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I built a PCB to run some stepper motors. In the past I created some PCBs to turn on/off LED lights but that's the first time I will supply a motor. The current will be higher than my previous experiences and it is the first time I need to really think about the track widths therefore I want to make sure some points before going further. My stepper motor is rated 1.5A per phase but I will limit the current to 1A on the A4988 driver (If required I can lower this limit).

  • As the worst case I assume the motors will be supplied with 2A (2 phases x 1A per phase)
  • I tried to make the track width for each motor about 1.5 millimetres. I cannot measure them properly in the final product but when I check with a ruler I see each of them are thicker than 1 millimetre.
  • My search on the web shows that 1 mm track width must be sufficient (if not I am OK to decrease the limit.

My concerns are mostly on the solder joints.

  • The drivers have pins and I need to solder it onto the PCB. The gap between the pins and the copper on the PCB needs to be filled with solder.
  • Also I have some small disconnected paths on the PCB. They are small enough to be filled with solder but as I want to be safe I want to connect this paths with jumper cables. However again I believe there will be some solder between copper tracks and the copper wire at some points.

How can I know that the solder joint will be able to handle about 2A? In my opinion the solder will be much thicker than the tracks, even though the conductivity of the solder is about 10% of copper I believe the thickness of the solder will compensate it. However as I am not experienced I just want to confirm this. Could you please give me some guidance?

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  • \$\begingroup\$ These are relatively low currents still. PCB trace width (including those for ground currents) is what you should worry about. \$\endgroup\$
    – Lundin
    Commented May 10, 2021 at 9:47

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If the trace/pad is sized correctly and you aren't shoving more current through the pin than the component designer intended (assuming they sized the pin appropriately for their part) you don't need to worry in most situations. The solder has much more cross sectional and contact area with the pad and pin than the trace cross section.

I wouldn't worry unless you are running in very hot ambient temperatures or running extremely high currents (50A? 100A? maybe not even then) and if you do, using silver solder increases conductivity by 2-4x which will probably get you out of it if you didn't bungle things too badly.

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