| The creep and fatigue damage mechanisms interact in lead-tin solder alloys that are used in the surface mounting of electronic components. This study concentrates on identifying and characterizing the damage mechanisms that impact on the design of accelerated tests and on the application of these tests to life prediction. Cyclic creep tests were performed on bulk cast solder of 63Sn-37Pb, the eutectic composition, and 81Pb-19Sn. In the eutectic solder, mean stress was varied from 1 MPa to 21 MPa, stress range was varied from 5 MPa to 28 MPa, temperature was varied from 0{dollar}spcirc{dollar}C to 42{dollar}spcirc{dollar}C, and hold time on load was varied from 0.3 seconds to 300 seconds and compared to static tests. Tests with unbalanced hold times, i.e., with the off-load hold time or the on-load hold time removed, were also performed. Strain controlled fatigue tests were performed from 2000 {dollar}muvarepsilon{dollar} to 20,000 {dollar}muvarepsilon{dollar} with hold times from 0.3 to 300 seconds. The 81-19 alloy and joints of the eutectic alloy were tested at hold times from 0.3 seconds to 300 seconds and compared to static tests. It is found that damage is stored as a function of time on load and stress, that is, cyclic creep damage. Because creep is the dominant damage mechanism, the severity of cyclic creep tests should not be ranked by an effective stress defined as the average of the stress raised to the n power, where n is the stress sensitivity exponent of the creep equation. At shorter hold times in both alloys and in eutectic joints, a significant part of the strain is recoverable, time dependent, and nondamaging, that is, anelastic. By this anelastic strain storage mechanism, life, measured either by cycles to failure or by time on load, is increased by at least a factor of 5. The unbalanced hold time test results show definitively that the anelastic mechanism is responsible for the increase in life rather than some other effect of cycling the solder. In the strain controlled tests, the anelastic effect is exhibited by a decrease in stress relaxation rate with decreased hold time. The implications of these findings on accelerated testing and life prediction schemes are discussed. The amount of creep-fatigue damage that solder has experienced has also been measured by the examination of grain boundary surfaces for void nucleation and growth. The volume of grain boundary voids is found to be proportional to the number of cycles of damage that have been stored. |
