Mechanical Strain
Lateral force generated when two bonded materials with differing thermal coefficients expand or contract at different rates during temperature changes. Thermal expansion shear is most prevalent at the interface between the bottom of a large ceramic component and the surface of a copper circuit pad. This force eventually results in the fracture of solder joints if not properly managed through material choice.
Flexible adhesives can mitigate the intensity of these stresses.
Interface Fatigue
Repeated cycles of heating and cooling cause this lateral stress to build up, which leads to work hardening in the metallic bond. Whenever thermal expansion shear occurs, it acts to pull the component away from the board horizontally. This mechanism is the primary cause of early failure in hardware exposed to outdoor environments or intense internal heating.
Thicker solder fillets can help distribute the load across a larger area.
Joint Durability
Solder alloys are selected based on their ability to resist deformation from these expansion forces over thousands of operating cycles. Designs failing to account for thermal expansion shear often show hairline cracks near the corners of large surface mount parts. Careful pad design minimizes the mechanical leverage that these differences in expansion exert on the assembly.
Life cycle testing confirms the limits of the connection.
