Molecular Relaxation
Viscoelastic model describes the non-linear return of a polymer or substrate material to its original shape after a mechanical stress is removed. Applying kohlrausch exponential recovery explains how PCB laminates behave when they undergo thermal expansion during the reflow process. This recovery follows a stretched exponential function rather than a simple linear path.
The rate of return depends on the internal molecular structure of the resin system.
Thermal Stress
Expansion of the board during heating creates internal tension that does not disappear immediately upon cooling. While simple materials relax quickly, the complex glass-reinforced epoxy in a circuit board undergoes kohlrausch exponential recovery over a longer period. This delayed movement can put stress on copper traces and through-hole barrels after the assembly leaves the oven.
Understanding the relaxation timeframe helps in predicting the long-term reliability of solder joints under cyclic loading. Surface mounting of heavy components can further influence how the board settles as it returns to a stable state.
Dimensional Stability
Warpage of the board often persists if the relaxation process is interrupted by rapid cooling or mechanical clamping. Because the recovery is non-linear, the board may change shape slightly for hours after the thermal event ends. This kohlrausch exponential recovery contributes to the residual stress that causes delamination in extreme environments.
