Layer Adhesion
Mechanical force acting parallel to the interface of stacked materials defines the failure mode within composite laminates. Interlaminar shear stress occurs when opposing loads generate sliding movement between adjacent plies of a printed circuit board or structural resin. Resin systems rely on these internal bond forces to maintain structural integrity under thermal cycling or mechanical vibration.
Excessive force triggers delamination, which separates the reinforcement fibers from the matrix and compromises the electrical insulation properties of the dielectric.
Bond Integrity
Measurements of this parameter verify the capacity of a curing cycle to provide sufficient crosslinking between prepreg sheets during lamination. Manufacturers perform short beam shear testing to determine the load point at which the transition from elastic deformation to permanent material separation begins. High values indicate a strong chemical bond, whereas low values reveal contamination, incomplete resin flow or trapped air pockets during the press cycle.
The test protocol applies a concentrated load at the midpoint of a specimen with a short span to isolate the failure mechanism to the mid-plane. Precision in this evaluation ensures that high density interconnects survive the aggressive thermal expansion experienced during solder reflow processes.
Structural Limit
Failure follows when the load applied to the material exceeds the peak shear strength provided by the resin matrix at the interface. Stress concentrations near drilled holes or board edges increase the risk of localized ply separation under operational conditions. Rigorous control of the layup orientation and curing parameters prevents these internal ruptures from developing during fabrication.
This property determines the absolute capacity of a multi-layer assembly to withstand mechanical shearing without permanent loss of layer registration.
