We have come up with several good guidelines to follow when laying out boards. Here are some tips we think everyone should know about.
Very General:
- Lay out the important stuff first
- Don’t use fabrication houses specifications
Keep traces short - Ground planes
- Decoupling caps
- guard rings
- shielding traces, etc.
More Specific:
- Identify critical signals / parts of the circuit and lay them out first.
- Don’t work with exactly the fabrication houses specifications. Just because a fabrication house can handle down to 6mil traces and 6mil space does not mean you should design with those sizes.
- Annular ring around the vias should be much larger than the drill hole, and a 2-5 mils for high end fabrication houses and 6 – 10mils for low cost fabrication houses.
- Label everything, all the time! It sucks to have a board and not know what is what.
- Understand the thermal activity on the board.
- Avoid directly joining two or more pins together, this can cause issues with production soldering.
- Silk screen “patches” can be for writing on later. This is good for testing for marking failed vs passed.
- One layer should act as a continuous ground plane (if possible).
- Avoid sharp corners (If this is required try to miter the corner).
- Keep signal traces and component leads as short as possible.
- If possible separate different types of components with shields. DC switched supplies, low speed components (audio), intermediate frequency components, and radio frequency components should be isolated from one another.
- High-speed communication (over >100MHz) generally requires point-to-point connections, rather than multi-point multi-drop bus connections. Communication over 500MHz should use RF layout techniques and proper connector selection.
- Try to keep all digital and analog signals isolated from one another; i.e. analog to one end of the PCB, and digital signals to the other end.
- All digital chips that have unused pins should be pulled to VCC or digital GND as appropriate. Floating connections can radiate and may cause FCC or feed through issues.
- Whitepapers exist for good reason, if available analyze the whitepaper design. Often these papers provide insight that data sheets do not.
- All power inputs should (must) utilize filtering capacitors.
- It is often good practice to use electrolytic and ceramic capacitors in parallel.
- Always place DC filter caps as close to the packages as possible, if possible refer to manufacturers layout recommendations.
- Via stitching should be used to tie different layers of the same signal together. This technique creates a vertical connection through the board structure which helps to maintain a low impedance and shorten return loops.
- If the electrical length of a trace is longer then 1/10 wavelength. The wavelength should be calculated by the highest frequency of the signal. This means for a digital system the rise time of the square wave should be known so that the maximum frequency of the square-wave can be evaluated.
- The faster the rise time of a wave (sharper corners) the electrical length of the line should be re-evaluated (square wave in time = sinc in frequency).
- Make sure all traces are terminated into something.
