Structural Loading
Surface mount assemblies experience lateral forces at the interface between the component leads and the printed circuit board pads. Land pattern shear stress describes the tension applied across the solder joint during temperature changes or mechanical bending. This force threatens the electrical and physical connection of sensitive electronic packages.
Thermal Expansion
Differences in the coefficient of thermal expansion between the silicon die, the plastic package, the solder volume, and the epoxy laminate substrate drive the deformation. When the assembly heats up, the board expands at a different rate than the component, pulling on the solder joints. Large components with many pins are particularly susceptible to this displacement.
Fatigue Mechanism
Repeated thermal cycles lead to the formation of microcracks within the intermetallic layer of the solder. The land pattern shear stress accumulates over time, causing these cracks to propagate until a complete open circuit occurs. High vibration environments accelerate this process by adding high frequency mechanical oscillations to the static thermal loads.
Reliability Criterion
Design rules limit the size of the pads and the thickness of the solder stencil to manage the load distribution. Reducing the stiffness of the component leads also helps by absorbing some of the movement before it reaches the pad. Verification typically involves destructive pull tests or specialized shear testing equipment.
