Material Distortion
The tendency of printed circuit board laminates to permanently expand or contract during thermal and chemical processing steps causes layer misalignment. This behavior, categorized as dimensional instability, appears when raw materials undergo stress relief during copper etching, lamination, and baking. If the core shrinks or expands non-uniformly across the panel, drill-to-copper registration will fail, ruining the interconnect yield.
Structural Source
Glass transition temperatures, resin content, and copper distribution all influence how much a substrate deviates from its original dimensions. When the resin is heated past its transition point, it softens and releases the internal forces that were locked in during laminate manufacturing. The absence of copper cladding on one side of a double-sided core further exacerbates this distortion by allowing the unconstrained resin to shrink or expand freely.
Mitigation Strategy
Fabricators counter this movement by pre-baking the laminate cores to relieve manufacturing stresses before the imaging and etching processes begin. They also apply scaling factors to the artwork and drill files, adjusting the dimensions to compensate for the predicted shrinkage of each specific laminate type. This compensation is based on empirical data collected from previous production runs.
Modern multi-layer designs require this predictive adjustment on every sub-assembly to keep the drill holes centered within their corresponding inner-layer pads.
