Joint Stress
Mechanical stress arises within a solder joint due to the difference in thermal expansion between a component and the PCB substrate during temperature changes. This thermal shear strain represents a primary cause of joint fatigue and electrical failure in assemblies exposed to repeated thermal cycles. The magnitude of this strain depends on the temperature range, the board thickness, and the physical size of the component.
Mechanical Deformation
As the circuit board heats up, it expands at a different rate than the ceramic or plastic component package soldered to it. This differential expansion forces the solder joint to deform plastically and elastically to accommodate the displacement. Over many cycles, this continuous deformation leads to micro-cracking and eventual solder joint failure.
This issue is especially pronounced in large ball grid array packages and leadless chip carriers, where the solder joints provide both the mechanical and electrical connection.
Reliability Assessment
Engineers evaluate this strain through thermal shock chambers and finite element analysis models to predict the lifespan of the assembly. These tests help select appropriate solder alloys and underfill materials to minimize the strain. Using flexible leads or compliant terminal designs can absorb the movement and extend product reliability.
