Vertical Strain
Dielectric material deformation occurs along the Z axis of a multilayer printed circuit board during temperature fluctuations. This out-of-plane thermal expansion generates internal stress between the copper foil and the resin substrate. Engineers quantify the phenomenon through the coefficient of thermal expansion measured in the direction perpendicular to the board surface.
High values correlate with the degradation of plated through hole reliability under thermal cycling conditions.
Dimensional Response
Copper features lower movement than the surrounding polymer matrix during operational heat cycles. Mismatches create mechanical tension at the interface of the barrel and the hole wall. Delamination or barrel cracking results when excessive strain exceeds the ductility limit of the copper plating.
Analysis of these dynamics informs the selection of low profile or treated copper foils to improve bond integrity. Production specifications for advanced packaging define thresholds for this movement to protect via connections from fatigue.
Structural Constraint
Glass reinforcement provides the primary mechanism for suppressing movement within the laminate layers. Resin content dictates the total magnitude of movement because excess polymer expands more readily than the fiber weave. Fabricators manage the thickness and uniformity of the prepreg layers to ensure consistent impedance and reliable interconnections.
Precise control of the material transition at the glass transition temperature minimizes risk during solder reflow processes. Controlled movement prevents electrical failure in dense interconnection geometries.
