Meaning
Potential energy stored within a material due to the deformation caused by opposing forces acting parallel to its cross-section provides a measure of the work done during loading. In the analysis of solder joints, shear strain energy is used to quantify the damage accumulated during thermal or mechanical cycling. This value represents the area under the stress-strain curve and acts as a fundamental input for fatigue life prediction models.
Cycle Integration
Integration of the shear stress over the shear strain throughout a complete loading cycle yields the total energy density for that period. Higher values of shear strain energy indicate that the material is undergoing substantial plastic deformation, which correlates with faster crack propagation. Engineers use finite element analysis to map the distribution of this energy across a complex assembly to identify the most stressed joints.
This mapping identifies the specific ball or lead most likely to fail first in a grid array.
Hysteresis Loss
Conversion of this stored energy into heat or permanent structural changes like grain coarsening leads to the degradation of the joint over time. Because solder is a viscoplastic material, it cannot fully recover from the deformation imposed by the expansion of the circuit board. The rate at which shear strain energy accumulates determines the reliability of the interconnection under harsh operating conditions.
Standoff Adjustment
Reducing the energy density in the solder joint is achieved by adjusting the stiffness of the component or the thickness of the printed circuit board. Increasing the height of the solder balls can also lower the shear strain energy by distributing the deformation over a larger volume of material. These adjustments extend the operational life of the electronics by slowing the rate of damage accumulation.
