I am working on this board design. I am designing it to be flexible (multiple possible BOMs such as FET or NPN power transistors.) And I have this additional NPN where I want to be able to use a SOT23 or SOT223 or bypass with a 0 ohm 1206 resistor.
Yesterday I was editing the board layout to connect the 1206 resistor collector-emitter to my NPN combi footprint. But today I realized that I will only use one of the three in an assembled board. So I have further edited my SOT23-SOT223 footprint to add the resistor option. And I have a symbol which I am calling “QR” to indicate transistor or resistor. This will save significant board space and ease routing on this 2-layer board as compared to a separate footprint for the resistor.
When working on the footprint, I realized that it can easily fit the resistor connected between any two pins. But this symbol shows only the collector-emitter resistor because that is the only option I anticipate in this circuit design.
So you associate this combined footprint with all three symbols and then set them Do not fit or Fit depending on which variant you are using ?
I don’t see any benefits just downsides I’m afraid . . . why not simply use the correct footprints for the devices used but arrange them in the same way as you have for your singular footprint ?
If you overlay multiple footprints on the same pads or courtyard during PCB layout, that gives you DRC errors. Perhaps added difficulties when routing tracks. A combi footprint ought not do that.
I agree you will not get DRC errors, but they can be set to be ignored in that instance. If the combination of 3 footprints is the same as the combined footprint the tracking is exactly the same.
With the combined foot print you have “incorrect” footprint info for 3 components, you have one reference on the PCB (assuming it’s used) for 3 components (no SOT23, SOT223 or 1206 footprint ref in the BOM) I don’t see any benefits, just drawbacks . . .
I do not understand what that means. I know (and the BOM can know) that one of three different component packages can be placed there, according to the build version.
Avoiding the DRC errors seems like a significant advantage.
Especially if I am doing this in multiple instances on a board, it seems like a significant help.
You have three devices, SOT23, SOT223 and 1206 but you don’t have those footprints for those devices, so you are technically using an incorrect footprint for them . . . thats all I meant.
I’d sacrifice the DRC errors to have correct reference markers on the silkscreen and correct footprints and references in the BOM.
I think that overlaying the physical footprints causes some violation of the most conservative practice, regardless of whether we do it in the pcb editor or in the footprint editor.
For a board that is for my own use, I can easily determine whether there is significant downside. If I were doing this with a board that is to be commercially manufactured and used, I would need to discuss it with the manufacturing engineers.
My reference designation is “QR__” to indicate a transistor or a resistor (probably a 0 ohm jumper). For example, “QR21”.
And the BOM would indicate any one of these three at QR21 (for example):
0 ohms 1206
PZT4401 (NPN in SOT223)
MMBT4401 (NPN in SOT23)
This board in question is for my own use. But if it were to be commercially manufactured, I could probably submit drawings to indicate how these three different device packages would fit onto the one footprint. If the manufacturing engineer gave me a thumbs down, well fire him (or her) and find a new manufacturing engineer. (I am kidding.)
One thing I know is that I am not the originator of this sort of thinking. When I was an apps engineer, my manager recommended using one combi footprint for small MOSFETs in either 3 pin SOT23 or SOT-6 (6 pin SOT23). The odd thing is that one package would be rotated 90 degrees relative to the other. I use such a footprint to accommodate either package.
