The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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61 By checking in with the PCB fabrication stakeholder (as Ian in our story forgot to do), the prudent designer will find the print-and-etch factors for trace widths must stay proportionate to each other to some extent. This means as the trace-width is reduced, the height (or thickness) of the trace must shrink proportionally along with it. This will indeed need to be reflected in the PCB stack-up detail. Due to the nature of the PCB printing and etching processes, the acids attack the sides of the traces in contact with the substrate material more aggressively causing a trapezoidal effect. If the copper thickness is not reduced, the width of the copper at the base may become too narrow and fail: Balanced Etching Between Layers Besides etching considerations for trace routing, imbalanced etching of larger copper areas of the PCB can contribute to layer stress and warpage too. PCB layers which are etched leaving heavy and lite portions of imaging cause an automatic balancing challenge for the PCB fabricator. Commonly, PCB core laminates are supplied with copper foil laminated evenly to both sides. The laminated copper expands and contracts at a much different rate than the glass-epoxy laminates, but when the copper exists on both sides of the glass-epoxy equally, the stresses are evened out, allowing the core to remain flat. However, a challenge is introduced when a large percentage of the copper is only etched off one-side of the core. If there are dense copper patterns on one side, try including copper 'fill flooding,' by using ground fills on the opposite side to balance out the copper on each side. However, in the case of multi-layer PCB stack-ups, it is usually bad practice to randomly flood every layer with copper flooding for the cause of balancing the stack. In general, every signal layer deserves a solid return path laminated adjacently to it in the stack-up for signal integrity purposes. This means as the trace-width is reduced, the height of the trace must shrink proportionally along with it. Click to Enlarge Click to Enlarge

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