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I’m currently designing my own breakout board for the RFM95W (LoRa module). I’ll be operating at ~900 MHz.

The board has two layers. The top layer contains all the signal tracks (no ground pour). The bottom layer is a ground plane.

I’d like to use an SMA edge connector (the kind that mounts on the side of the PCB, and runs parallel to the board). I’ve added 3 copper pads on top of the PCB, and 2 pads on the bottom.

So far, I’ve designed a microstrip that connects the RFM95’s antenna pin to the SMA. It’s an extremely short track (half a centimeter maybe), so in practice, it shouldn’t qualify as a transmission line. Still, I’ve calculated the track width to match the 50 ohm characteristic impedance of the SMA connector (somewhere around 110 mils wide). It’s probably overkill, but it’s fun and I’m learning a lot.

Now, I need to route the SMA ground pads. The bottom two ground pads seem trivial — they are already touching the bottom ground plane. No work necessary.

But I’m confused about how to properly ground the top two pads.

I found some examples online that feature coplanar waveguides. For 2 layer boards, they typically have various patterns of vias connecting the top ground pour to the bottom ground layer.

The problem is, I’m not using a coplanar waveguide; I don’t have a ground pour on the top layer.

My instinct is to use vias, of course, to connect the top 2 ground pads of the SMA to the bottom ground layer of the PCB. The thing is, I don’t know how to account for impedance. Is this something I need to worry about, in practice? Do the vias need to be a certain diameter? Do the tracks need to be a certain width? Should I avoid tracks and place the vias directly on the ground pads (via-in-pad)? Should I use multiple vias for each pad?

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A connector usually has to introduce some mismatch; in fact the edge launches (the 'official' name for your connector) was designed to minimize that.

In practice I agree with you, the line is short and you wouldn't have issues. Since the connector is more or less a solid piece of brass I'd suggest to properly ground the lower pins and use the upper ones only for mechanical support. Via are surely helpful, as is via stitching with the upper layer ground.

Don't worry about the 'impedance' of the vias since it's ground and you are running a single ended line (a differential line would have two balanced traces running in the 'ground channel'). In RF you can't have too much ground; it's the reference plane about which everything is measured against. It's the signal trace impedance which matters.

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