Prepreg Rheology Control in Thin Core Laminate Stacks
Precise control of prepreg minimum viscosity and press temperature ramp rates prevents core squeeze out and micro voiding in thin laminate stackups.
Inner Layer Movement
Layer Misalignment
Dimensional shifts occur when internal copper layers change position during the heat and pressure of the lamination process. This inner layer movement can lead to the misalignment of buried vias or the loss of annular ring integrity. Fabricators use specialized software and registration systems to predict and compensate for these material changes.
Thermal Expansion
Resin liquefaction and subsequent thermal expansion create internal forces that push the copper layers outward or cause them to shrink upon cooling. This inner layer movement varies based on the glass weave orientation and the copper density of the specific design. Pins or optical alignment systems attempt to lock the layers in place, yet some degree of shifting is inevitable in high layer count boards.
Post-lamination x-ray inspection reveals the actual location of the features inside the solidified panel.
Registration Tolerance
Failure to control these shifts results in broken connections or short circuits when holes are drilled through the stack. If the inner layer movement exceeds the design allowance, the drill will miss the internal pads entirely. Scaled artwork is used during the imaging phase to counteract the expected shrinkage or expansion of the material.
Boards with excessive internal shifting are scrapped because the defect cannot be repaired after the resin has cured.