It's a tricky subject and not well documented.

You have not said anything about the edge speed of any currents flowing through the vias(s). This is important if you are using them in the high current signal paths for a switch mode circuit such as a PSU or Class D amplifier since the impedance due to the via inductance may dominate the DC resistance and even the AC resistance due to skin effect.

• About the clearest information I have found is in the references

https://www.ti.com/seclit/ml/slup230/slup230.pdf

and:

https://www.ultracad.com/articles/viacurrents.pdf

given in:

http://circuitcalculator.com/wordpress/2006/03/12/pcb-via-calculator/

• This is also quite helpful:

https://www.dataweek.co.za/59129n

You may be able to find copies of the book it refers to from a number of online sources.

BTW, I assume that "The via is 12mm hole and 24mm pad." should be:

"The via is 12mil hole and 24mil pad."

To my mind the default 12/24mil via size is too small so I always change it to 15/30 mil for any new PCB before I start.  For high current lines I then use 1 via per 100mA of current in the line.  Might be overkill but better than a burnt board.

@andyfierman

Andy,    Thank you very much.   I will read all of these.  I do not know anything about edge speeds.   This is a very simple flip flop relay board for a 2 am motor.  Thank you!!!

Charlie.

@MikeDB

Mike,   I have read some varying opionions on the value of increasing the hole size and pad diameter.  But it makes sense to me.  I also do like the idea of over lapping traces from top to bottom by say 30-40 mils and have a via on each end.   Seems to me to off some redundancy that has got to help with current flow.

@charosenz

Yes if it's a normal high current line then overlapping and placing vias 50mils apart works well and I do this all the time for the +3V3 line for instance where necessary.  However if it's for a very high frequency high power line (which you say yours isn't) then you usually fan the track out and put all the vias in parallel so the signal isn't degraded by creating odd reflections between the vias.

@MikeDB

Excellent, thank you!!!