Silica Filled Resin Viscosity Minimization during High Pressure Lamination Ramps
Silica-filled resin viscosity minimization requires synchronizing heating ramps with press pressure timing to ensure complete void encapsulation without core shift.
Trace Swim Distortion
Feature Displacement
Substrate features move during the transition of resin from a solid to a liquid state. During the high-pressure lamination of a multilayer board, trace swim distortion happens when the liquefied resin carries the thin copper lines away as it flows toward the edges. It is most common in boards with thick prepreg layers and very fine signal traces.
The result is a mismatch between the drilled holes and the internal pads.
Flow Mechanism
Resin velocity increases as it moves through the narrow channels between copper features in the stackup. This movement exerts a lateral force on the traces that can overcome the bond between the copper and the core. Trace swim distortion is often asymmetrical, with more movement occurring at the corners of the panel where resin flow is highest.
Using low-flow prepregs or reducing the lamination pressure can mitigate this movement. Designers also add dummy copper pour in open areas to balance the resin flow and anchor the signal lines.
Registration Failure
Even a small amount of movement can cause a drill bit to miss an internal pad or hit an adjacent trace. Automated X-ray inspection is used after lamination to detect trace swim distortion before the board proceeds to drilling. Precise scaling factors in the drilling program can sometimes compensate for uniform expansion but not for localized swimming.