Calculating Microvia Capture Pad Tolerances across Sequential Lamination Cycles
Capture pad size equals laser diameter plus twice the root-sum-square of tool, drill, and lamination movement tolerances.
Differential Shrinkage
Material Deformation
Dimensional variances occurring during the thermal processing of multi-layer laminates describe the phenomenon where different constituent materials contract at unequal rates. Substrate materials exhibit differential shrinkage when the resin system and the reinforcement glass styles respond differently to the heat and pressure of the lamination cycle. This movement occurs because the coefficient of thermal expansion for epoxy resin is much higher than that of the woven glass or the copper cladding.
Internal layers may shift or contract while the core remains stable, creating misregistration across the stack.
Positional Displacement
Movement of features relative to a fixed datum point complicates the drilling and routing of the final board. If differential shrinkage is not modeled accurately, the internal pads will not align with the holes drilled through the outer layers. Fabricators measure this effect by comparing the positions of targets before and after the press cycle.
They then apply scaling factors to the artwork for future production runs to move the features to their predicted post-press locations. The extent of the movement depends on the resin content and the orientation of the glass weave within the prepreg.
Process Control
Strict monitoring of the lamination temperature profile and cooling rate minimizes the internal stresses that drive these changes. Controlled cooling prevents the sudden contraction that exacerbates differential shrinkage in high layer count designs.