Thermal Reliefs on Gnd to Meet Current Demands


#1

I created a board that is planned to “stack” on top of a second pcb of the same design. This means that the current from Gnd on the first board needs to carry the current demands of the piggybacked pcb.

Of course I put Gnd Fills on both layers, and no dedicated trace to the opposite side of the board. Oh, yes, I had a dedicated trace, but as I kept the spacing for the Gnd Fills it became obvious that I did not actually need a dedicated trace; there is significantly more copper then needed.

The oversight was the small thermal reliefs on the connecting pads.

I know I can alter the Footprint assigned to the Pcb, but I’m not certain the best practice to fix this issue. Anyone been down this rabbit hole?


#2

It is pretty common to design a pcb to piggy back on another pcb, for example bluetooth modules or esp8266 to name a few. These usually have castelladed pads to make the connection.
I think 2 layers of solder mask is considered enough isolation or else a piece of tape or similar can be put between them and the copper can be put to good use.

What exactly is your issue with the “thermal reliefs” ?
How do your PCB’s look like?
Can you post some screenshots to clarify?


#3

If I understand correctly, your concern is that the connector pads with thermal relief will not be able to handle the current for the daughterboard? There are many resources on the web for calculating heat rise in traces, and the “wings” of your thermal relief pad are in essence traces. Heat rise depends mostly on copper thickness, width, length, and current. You have four “traces” so the current will divide pretty equally between them. Usually it’s not a big concern because the trace length is so short, unless your daughterboard draws a lot of current. You can modify the shape of the thermal reliefs in Design Rules.


#4

I don’t want to do that; it will cause many problems in other areas of the board.

I just have 1 pin, on 2 connectors, that should have the thermal relief connection increased in size. Like I said, I know I can do it, I just don’t know the “best practice” method.

I can create a new Footprint and rename it just for this project.
I can alter the Footprint in the board without changing the Footprint in the Library (i think).
Other??? I don’t know.

I can’t be the only one that needed more ground current on a pad with the thermal reliefs.


#5

Edit the pad’s Thermal Relief properties under “Copper Zones” of the second tab of the Pad Properties dialog.


#6

That’s what I meant, sorry :slight_smile:


#7

How would you document that this change was implemented to the on-board Footprint?