Limitations of Norris-Landzberg equation and application of damage accumulation based methodology for estimating acceleration factors for Pb free solders

With the increasing use of Pb free solder in Electronic assemblies the estimation of their reliability under field use conditions is becoming important. The acceleration factor for solder depends on delta T, dwell times, ramp rates, actual values of temperature extremes, and the type of package. During the last few years, a number of papers have been published to determine the acceleration factors using Norris-Landzberg equation by varying some of these parameters. However, no consistent Norris-Landzberg parameters have been found yet for SnAgCu solders. This paper seeks to relate the field life with accelerated life by using simulations and a damage accumulation methodology. It is shown that the damage accumulation process in solder joints is much more complex to be captured by a simplified equation, such as Norris-Landzberg equation. The data presented here also shows that the acceleration factors depend on the package type and basing reliability requirement on accelerated test conditions, e.g., 1000 cycles for -40 to 125°C, irrespective of package type might preclude a package for a certain application even though the package might pass field level reliability requirements.