Michifumi Kawai

Highly accurate design of thermal fatigue life for flipchip joint

As electronic devices become smaller and their density increases, there is increasing need for high density packaging with the flipchip connection. But according to prediction of thermal fatigue life for flipchip connection, there are strong restrictions on the available materials which can be used in packaging and for temperature condition, and it is thought that flipchip connection cannot be widely applied. There are two main reasons which limit the use of flipchip connections; one is that the strain level under field use has not been made clear yet, and the other is that the temperature range of the accelerated fatigue life test (the thermal cyclic test) which is usually used to predict thermal fatigue life of solder joint is close to the melting point of the solders. In this study we developed the methods to solve these problems. One of the methods is the method to analyse the strain distributions associated with the temperature distributions at various time under field use by FEM. It is clear that the equivalent total strain range under field use of a computer is near 0.2%, which is much lower than the usual thermal cyclic test (about 1%). We then developed a new experimental method to carry out thermal cyclic tests to get the thermal fatigue life in a reduced time which models the conditions of actual field use better than before. In this method, the temperature range is made close to that of field use. The strain range is also made close to that of field use by selecting the proper substrates. Consequently the effect of the temperature dependence can be removed. The results demonstrate that the thermal fatigue life of our high density flipchip package has higher reliability than previously predicted.