Optimizing Press Thermal Profiles for High Density Interconnect Coreless Assemblies
Optimizing coreless HDI thermal profiles requires synchronized heating ramps, controlled viscosity dwell windows, and low-rate cooling to prevent warp.
Residual Strain Relief
Stress Recovery
Internal mechanical forces stored during the lamination and drilling phases dissipate when the board is heated above its glass transition temperature. Successful residual strain relief prevents the circuit from warping or twisting during later assembly stages. This movement is a natural reaction of the composite material as it seeks a state of equilibrium.
Dimensional Change
Dimensional shifts occur as the copper and glass fibers find their new positions after the bonding pressure is removed. While residual strain relief is necessary for long term stability, it can cause registration errors if the movement is not predicted during the design phase. Manufacturers often use a bake cycle to force this relaxation before the final imaging of the circuit layers.
Thermal Stabilization
Annealing the board at specific temperatures for a set duration allows the polymer chains to reorganize and shed the energy trapped during high pressure cycles. The mechanism of residual strain relief is particularly important for thin cores where the ratio of copper to laminate is high. If these stresses are not addressed before component mounting, the heat of the reflow oven will trigger the movement then, leading to cracked solder joints or misaligned pads.
This process involves a controlled heating ramp followed by a very slow cooling period to ensure the board remains flat. Final inspection confirms that the substrate has reached a stable geometry that will not change during subsequent thermal cycles.