Surface mount and micro soldering

Surface mount and micro soldering tools and techniques.

Summary

Modern electronic gadgets of all sorts tend to make use of surface mount techniques, where components are soldered onto the surface of a circuit board, rather than with leads passing through holes in the board and soldered on the other side ("through-hole" construction). Surface mount devices are generally smaller, often much smaller than their through-hole counterparts, making repair much harder. Nevertheless, much is still possible, as described here.

Safety

Take care with a soldering iron - it can give you a nasty burn. Avoid breathing fumes from flux or from any adjacent plastic which might get overheated.

Tools required

For serious surface mount work a proper surface mount solder station is essential. This will include a fin tipped soldering iron, a hot air soldering tool, and ideally a pair of soldering tweezers, and will cost a 3 figure sum. But if you can't justify that, you can do a lot by supplementing the tools you probably already have with just a few more, at a very modest cost. The following is a minimum:

• Soldering iron with a 1mm bit. All but the cheapest irons have interchangeable bits, and you can probably get a 1mm bit for the iron you already have. But it needs to be a small soldering iron which you can manipulate more like a pencil than a crowbar.
• Thin solder - 22SWG (which is 0.028in or 0.711mm) or thinner.
• A solder flux pen.
• Solder braid.
• A magnifying glass and/or close working glasses.
• Fine tweezers.

For normal work, people may tell you that you don't need flux as solder has flux in it. In fact it only has a minimum which is insufficient for surface mount work.

If your eyes can focus at 20cm (8in) then you will still need a magnifying glass for inspection. One that usually comes included with a "helping hand" will do.

If not, get yourself a cheap pair of "ready specs". Test the ones on display in many shops, and find ones that allow you to focus at a distance of 20cm. If you have needed reading glasses with only a simple prescription for some years (minimal astigmatism or difference between the eyes) then a strength of 3.5 dioptres will be about right.

Additionally, if your near vision is no longer as good as it used to be a jeweller's eye loupe is well worth getting. After a little practice holding it in your eye socket you will find it invaluable for close inspection.

The correct way to use any kind of magnifying glass is to hold it as close to your eye as you comfortably can, then position the work so that it's in focus.

Surface mount soldering

With normal soldering you can see any bad joints relatively easily and rework them, but with surface mount it's much more important that all joints are good first time. This is why you can't get away without using flux.

Resistors, capacitors, inductors and diodes are 2-terminal devices, and there will be 2 pads that the ends must be soldered to. ICs (Integrated Circuits) have more legs, with one pad per leg.

Apply flux to both or all the pads, then lightly tin one of them only, using no more solder than necessary. In the case of an IC, choose a corner pad.

Now position the device correctly over the pads. Hold it in place by pressing down on it lightly with the tips of a pair of fine tweezers and apply the soldering iron to the pad, touching the IC leg or the end of the 2-terminal device. Hold it there for a second or so until the solder melts then apply a tiny bit more solder to the leg or the end of the device.

It will now be secured by one end or one leg. Examine it very carefully to ensure the other end or all the other legs are correctly positioned. If an IC is very slightly mispositioned you can just push it, flexing the soldered leg, otherwise reapply the iron and gently reposition as necessary.

Now apply some more flux and solder all the other legs, starting with one at the opposite corner. In each case, apply the iron for a second or so to heat the pad and the leg before applying the tiniest amount of solder.

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