Fatigue damage prediction during thermo-mechanical cycling for lead-free solders

Since a significant number of failures of solder interconnect is caused by fatigue damage the predictions of life degradation helps to design for reliability. Techniques that predict fatigue life often do so using an approximate calculation that is based on the strain energy density of a constant mesh i.e. the original joint shape. Currently the evolution of a crack, is not considered, and using true stress values is not undertaken, as the true stress cannot be measured. What is proposed here is to use a resistance measurement to estimate the remnant area that has not been cracked, and therefore a true stress value can be calculated. Testing a single joint, as is carried out in the NPL interconnect testing properties testing machine (IPTM), then the load, elongation and resistance can be measured simultaneously. In the TMF mode the solder joint is not only subjected to a mechanical displacement profile but also to a matching thermal profile. Here we present measurements of the strain energy for the equivalent of each thermal cycle, by summing each of these cycles the total strain energy density to failure can be measured. Measurement of strain and stress during each thermal cycle can give an insight into thermo-mechanical history of solder joints. This new approach reflects the true stress (measurement of force and actual load bearing area) developed inside a solder joint as well as true strain. The load bearing area can be obtained by monitoring of solder joint structure resistance, which can be correlated to the crack area.