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 Flow Displacement
Viscous Movement
Viscous movement occurs when liquefied epoxy resin migrates during the lamination process to fill the gaps between the copper traces and the glass fibers. Resin flow displacement occurs when the heat and pressure of the press force the semi-solid prepreg to move from high-pressure areas to low-pressure areas. This movement is necessary to create a solid, void-free bond between the layers of the circuit board.
Controlling Volume
Controlling the amount of resin that moves is critical for maintaining the target thickness of the dielectric layer. If resin flow displacement is too high, the resulting layer might be too thin, which changes the impedance of the traces and can lead to short circuits between the copper planes. Conversely, if the resin does not flow enough, it will leave air bubbles trapped inside the board which can cause delamination during soldering.
Temperature Variable
Temperature ramp rates and the timing of the pressure application are the primary variables that dictate the viscosity and the movement of the resin. A well-managed resin flow displacement ensures that the copper is completely encapsulated and that the final thickness of the board is within the specified tolerance. Fabricators use specialized test coupons to measure the flow characteristics of a specific batch of prepreg before using it in production.
This testing helps in adjusting the press cycle to account for variations in the age or the storage conditions of the material. It also confirms that the resin has not advanced too far in its cure cycle during shipping.