I’m working on a relatively large PCB with potentiometers, buttons, and LEDs. Due to cost constraints, the PCB is limited to two layers. The bottom layer is predominantly ground, with some “underpasses” to accommodate routing on the top layer. However, this setup has resulted in relatively long traces, such as the 1800mm supply voltage line for the potentiometers. The signal traces are about half that length at most.
I’m seeking advice on precautions to consider when dealing with traces of this length. I’m contemplating adding 100n capacitors at the end of each spur, but I’m uncertain about their effectiveness. All my traces are 30mil wide and spaced 2mm apart to minimize crosstalk.
I don’t know the frequencies involved in this design which would determine your emissions but you must also consider immunity. What might couple onto these traces from outside sources and crosstalk coupling from other signals on this board. Having a good ground plane on the bottom is a good start. Where you need to put a slot in it to route a trace be sure to put connections across this slot on the top side of the board. Put in as many connections as you can to keep the openings in this slot as short as possible.
You might also add some bulk decoupling at several places along this long power buss and put small bypass caps as you suggested at every place a component connects to this buss. Place these bypass caps as close to the component pins as possible.
Thank you for sharing your input; I will address all the points you mentioned. Another concern is whether the long lengths will result in a significant voltage drop along the way.
Yes, the voltage drop can be a concern; you can calculate it using the Trace Width and Current Capacity Calculator based on your project requirements.
Why 2 layers?.. 2 or 4 layers is the same cost. I don’t buy the argument of “due to cost”…Two layers were boards of the '90s and due to EMC issues, every board was transformed into 4 layers or more. In addition, will be silly to have a nominal 59-65 mil 2 layers in a core. Don’t bother to continue, and convince your boss for more layers.
Thank you for your input! I agree that using 4 layers instead of 2 layers could be beneficial. I will explore the possibility of incorporating 4 layers into the board design.