Meaning
Localized pressure occurs at the microscopic peaks of a material surface when two objects slide against each other. This micro asperity shear stress determines the rate of material deformation and the eventual formation of a stable electrical contact area. Real surfaces are never perfectly flat and only touch at these high points.
The total contact area is a small fraction of the apparent geometric area.
Peak Deformation
High pressure at the tips of the asperities causes the metal to flow plastically. This flattening increases the contact surface.
Material Displacement
During the sliding motion of a connector pin, the lateral force creates a shearing action on the engaged peaks. This micro asperity shear stress can tear small particles of metal away from the surface and create debris. This process cleans the surface of oxides but must be controlled to prevent excessive wear.
The hardness of the plating material influences how these peaks respond to the applied load. Soft metals smear while hard metals might fracture under intense stress. Calculations for the friction coefficient rely on the ratio of the shear strength to the yield pressure at these points.
Optimization Strategy
Optimization of the surface finish ensures a consistent electrical path over the life of the product. If the shear stress is too high, the plating may delaminate from the substrate. Design teams optimize the surface finish to balance the need for cleaning with the requirement for long-term durability.
