What's the best way of breaking a closed solder-jumper (like the two shown below) on a PCB? ... is there a special tool available that would make things easier and less error-prone? ... or would an ordinary x-acto knife suffice?

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i use a scalpel

 

Use a scalpel and cut twice, once down the side of each pad and remove the section of track between them.
If you warm the area with a hot-air desoldering tool first, it makes softens the board and makes it easier to cut through the copper.

 

I find that it is all too easy for a knife to slip and damage nearby tracks. I have a scalpel with a round handle and a pointed blade. I put the tip of the blade on the track to be cut and then rotate the handle so the blade works like a drill. This way I can break the track without any risk.

 

I've used an exact-o knife. Break the tip of the blade off then use the back side of the blade (the flat side) to scrape away at the trace. The sharp break will act like a chisel and remove fine layers of copper until you've cut completely through the trace. It's a narrow cut but it will break the connection. And in the future if you want to you can solder bridge the short gap.

To break the tip use a pair of good pliers and grip as little of the tip as you can. Bend it till it snaps. You don't have to break the blade in half and you don't want to try and cut the blade. Breaking the very tip of the blade will suffice.

 

I use a dremel and small disc cutter, as long as it is just two layer board.
I wear my mgnifier illuminator head band, ensures there is no stray copper!

 

I use a dremel and small disc cutter, as long as it is just two layer board.
I wear my mgnifier illuminator head band, ensures there is no stray copper!

View attachment 353268

I did do that but the scalpel easier I also a pencil grinder. It’s real good for cleaning off the solder mask.

 

would an ordinary x-acto knife suffice?

That's usually what I use. With an unmodified #16 blade.

 

If there has already been solder applied to close the circuit, then use a "solder-wick" product to remove the solder. If removing a copper connection, the previous suggestions are applicable.

 

I use a small dental burr in a 'Dremel' to grind away the unwanted copper. Leaves smooth edges with no burrs and minimal damage to the underlying FR4. A friendly dental nurse saves me a few 'dead' ones that are no longer useful for teeth (tooth enamel is a lot harder than copper) after they've been through their sterilizer.

 

I use my small carbide tipped scriber to cut PCB copper, either traces or solder blob switches, after using solder wick to remove the solder. No small particles going anywhere. YES, it does take a steady hand and good control.