I learned this technique a good few years ago from an old boy who was a prototype wireman for a large electronics company. I’ve never seen it mentioned on t’Internet so I thought I would write it up as a how-to.
You will need the following:
- Your circuit board – after etching and before drilling*
- Acetone/nail varnish remover
- A large flat bastard-cut file
- A wire brush
- A small tub of brown flux jelly
- A small lump of tin/lead solder
- gloves, paper tissues/clean rags, etc
- A 25W soldering iron with a largeish (say 5mm) chisel tip in good condition
- A clean sheet of white A4 paper
*Note that you can do the drilling before tinning if you wish but then it’s difficult to clean the excess flux out of the holes.
- Acetone and jelly flux are potential skin irritants so wear gloves.
- You’re going to make a fair volume of flux fumes which you should try to avoid breathing in.
Note that if you’re thinking of using a desk fan to blow away the fumes, make sure to keep it away from your soldering iron – otherwise your soldering iron will be cooled (drastically) by the air flow from the fan.
First a little prep work:
1. Using the wire brush clean the debris out of the grooves of the flat file. Bastard-cut files are generally single-cut so you can brush along the grooves to get rid of all the swarf. This is an important first step and shouldn’t be skipped.
2. Next, get yourself a small slug of solder. You can either melt a couple of feet of 18 or 22 SWG cored solder or use a solder bar or (if you’ve been doing some de-solder work lately) you can make a slug of solder out of the debris ejected from your de-solder pump. The slug in the photos is about 15mm in diameter and 4mm thick.
3. You will also need a small tub of brown flux jelly. Apparently this is also called red flux jelly. Anyhow, you can buy it in the UK from Rapid Electronics. Other flux products may do but you should make sure your product is non-corrosive and suitable for light electrical soldering.
4. Switch your soldering iron on so it has time to get nice and hot.
5. Clean the etch-resist off your circuit board. If you used photo-resist, acetone will clean it off. If you used laser toner then elbow grease and an abrasive is the order-of-the-day. Either way finish off with a solvent cleaner such as acetone, isopropyl alcohol or mineralised methylated spirit.
6. As soon as you finish cleaning the board, apply a fairly liberal coating of jelly flux. Don’t wait because otherwise the copper will start to oxidise.
7. Lay down a clean sheet of A4 printer paper on your workbench and place the flux-covered circuit board in the middle. Using the file and the slug of solder “grate” the solder over the circuit board and surrounding area. You are looking for a very light coating of solder filings. Less really is more! Don’t be tempted to create solder swarf by another method and then sprinkle them on – you will overdo it.
8. Make sure the tip of your soldering iron is clean and free of debris (it doesn’t need to be tinned – and it shouldn’t have a huge blob of solder on the end). Hold the soldering iron tip flat to the surface and rub the iron back and forth across the circuit board. As you do so, the tip will pick up the particles of solder and spread them thinly over the board.
9. The trick is to use plenty of flux and get the right amount of solder filings on the board. It’s best to start with too few solder filings because you can always add more (or pick up some of the filings from around the edge of the board with your soldering iron – this is why you have a clean sheet of white paper under your board). If you get the amount of solder just right you will get a very thin layer of solder. Too little solder and the effect doesn’t work; too much solder and, well, too much is too much. Experiment to find the best result.
10. When the board is completely covered with a thin layer of solder, wash off the excess flux with acetone.
11. You’re done!
p.s. Click on the pictures for larger versions.