Managing Unprobed Net Fault Escape Liabilities across High Speed Dual Sided Surface Mount Batches
Unprobed high-speed nets demand combined boundary scan and thermal screening to bound latent defect escape liabilities before batch signoff.
Coffin-Manson Model
Fatigue Estimation
Thermal expansion mismatch during power cycling induces mechanical strain in surface mount solder joints. The coffin-manson model predicts the number of cycles to failure for these joints based on the accumulation of plastic strain. It describes the relationship between the applied strain range and the fatigue life of materials subjected to low cycle fatigue.
This formula relies on the plastic strain amplitude and the ductility coefficient of the solder alloy to determine the expected durability. Designers employ this empirical relationship to establish the reliability of interconnections before proceeding to volume production.
Strain Quantification
Solder joints experience varying levels of stress whenever temperature gradients shift across the printed circuit board assembly. The coffin-manson model calculates the impact of these shifts by comparing the total plastic strain per cycle against the fracture ductility of the material. A high strain amplitude significantly accelerates the degradation of the metallurgical structure within the joint.
Smaller temperature swings result in lower plastic strain levels, which extends the operating lifespan of the electronic component. Engineers perform finite element analysis to derive these strain values from the geometry of the component and the coefficients of thermal expansion for the package and the substrate.
Boundary Limits
Linear assumptions inherent in the primary equation apply only to materials where plastic deformation dominates the fatigue process. The coffin-manson model loses predictive accuracy when the strain levels remain purely within the elastic region or when creep effects become the primary driver of failure. High frequency vibrations often introduce additional fatigue modes not captured by this specific mathematical framework.
Temperature dependent material properties fluctuate during thermal cycling and require precise calibration of the input coefficients to maintain validity. Extreme thermal conditions shift the failure mechanism from fatigue to intermetallic growth or brittle fracture, which renders the model insufficient for full structural integrity assessments.