Cleaning up solder

Making a soldered joint invariably leaves some solder exposed in the region of the joint line, and if this is a visible part of the model, it should be removed. The best way to clean up soldered joints is to cure the problem at source. Don’t use more solder than is necessary to make the join. Most of the strength comes from the solder that is between the two parts being soldered. Leaving large fillets of solder outside the join is usually unnecessary for strength. And of course, wherever possible, solder on the inside or the underside where it cannot be seen.

However, there are times when none of these things is completely practical, and it is necessary to clean up the excess solder. It is better not to use needle files because the solder clogs them and it is necessary to stop and clean the file too often, and they do not get into really sharp inside corners. Wet and dry paper avoids the clogging problem, but also does not get into inside corners. Both methods are also liable to mark the underlying material, and the marks then have to be polished out for a really good finish. Fibreglass brushes are great for removing the last traces of solder and providing a very fine finish, but are too slow for removing solder in any quantity.

Solder can be removed using a scraper, which largely overcomes the problems associated with files and abrasives generally. Figure 1 shows two scrapers made from worn out needle files. Grip the blade of the file in a vice, leaving the handle and just a short length of blade showing, and tap it with a hammer to break it off. The remaining teeth on all surfaces of the blade must be removed. An oilstone is better for this than a bench grinder for this. It is slower, but it produces a better finish. The end should also be ground to a chisel shape. Conventional scrapers used for metal (as opposed to solder) removal generally have a rounded edge, but this is not necessary for material as soft as solder.


Figure 1. Scrapers made from old needle files.

Unless the needle file came fitted with a proper handle originally, it will need one, because the effort required to push a scraper is greater than for a file. Handles can be simply pieces of tube of the right dimension. The file handle is secured in the tube with epoxy.

Scrapers like this can be made from any shape of file. Flat, triangular and square files are all useful shapes. Being made of hard steel, they cut soft solder very well and rarely need sharpening. Unfortunately, they also mark the underlying brass or nickel silver (both much softer than steel), and must be used very carefully to avoid unsightly scratch marks that can be hard work to polish out. The brass scraper in Figure 2 solves this problem.

The blade is filed up from a piece of hard brass. Brass is harder than solder so it still cuts the solder away, but not so hard that it marks the underlying parts. The only real disadvantage is that it requires sharpening more often than a steel scraper, but it only needs a few strokes of a file to restore the edge. Figure 2 shows a small, square scraper which is useful for getting in to tight spaces. In this case, the handle is probably more sophisticated than it needs to be, being turned and drilled at one end for the chisel bit, with a locking screw to hold the bit in place. I have made several bits for different purposes, which can be seen in Figure 3. The triangular one, although it takes a bit more making, has proved to be the most useful general-purpose shape.

The round bit shown in Figure 3 intentionally leaves a uniform fillet of solder when used in inside corners. If the prototype component was a casting with fillets rather than sharp corners, this tool reproduces them. Under a coat of paint, the final effect was quite realistic.

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Written by Nick Baines.