Designing Balanced Inner Layer Copper Thieving to Mitigate Thermal Lamination Shift
Balanced inner layer copper thieving mitigates thermal lamination shift by equalizing dynamic resin flow pressures and mechanical strains during press cycles.
Resin Hydrodynamics
Fluid Physics
Molten resin behaves as a non-newtonian fluid during the high temperature and pressure stages of board fabrication. Studying resin hydrodynamics helps manufacturers understand how the liquid epoxy wets the glass fibers and copper surfaces. The viscosity of the resin changes rapidly as it heats up, affecting how well it fills the internal cavities.
Viscosity Change
Heat causes the resin to thin out before the chemical curing process begins to thicken it again. In the context of resin hydrodynamics, this window of low viscosity is when the most effective bonding occurs. If the temperature rises too slowly, the resin may never become fluid enough to penetrate the tight spaces between traces.
Wetting Action
Surface tension and capillary forces pull the liquid resin into the microscopic texture of the copper foil. Good resin hydrodynamics ensure that the interface between the metal and the plastic is free of gaps. This bond is what provides the mechanical strength and moisture resistance required for long term reliability.
Without proper wetting, the board is susceptible to delamination during subsequent soldering operations. Moisture can seep into any voids and expand rapidly during heat cycles, causing the layers to separate with enough force to break copper traces.