Stress Concentration
Amplitude of the stress field near the tip of a crack or notch in a solid material characterizes the susceptibility of the structure to rapid failure. This parameter, known as stress intensity factor, depends on the applied load, the size of the defect, and the geometry of the component. It is a fundamental concept in fracture mechanics used to predict when brittle materials or solder joints will fail under stress.
Crack Propagation
In printed circuit board assemblies, the stress intensity factor increases at the corners of plated through holes and along the edges of solder joints during thermal cycling. When this value reaches the critical fracture toughness of the solder or resin, the crack propagates rapidly and causes electrical open circuits. Finite element analysis models calculate this value under different mechanical loading scenarios to identify high risk areas on the board layout.
By adjusting the trace geometry or pad fillet radii, engineers can reduce this localized stress concentration.
Assembly Integrity
Designers use these mechanical calculations to optimize the fatigue life of solder connections on leadless packages. The resulting designs are less prone to mechanical shock failures during drop testing. It provides a numerical baseline for evaluating the safety factor of critical solder joints.
Board assemblies built with these design rules survive longer in demanding environments.
