Cutting Traces

[zz28zz]

Looks like I'll need to cut some traces to troubleshoot an issue.
I'll have to reconnect them later so I'm looking for any tips, tricks or pitfalls to this.

[TAMHAN]

Two or more layer planar?

Large currents?

[tautech]

Looks like I'll need to cut some traces to troubleshoot an issue.
I'll have to reconnect them later so I'm looking for any tips, tricks or pitfalls to this.

Called 'razor blade technique', old trick. 

Used it for insertion of a jumper wire for use with a current probe.

Mostly you can get away with just cutting one trace to a device.
I keep a scalpel just for this, razor blades are a but flimsy IMO and I'd rather be a bit further away from the sharp bit. 
Before you finish up just check for slivers of copper that may have got away on you.

[KL27x]

If you have a belt sander or a grinder, you would just make your own chisels for this out of hex or round bar.

Short of that, take your exacto knife and grind the tip against a rock. Directly on the edge of the blade. What you want to end up with is a chisel point that is the width of the exacto blade's thickness. This will be able to cut down to 12/12 traces, individually.

What I do is first press the chisel over the trace at point A and make what I guess a woodworker might call a knife wall. Essentially, you are digging out a very tiny and super shallow mortise, here. (Bear with me, I'm a wannabe woodworker.) This first mark is to cleanly delineate the end the cut. This prevents lifting of excessive amount of copper. Then move over to point B, a few mils away from point A, and pare out the section between point A and B.

I had some recent practice of this. I got impatient and took the board out of the etch tank too soon, leaving little flakes of copper. I found not one but TWO shorts between power and ground, after dicing up the power rail into bits. Got it all sorted out in the end, though. I actually only ended up doing 3 cuts. Once you have verified that one side is actually fine, and the other is not, something magic happens, and your eyes start to work.

Reconnect by scraping a little of the soldermask away and using 30AWG wire for the small traces. The tiny chisel is the way to be, here. If you have trouble getting a connection with flux, you are probably doing it wrong. But if the traces are really that tiny, you can use just a few swipes with a fiberglass brush to shine up the copper and remove trace mask, but only after scraping the trace as well as you can with the chisel. If you brush too hard, you'll expose copper in the surrounding area. 

[jpanhalt]

Once you cut the trace and need to fix it, I found that fiberglass "scratch" pens like this: https://www.amazon.com/SE-7616SB-Fiberglass-Scratch-Brush/dp/B003NHDITW make removing the solder mask easy and neat.

John

[DaJMasta]

That scratch pen is an interesting idea, I've always just used light pressure with an xacto or similar to remove the solder mask, similar to what you'd do for enamel wire insulation removal.

A lot of times I'll cut with a knife and then run some other tool (or the back of the blade) through a few times to clean up how it looks, then inspect it to see if I've made a clean break.  It's easy to have little burrs and you don't want a potential short for the one that's supposed to be cut.  Keeping the cut as narrow as  possible makes the replacement jumper real small and keeps it out of the way of other work, but you want to be sure you can see an area with no conductor there.

[Neilm]

I use a scalpel to cut the trace. I scrape the resist off the trace then make two parallel cuts about 1mm apart. A soldering iron on the trace in the middle quickly lifts the trace rememant and the trace is separated. When I want to reconnect, just tin the scraped off trace either side of the gap and either solder bridge the gap or fit a jumper link, either 0603 reistor or a thin bit of wire.

[SeanB]

To dress the blade take a ceramic cup and turn upside down, and use the non glazed base rim as a whetstone. I have done that, though I do have some very expensive precision ground Ceramus plungers that I use for knives, very expensive because the Ceramus dispensers do not take kindly to being dropped, the glass precision ground cylinder sleeve always breaks, leaving the cylinder undamaged, and the cost of the sleeve is more than a new one after you consider shipping to Germany and back.

[KL27x]

That scratch pen is an interesting idea, I've always just used light pressure with an xacto or similar to remove the solder mask, similar to what you'd do for enamel wire insulation removal.

It is great for very specific things. One thing is if you are ever hacking something with the carbon covered button pads on it. If you want to solder to this pad (and you don't care about destroying the original purpose) I find it is great for cleaning up the copper after scraping an area of carbon with a chisel. It is good for cleaning up crusty contacts. For most pcb rework, I find it is not particularly helpful and not a must have, for me. I.e., since my last one broke, I'm in no hurry to replace it. The major problem with it is it removes soldermask very well... but very indiscriminately over what is a gigantic area if the traces are any much smaller than huge.

[KL27x]

@seanB I'm pretty handy with the ceramic cup trick. But it's worth it for me to have an actual sharpening stone and strop at the electronics bench. Takes just a small sliver of Arkansas stone and a little strop on a small block of wood to keep my rework chisels and knives sharp and burr-free. Fiberglass is not kind to edges.