Crack Propagation
The relationship between the rate of cyclic crack growth and the range of stress intensity factors during mechanical loading governs the long-term reliability of structural joints. Under repeated thermal and mechanical cycling, Paris-law fatigue provides the empirical equation that models the crack growth per cycle. Understanding this progression allows engineers to estimate the remaining useful life of solder interconnects in printed circuit boards.
Lifetime Prediction
Predicting when a solder joint will fail under the thermal stresses of repeated power cycles requires calculating the progressive crack length using these growth rate equations. In electronic boards, Paris-law fatigue operates in the stable crack growth region where the stress intensity range dictates the damage rate. If the board is subjected to rapid temperature changes, the expansion mismatch between the component and the board drives the crack further.
Engineers apply these modeling techniques to design solder geometries that prolong the period before the crack reaches a critical, unstable size.
Material Testing
Experimental testing of test vehicles on vibration tables yields the material-specific constants required to populate the fatigue equations. These constants depend on the specific solder alloy composition and the reflow profile used during assembly. Knowing these parameters helps designers select materials that resist fatigue in harsh environments.
