Hybrid Multilayer PCB Fabrication Process Window Optimization Fundamentals
Optimize hybrid PCB fabrication by matching laminate cure kinetics, deploying plasma desmear for mixed resins, and applying layer-specific scaling factors.
Scale Factors
Geometric Adjustment
Fabricators modify the dimensions of the circuit artwork to compensate for the shrinkage and expansion of the laminate during processing. Applying scale factors ensures that the finished board matches the original design specifications after it has gone through lamination. These adjustments are typically applied in both the horizontal and vertical directions to account for the grain of the glass reinforcement.
Thermal Expansion
Epoxy resin and glass fibers respond differently to the heat and pressure of the lamination press. Without the use of scale factors, the internal layers would be slightly smaller than the outer layers, leading to misaligned pads and vias. The amount of scaling required depends on the material type, the copper density and the thickness of the board.
Heavier copper layers tend to resist the shrinkage of the resin more than thin layers do. Designers and fabricators collaborate to determine the best initial guess for these values based on historical data from similar builds.
Verification Method
Measurements taken from the first production run are used to refine the artwork for future batches. If the measured holes on a test panel are slightly out of position, the scale factors are adjusted to move the pads back into alignment. This iterative process is necessary for maintaining high yields on dense boards with small annular rings.
Small errors in the alignment of each individual layer add up to a large total displacement in the finished product.