A new methodology to measure the damaging strains stored in lead/tin solders during cyclic loading

A new methodology aas developed to isolate and measure the damaging strains in lead/tin solders [1]. The new methodology is based on load controlled cycling as opposed to strain controlled cycling. Under load controlled cycling conditions it is easy to separate out the effects of the different strain components. It was determined that the damaging strains were primarily creep strains, even at a cyclic loading rate of 0.5 Hz at a temperature of -40 degrees C. Fractographic analysis of the fractured specimens confirmed that failure even at -40 degrees C occurs by creep processes. The envelope strain curve generated from load controlled cycling data is a direct measure of the damaging strains stored in the material The damaging strain stored per cycle is identified by a new parameter, gamma D. The damaging strains per cycle measured for various solder compositions tested at varying frequencies, temperatures and microstructures could all be correlated by simple linear relationships. It was observed that the total damaging strain stored during the secondary creep region of the envelope strain curve ranges from 2% strain to 18% strain depending on the test conditions.