The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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62 PCB flatness is achieved by design. While it would be easy for the PCB fabrication engineers to mechanically warp the boards in the opposite direction to meet any flatness specifications for problem designs, it would also be very unethical. It is a simple fact that upon experiencing heat in the wave solder or reflow oven, an unbalanced PCB will again warp to its unconstrained equilibrium, and the problem will be exposed. Therefore, fabrication engineers appreciate a designer who is concerned about properly balancing the copper in the design stack-up during layout. It is the best preventative method to achieve PCB flatness. Stack-up Best Practices: F F Always provide a closely-spaced, adjacent, solid plane return path for signal traces to reduce EMI. F F Alternate signal and plane layers symmetrically about the centerline of the PCB to create a balanced stack-up. F F Use an impedance calculator to determine general widths for impedance control. There are many free impedance calculators available on-line which can serve this purpose. The designer's job is to get close, so the fab shop can then dial it in. F F Allow the PCB fabricator to adjust trace width, dielectric thickness (distance to reference plane), and dielectric strength to achieve specified impedance. Do not specify physical material dielectric properties of a PCB stack-up unless absolutely required to do so. F F Never allow the fabricator to adjust the copper thickness to control impedance. It is ineffective and if adjusted thinner, may devalue required current-carrying capacity of conductors; decreasing performance.

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