New thermal fatigue life prediction method for BGA/FBGA solder joints with basic crack propagation study

Evaluation of the thermal fatigue life of the solder joints that connect the BGA package to the system board electrically and mechanically, and the improvement of precision are important issues in the development of BGA/FBGA packages. The fatigue life of the BGA solder joint consists of a process of micro-crack initiation followed by its propagation. This results in the increase of electrical resistance and the lifetime of solder joint. It is necessary to develop an evaluation method using crack growth - which defines most of the total life - for more accurate prediction of the lifetime of BGA/FBGA solder joints. The fatigue crack growth rate of BGA solder joints was measured by using actual BGA solder joint specimens in order to evaluate the thermal cycle life of BGA packages. Eutectic solder was used for the solder joint material. Isothermal mechanical fatigue tests were performed on these specimens and thermal cycle tests were performed under the temperature condition of 0&lrarr2;+100°C. The crack propagation length was measured by observing the fracture surface of the solder joint before the joint was broken interrupting the fatigue test. The fatigue crack growth rate was then evaluated. A structural analysis using FEM was made to determine the equivalent plastic strain and the plastic strain energy density of BGA solder joints in consideration of the nonlinear stress-strain relation of the solder material. The fatigue crack growth curve of BGA solder joints was evaluated combining the above experimental and analytical results.