I am trying to build an LED PCB on aluminum and one of the pins in the LED is a thermal pad. The LED in question is Wurth 158353060. Pin 3 is the thermal pad, and it is not connected to ground (symbol image below)
I am wondering, does KiCad automatically solder pin 3 to the board with no connections to the ground? (PCB editor screenshot below)
When I try to update the PCB from schematic I get this warning
Warning: No net found for component D1 pad 3 (no pin 3 in symbol).
Your LED is a 3 pin device, so so should your symbol be. The datasheet should specify if the thermal pad is totally isolated from the anode and cathode or not. If it’s truly isolated, you should be able to connect it to the anode or cathode. Since your cathode is connected to the larger area ground pour, the best thermal performance would be thermal pad connected to cathode.
Also, for best thermal performance, you might consider a solid fill as thermal spoke reliefs work both ways. Thermal spokes make hand soldering easier as the heat from the soldering iron doesn’t dump into the relative large thermal mass of the pour, but also the LED can’t dump it’s thermal dissipation into that larger thermal mass as effectively as a solid pour would allow. If you do a solid pour, you’ll likely need to assemble with a reflow oven or use a preheater under along with hot air on top so it’s a trade off between ease of soldering or the lower thermal resistance of a solid mass.
I cannot see where the Wurthless data sheet indicates what to do with that center pad. But I bet that it is similar to some 1W LEDs which I purchased years ago and I used 4 of them in the laundry room “dimlight” which I made years ago and operates 24/7. In these no-name LEDs, the center pad is a heatsink which is electrically isolated. I think you can probably connect the pad to either electrical terminal without interfering with the operation. Of course you want some copper to be well-connected to pcb copper if you want to run your LED at significant power.
I run my 4 LEDs at far under their power rating (17 mA if I remember correctly.) The heatsink pad is soldered to some copper sheet, and I hand wired the terminals to an old cordless phone power supply with dropping resistors. My purpose is for it to provide enough light so I can walk through the laundry room with the doors closed and maybe get something out of the closet. This thing is brighter than a night light but much dimmer than normal room illumination.
If you cannot get clear information from Wurth, I suggest testing one sample before laying out your board. I bet that the center terminal is floating as on those that I have.
If one makes an assumption that these Wurth LEDs are similar to variants like Cree, then Cree seems to specify it’s usage better than the Wurth datasheet does. Cree says it’s electrically isolated and can be connected to GND. Since your cathode is already grounded, so could the thermal pad.
If you’re going to be soldering these to a metal core board, you’ll have a hard time hand soldering them regardless of thermal relief spokes or not. You’ll need oven reflow or use of a preheater under and hot air over either way so my $0.02 suggestion would be to make as solid of a connection to as large of Cu area as you can and make use of the lower thermal resistance of the MCB dielectric layer that you’ll be paying a premium for.
True, the Wurth datasheet doesn’t show any information about it, it just says that it is thermal pads. I am not sure if its electrically isolated
I think I need to add an isolated copper and solder to it. https://www.reddit.com/r/ECE/comments/5k0wkd/if_the_datasheet_doesnt_say_do_you_connect_the/
My point is that if the heatsink pad is isolated, that means that you could connect it to either cathode or anode (if it is convenient for you to do so) without hurting anything. But I am only 95% certain and I recommend testing one LED before you commit your pcb layout to this.
FWIW, this is the symbol I use for Cree XP-E LEDs, which have the same footprint as these Wurth types.
The associated footprint has the thermal pad extended, based on an Application Note by Cree. This has proven to be quite adequate to drive them at 500mA when mounted on an MCPCB.
