Multiplier Ratio
Mathematical equations that project the field life of solder joints based on accelerated thermal cycling test data determine the relation between lab results and long-term service. Applying the Norris-Landzberg acceleration factor adjusts the cycle-to-failure ratio by accounting for the frequency of thermal cycles and the maximum temperature during the cycle. This model modifies the basic Coffin-Manson relation to incorporate the non-linear effects of temperature and cycle frequency on lead-based and lead-free solder alloys.
By calculating this value, reliability engineers estimate how many test cycles correspond to one year of actual operation.
Stress Calculation
Calculation of this factor requires inputs for the temperature extremes and cycle frequencies of both the test environment and the field environment. The equation includes an activation energy term that accounts for the diffusion-controlled creep mechanisms in the solder joint. This mathematical adjustment ensures that the accelerated test results are not over-optimistic or overly conservative.
Model Boundary
Applicability of the Norris-Landzberg model is limited to solder alloy joints that fail due to creep-fatigue interaction under thermal cycling. If other failure mechanisms like vibration or shock dominate, the calculated factor is no longer valid. This boundary prevents the misapplication of the model to unrelated mechanical stress scenarios.
