Detachable "debug" portion of PCB?

Question

I'm trying to add "optional" debugging test points to an existing, very dense 4 Layer PCB design (signal - GND - PWR - signal). I'd like to change the existing PCB as little as possible. My idea was to change the outline as shown below, adding a small debug area through a small neck (approx 5 mm tall by 1 mm wide) to the existing PCB:

This way, in an ideal world, when I get my PCBA's back from the manufacturer, and everything works, I can simply snap off the debug PCB and use the existing PCB as intended in end use. The current end-use mechanical dimensions don't allow for changing the PCB size. Then, before doing a bigger spin of PCBs, I delete the debug PCB, clean up the debug traces, and am ready to go.

Or, more realistically, if things aren't working as intended, I can attach logic/scope/meter probes easily to the traces via a 0.1" header (or similar) I've routed here. Figure out what went wrong, fix any mistakes and try again.

So the questions:

1. Is this a reasonable approach, if trying to minimize the changes to the existing PCB? Other suggestions welcome.
2. How can I minimize the chance that snapping the connection point will damage any existing PCB traces? Right now the route-keepin to board outline spacing is 0.25 mm = ~10 mil. I suppose increasing the width of the "part to snap" and aiming to snap closer Debug PCB should help, but other ideas welcome.
3. I don't intend to extend L2 (GND) and L3 (PWR) to the Debug PCB, but should i still worry about them shorting at the "snap" area? Any ideas to mitigate this?

Answer 1

Don't make the middle part too small. PCBs are produced in a 'frame' and removing the PCB from the frame has the danger that the debug portion breaks off easily in the last stage of manufacturing. I would use two sots and leave a small connections at the outside and a bigger one, with routing in the middle. Below is a picture I took from the WWW of PCBS in a frame.

I would also make some solder points that you can reconnect the debugger later on.

Answer 2

It is not clear how big a run this PCB is or how complex the circuity is on the test part.

If it is a big run and the test part is fairly complex this is not a great idea in my opinion.

First you are paying to duplicate a circuit on each board that presumably you only need one or two of. Second if it fails, you need to check on each board if it is the main board that has the problem or the debug board. Also, as you have mentioned, you suffer from the risk of damaging the main board when you remove it.

You would be better to create a small run of a separate debug board which mates into a debug connector you add to the main board. You can then keep those debug boards as your gold standards.

If the debug board only contains traces... the connector alone, with an appropriate harness should be enough.

Later, you can unpopulate the connector from subsequent runs of the main board as a cost reduction.

However: if you have to run traces across the board so they can come out on the debug section, you may be adding more trouble than you are solving. Much better to have local test points and or test pins you can hook a scope onto where the trace is actually used.