PCB Design Checklist
An ever expanding list of tips, tricks and things to keep in mind when designing PCBs
This page is a repository, of sorts, of various tips and tricks I've picked up myself through books, app notes, or just experience doing everything the wrong way. It's neither authoritative nor can I promise that everything here is the best solution (assuming there even is a 'best' solution), but it comes out of my own dissatisfaction with many of the tutorials and resources on the web that don't always do a great job of explaining certain important subjects as clearly as they could or in one place.
Useful Numbers
While not design tips, per se, the following ballpark figures and numbers may be useful to keep in mind designing your PCB.
Show Your Work
It's high school math class all over again! If you want full points, you need to show not just the final calculated results but also include your work, meaning any formulas or information required to arrive at the values you've chosen.
For example, rather than just saying that a 50 ohm microstrip antenna trace should be 70 mil wide, show the formula or some basic information why it should be 70 mil since this will depend on the substrate thickness and material chosen (FR4, etc.), the thickness of the copper (1oz/35um or 2oz/70um), the frequency, etc.
Example: Calculating resistor values for 5.0V and 3.3V output on a DC/DC converter
Keep high-speed signals close to the GND plane
High speed signals should be kept as close to the GND plane as possible, ideally with a signal layer on the top and a GND plane on layer two directly underneath. This helps avoid crosstalk, as can be seen in the diagram below illustrating two traces at varying heights from the GND plane. The potential crosstalk varies according to the amount of magentic force that passes beneath the trace and the GND plane, indicated by the number of lines beneath the trace. Making the traces an appropriate width, and placing them closer to the GND plan helps reduce crosstalk.
Crosstalk is extremely sensitive to trace height (you can see that trace D has 1/4 the crosstalk of trace B in the diagram below). Keeping the trace height to a minimum is the single most effective thing you can do to reduce crosstalk.
Source: EETimes Fundamentals of PCB Design by Howard Johnson (course 3767)
Route differential signals adjacent to each other and keep them identical in length
Differential signals like D+ and D- with USB have better signal integrity and are protected from glitches since the lines are always complementary. A 1 on the first line is mirrored by a 0 on the second line, so even if there is a glitch on the lines, if both lines go high the data will be invalid.
In order to take full advantage of magnetic field cancellation, and to ensure reliable data transfers, route differential signals adjacent to each other, and place a GND guard traces on both sides of the entire length of the differential-pair.
To ensure accurate and high-speed data transfers, make sure the differential lines are identical in length. For USB, for example, ensure that the D+ and D- lines are both the same length to ensure that the signals are sent and received at the same time.
Eagle Tip: You can see the length of any trace in Eagle by running length-freq-ri.ulp (just type 'run length-freq-ri' in the command bar):
This is important for USB, particularly at high-speed
Use several package sizes for decoupling caps with sensitive parts
If a sensitive part requires several decoupling caps, try to use different package sizes for each part because each package size has different parasitic properties or may cause problems only at a certain frequency. For example, if you require a 0.1uF and a 10uF ceramic cap for decoupling, use, for example, 0603 for the 0.1uF and 0805 for the 10uF caps. Place the caps from smallest value to largest value going out from the pin, placing the 0.1uF as close as possible followed by the 10uF cap. Connect the other half of the caps directly to GND as much as possible, placing a via as close as you can get it to the cap.