Damage mechanics of microelectronics solder joints under high current densities

The electromigration damage in flip chip solder joints of eutectic Sn/Pb was studied under current stressing. The height of the solder joints was 100 μm. The mass accumulation near anode side and void nucleation near cathode were observed during current stressing. Surface marker movement technique is used to measure the atomic flux driven by electromigration and to calculate the product of effective charge number and diffusivity, D×Z*, of the solder. Subsequent experiments reveal that the presence of thermomigration due to joule heating makes the extraction of the product of effective charge number and diffusivity erroneous when using marker movement technique. Pb Phase growth is observed under different current density and temperature. Higher current density leads to faster grain coarsening. Based on the test results, a grain coarsening equation including the influence of current density is proposed, dn - dn0 = Kjmt. The current density exponent m is found to be 3, and phase growth exponent n is found to be 5.5. Within our test temperature range, electric current seems to have greater influences on phase growth of solder joint than temperature or thermomigration caused by the temperature gradient due to joule heating during current stressing. Nano indentation tests suggest that mechanical property, e.g. Young's modulus, degrades in the localized area where void nucleates during current stressing.