Thermal pad with no electrical connection

Making PCB Layouts, Manual routing, Auto-routing, Copper pouring, Updating from Schematic, Manufacturing Output
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jtowich
Posts: 15
Joined: 05 Jun 2014, 12:11

Thermal pad with no electrical connection

#1 Post by jtowich » 26 Sep 2014, 13:06

I have a SOT223 device that the data sheet specifically says that the thermal pad should not be connected to anything electrically, including GND. I am having trouble connecting a copper pour to it.

If a small trace (going nowhere) is connected to the pad in schematic it generates a net, but that net is not carried over to the layout program. To get a net in the layout program a small trace is placed on the thermal pad which generates a net. Then the copper pour and pad are set to this net. That works and DipTrace connects the copper pour to the pad with thermals. However, DipTrace keeps automatically removing the pad from the net, resulting in verification errors at the connection points.

I can ignore the verification error, but that seems like a kludge. What would be the proper way to handle this situation?

EEnh
Posts: 38
Joined: 25 Sep 2011, 10:01

Re: Thermal pad with no electrical connection

#2 Post by EEnh » 28 Sep 2014, 22:51

At my normal job (non Diptrace), thermal pads are normally assigned as pins, as are mounting pins, SMT hold-downs, etc. that are not counted as pins by connector mfgrs.

I would hope that Diptrace would handle it properly if you do create it as a pin and leave it as a no connect. The pin should be visible on the schematic symbol for reference (my opinion) with a name typically like "EPAD" for exposed thermal pad.

Haven't had occasion to try this in Diptrace.

Den

jtowich
Posts: 15
Joined: 05 Jun 2014, 12:11

Re: Thermal pad with no electrical connection

#3 Post by jtowich » 14 Oct 2014, 11:27

Yeah, I agree that is the way it should work, but it doesn't in Diptrace. I guess I will live with the verification errors.

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