Laying Tracks

For a beginner in pcb design determining track widths can be a bit tricky. There are only a handful of factors that must be determined so the proper track width is chosen. Remember a track is copper and has electrical characteristics like DC resistance and inductance which can effect your circuit design.

PCB Manufacturer Limits

You absolutely must check with your pcb manufacturer if your design needs trace widths smaller than 8 mils. Some pcb houses haven’t made the switch to processes that support traces smaller than 8 mil. Also you’ll be paying a premium for anything smaller than 8mil. The lower limit for tracks is 4 mil. Smaller than that and the copper etching becomes flaky resulting in hair line shorts or opens. These will cost you hours and hours of troubleshooting time and a respin of the printed circuit board. Most manufacturers will specify in terms of width/space. So a 8/8 would mean a minimum of 8 mils track width and a spacing between tracks of 8 mils. Within your pcb design software you can specify the design rules to keep these widths and spacings. This makes the job a pcb layout much easier.

Temperature Rise

Remember in the introduction when I mentioned traces have DC resistance? The wider your traces the less resistance they have. The less resistance, the less power is dissipated in the trace. Remember, P=I^2 X R? This means that wider traces can carry more current and keep temperature rise to a minimum. In general you’ll want to keep the temperature rise to less than 10°C. There are some fancy calculators to determine the temperature rise in a trace. Use them! There is also one other factor that determines temperature rise in tracks. Thats the copper thickness of the pcb. Copper thickness is specified in ounces of copper per square foot. The 3 common thicknesses are 0.5oz, 1oz, and 2 oz. If you need to haul some major current across you board, like in a power supply design, you’ll want to go with 2oz copper. Doubling the copper helps heat, but it also increases cost, so use only what you need.

General Design Guidelines

For the majority of your signal traces you can use anything from 8 – 25 mils. As with most things in the world, the bigger the better. The same goes for trace widths. If you’re carrying power across the power you’ll want to use some larger 50 – 100 mil traces. One last general pcb design practice to mention here is ‘necking down’. Necking down a trace is going from a 25 mil track down to 8 mils (to get between IC pads) and then back up to 25 mils. This helps to route your board will nice low impedance tracks.