Degradation Mechanism
The progressive damage and crack growth that occurs in solder joints under cyclic temperature changes limits the operational lifespan of electronic assemblies. This phenomenon, known as thermal fatigue, results from the repeated expansion and contraction of the different materials in the circuit board and components. It leads to the eventual electrical and mechanical failure of the connection.
Stress Distribution
Shear stresses accumulate at the interface between the solder joint and the printed circuit board pad because of mismatch in thermal expansion coefficients. During each thermal cycle, the solder joint undergoes plastic deformation, which slowly damages the metal matrix. This damage is concentrated in regions that experience the highest temperature shifts or have the largest differences in material properties.
Lifespan Prediction
Analyzing the rate of grain structure coarsening helps estimate when a joint is approaching failure. Designers use thermal cycling tests to accelerate the onset of thermal fatigue and evaluate the endurance of new board assemblies. By placing the boards in test chambers that alternate between extreme hot and cold temperatures, engineers can measure how many cycles the joints can withstand before the resistance rises.
Incorporating copper anchor points or underfill materials distributes the stress more evenly and delays the start of cracking.
