Thermal, isolated SM pads anyone?


#1

Hi,

I’m using power resistors in a design. Their package is a 2 pin ‘D-PAK’.

The two pins are electrically isolated from the heat slug on the back.

The component obviously needs to sink heat to a pcb copper plane, but not necessarily to a signal (Does this still need a ‘pin number’?).

The symbol & footprint combinations in the supplied libraries don’t quite cut it, so I’m sure I’ll have to draw my own.

What’s the recommended way of handling this?

Thanks,

Mike


#2

TO_SOT_Packages_SMD:TO-252-2 should be what you search.
It has pin 1 and 3 as the connection pins and pin 2 as the tab -> isolated from the other pins.
So if your symbol then only assigns pins 1 and 3 pin 2 is left floating. (you can also add pin 2 with electrical type not connected to your symbol)


#3

Hi Rene,

Yes, I saw this.

It seems that whatever I use I’ll have to modify: Either the footprint (swap pins 2 & 3) or the symbol (add ‘pin 3’).

I thought maybe I could give the tab a nonsensical number - I’ve seen a TO-220 with a ‘0’ for the mounting hole.

Thanks, I’ll have another go.

I asked the question, because there is such a variation in the way things are handled within KiCad libraries, what the approved method was.

mike


#4

hi mike

i do all my libs myself.

i understand that for many people that is not an option, not initially.

generic libraries will be generic, so you will have to do some type of home made libraries at some point.

if and when we get libraries that are non-generic and states clearly that this is part # x from manufacturer y
so to pinpoint the exact part, then it will be solved, this would require more work so its avoided , but in the long run…

95% of my libraries are non-generic, they sort under /manufacturer/real-world order-able part-no

even in the cases where i use generic libraries the parts always carries a real partno

you cannot send it to production otherwise, the factories will not guesstimate the parts # in your BOM


#5

I haven’t tested this behavior yet, but you may be forced to modify the schematic symbol by adding another pin.

If you enhance the thermal capability of the tab by adding a fill zone (“copper pour”) on the board . . . the zone must refer to a net name. If the tab does not appear on the schematic, then no net name will be assigned, and you won’t be able to connect it to a fill zone.

Dale