Via on top of smd pad, do I need to modify something?


#1

Hey, so what I’m wondering is that if I need to do something when placing via on smd pad?
I would like to place bypass caps under the MCU (bot side of PCB) and connect it by placing via on the pad of capasitor’s to save some space, but I’m not quite sure if I then need to use the option “do not tent vias” when making Gerber files? Or will kicad automatically remove tenting from the vias on top of pads?

Or is there some other trick I should use to make sure there will be no solder mask or anything in the pad because of the via.


#2

“Do not tent” just creates a dot in the solder mask layer for a via, i.e. removes the solder mask from the physical board. And there are mask holes already in the pads (if the pads are made correctly). There’s no need to do anything except ask from the manufacturer how they handle vias in pads. Warning: it may be expensive. The vias must be plugged with some substance to prevent solder paste flowing into the holes. If you solder manually at home that may not be needed.


#3

Thx for the reply. I’ve asked the board house and they said it’s fine to have vias on pads, it’s only a prototype pcb and will be soldered by me.

I don’t know if i was too specific before, but I’m planting the vias middle of the pads when routing tracks, does the same apply in that case? Or should I modify the footprints to have the vias allready in place?


#4

It’s better to do it while routing unless you’re going to reuse the footprint with holes inside pads.

If you do it in the footprint you need to add a through hole pad with the same pad number than the SMD pad. If you define the same dimensions for it than the via would be there’s probably no difference in the generated gerbers (the gerber file format doesn’t know about pads, tracks or vias, it knows only copper shapes, and the holes are just drill marks).


#5

Having via holes suck up solder paste is often quoted as a reason for not having via’s in SMD pads.

A much less often qouted but very valid reason for the opposite: adding via’s to SMD pads, is for mechanical strength. Lots of SMD connectors have mechanical pads and having a few via’s in these pads give a significant increase in strength to the adherance of the connectors to the PCB.

These mechanical pads are often also quite big, so lots of room for solder paste to be sucked into the via, which adds additional strenght.

Pads for soldering wires directly to the PCB also benefit from this.
This is often used in for example ESC’s where very high currents are used and there is no room for the additionals size or weight of connectors.