Material Distortion
Mechanical deformation describes the lateral displacement between parallel planes within a multilayer laminate board under thermal stress. Subsurface shear strain quantifies the internal sliding occurring beneath the surface layers during reflow cycles or environmental exposure. This deformation represents the relative shift of adjacent layers when the internal resin and copper structure experiences differential expansion.
Industry standards establish thresholds for this displacement to prevent latent cracking within plated through holes or delamination between dielectric layers.
Inspection Protocol
Automated optical microscopy detects these internal irregularities by identifying trace misalignment or pad shifting in cross sections taken after thermal cycling. Technicians utilize high resolution imaging to measure the offset distance between fiducial markers on internal copper planes compared to their original design coordinates. A cross section reveals the magnitude of the displacement through a destructive sample taken from a sacrificial coupon or an aged board.
This measurement serves to validate the adhesion properties of the prepreg and the accuracy of the lamination process.
Geometric Consequence
Excessive movement creates permanent structural fatigue that compromises the conductive paths within the board. Rigid constraints on the expansion of the dielectric material help keep these internal shifts within tolerances during cooling. Smaller offsets translate into higher reliability for high density interconnects because they reduce the cyclic stress applied to delicate copper barrels.
Long term stability of the internal layout relies upon keeping this internal sliding within limits defined by the coefficient of thermal expansion mismatch between the resin and the conductive glass fibers.
