling rate of thermal cycle loading, reducing the holding time during the highest and lowest temperature.Finally, the ATC Orthogonal Experimental has been designed to analysis the factors that influenced the fatigue life of BGA composite solder joint. With the optical microscopy and the pulling up experiment after staining, the location of fatigue crack and the style of propagation have been observed. The results show that the cracks usually initiated in the corner joints of the whole array and then extended to the center gradually. And it also shows that the three factors have different capability to influence the thermal fatigue life and the order in descending are the heating and cooling rate of thermal cycle loading, the holding time during the highest and lowest temperature, and the Maximum size of solder ball.