I’m experimenting with footprints which can accept either a surface-mount part or a through-hole part. Usually this means a through-hole pad is placed on a surface-mount pad. This results in the 3D model showing the surface-pad preempting the through-hole pad. Is there some way around this?
Can you show an example? Generally that should be supported just fine by the 3d viewer.
Also generally placing a through-hole in an SMD pad is not a good idea as the solder flows into the hole, which can lead to unpredicable solder quality.
Ah, I see the problem. The solder paste layer is visible in the 3D viewer, which shows (correctly) that you’ll be adding solder paste on top of the holes. Do you want to apply solder paste to the pads? It not, you can uncheck solder paste from the SMD pads. Or just hide the soldermask in the 3D viewer if the problem is purely visual.
As I said, if you were producing this with SMD components as is, the solder paste/solder would flow into the hole, which wouldn’t be great.
Agree with Jonathan,
This is possibly a better footprint layout, no solder wicking:
I’m pondering this kind of footprint because larger-valued electrolytic capacitors and inductors have nearly the same footprint areas in SMD formats as they are in through-hole formats.
For the inductors I’m using (10mm x 10mm similar to Bournes SRR1260), the pads for the SMD parts are further apart than they are for similar through-hole parts. So the alternative throught-hole pads won’t interfere with the SMD pads. For the electrolytic capacitors, the pads look really long. Would I be safe if I reduce the length of the pads you see here from the inside out enough for the through-hole pads to show through?
What about offsetting (rotating) the THT pads, say, 45°. 90° may be too much. A THT electolytic is round so that won’t change the real estate and you should still be able to run at least one track through.
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I thought of that, but I need to run through three traces.
This is definitely not recommended and will cause problems in real production, but if its just for hobby prototyping, maybe you could do something like this:
(You could also considering putting solder mask on the thin traces between the pads, but I left them out here for visibility reasons)
But then you could of course also use your original design and solder that by hand.
What sort of troubles would be caused? I want to hand-assemble the first few boards to make sure there are no errors. Once I’m sure, the production boards would be made with a pick-and-place. I also have half a mind to offer unpopulated boards.
In my image, i halved the size of the SMD pads to make room for the THT pad, so if you put an SMD component on it using standard pick and place, it will probably have not enough solder and create a very weak connection. In your original design, there is enough solder paste, but half of it will flow into the hole, which creates an equally bad problem. It’s not a problem for hand soldering as you can just add more solder, but for production boards, you should just use standard SMD pads and remove the THT holes.
As Jonathan_Haas also already mentioned, for hand assembly it won’t be a problem. You are inspecting each solder joint when you’re making it, and humans have a built in autocorrect feedback loop built in for adjusting the amount of deposited solder.
Automated production is different. When you use a solder stencil, the amount of solder on each pad is fixed, and if you have a THT hole inside a SMT pad, then the solder will wick into the hole, and it is very likely there is not enough solder left on the SMT pad to make a good solder joint. There is also no feedback loop that automatically adds more solder to guarantee a good joint. There can be AOI (Automatic Optical Inspection) But even if it flags all the dubious joints, it still needs rework, and thus human intervention to correct it.
So, remove the THT pads before generating gerbers for production?
Yes, or just replace the footprint reference with a standard kicad footprint of the right size.
You can also rotate the SMT footprint a bit. Then you can still have it in the same location, but just with the pads not overlapping with the THT footprint.
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