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 fine tipped soldering iron, a hot air soldering tool, and ideally a pair of soldering tweezers, but will cost a 3 figure sum. For the occasional 1-off job you may not be able to justify that, but you can do a lot by supplementing the tools you probably already have with just a few more, at a cost of a few pounds only. The following is a minimum:

• Soldering iron with a 1mm bit.
• Thin solder - 22SWG (which is 0.028in or 0.711mm) or thinner.
• A solder flux pen.
• Solder wick.
• A magnifying glass and/or close working glasses.
• Fine tweezers.

Iron

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.

Whilst working, rest your wrist on the bench or other firm support to minimise hand shake, and hold the handle of the iron close to the element, manipulating it with finger movements rather than with your wrist or arm.

Flux

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.

Magnifying glass

Check whether your eyes can focus at 20cm (8in) with reading glasses if worn. If you can't focus that close and if your prescription is simple (if you wear glasses, you have minimal astigmatism or difference between the eyes) then get yourself a cheap pair of "ready specs".

Test the ones on display in many shops, and find a pair that allows you to focus at a distance of 20cm, up to about +3.50D if your far vision is still good without glasses.

However good your eyesight you will still find a magnifying glass invaluable for inspection. The one that is usually included with a "helping hand" will do.

To get a really clear view of the tiniest components a 5x or 10x jeweller's eye loupe is well worth getting. With a little practice you can hold it in your eye socket, leaving your hands free.

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

Types of surface mount components

A Wikipedia article on Surface mount Technology gives much information about devices and procedures.

Resistors and capacitors come in several standard sizes, the size indicated by a code, e.g. 1206 indicates a device 0.12 x0.06 inches. Other common sizes are 0805 and 0603.

ICs come in a variety of packages, not all of which can be soldered or unsoldered without special tools or equipment.

SOIC (small outline integrated circuit)devices are easiest, with a pin spacing of 1.27mm (0.05 inches).

SSOP or TSSOP ([thin] shrink small outline package) devices generally have a pin spacing of 0.65mm (0.025 inches) and are challenging to solder by hand, but still possible with considerable care.

LCC (leadless chip carrier) devices have no leads or leads which fold under the device and can be soldered by allowing the solder to flow up from the pad onto the contact on the side of the device.

QFN (quad flat no-lead) and MLP (micro leadframe package) have pads on the underneath extending to the edges, and whilst it should be possible to solder these it would be hard to tell by inspection that all the pads were properly soldered.

BGA (ball grid array) devices have solder balls in a grid on their underside which are soldered onto pads underneath the device. These can only be removed with a heat gun and it would be extremely challenging to attempt to install a replacement.

Surface mount soldering

With normal soldering you can see and rework any bad joints relatively easily, but with surface mount it's important that all joints are good first time as far as possible. 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. 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. The solder should flow over the leg, joining it to the pad. Continue to hold the device in position for a second or two after removing the iron, and until the solder solidifies.

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. Too much solder will cause a bridge between two pads.

You can remove solder bridges between pins using solder wick. Apply some flux to the wick (it won't work without) and lay it across the bridged pads. Now press it down onto the pads with the soldering iron, and once the solder has melted, draw the wick and the iron together away from the pads with a wiping motion. You may need to repeat the process.

Rework

Removing components

Removing surface mount components is much harder than soldering them to the board in the first place since ideally you would have to heat multiple solder joints at the same time. Lifting a device before the solder has melted risks lifting the copper track from the board, which may be hard to repair.

In the case of resistors and capacitors your standard soldering iron bit is probably wider than the device itself and you can apply the iron to both ends simultaneously and flick it off its pads when the solder melts.

With ICs and connectors, if they are dead or easily replaced the best way is simply to cut off the legs with a craft knife, then remove the amputated feet individually of a few at a time with the soldering iron.

An alternative is to use a heat gun with a small nozzle such as may come with a good surface mount solder station, or a gas soldering iron with the heat gun attachment. But you need to take care not to apply too much heat to adjacent components.

If none of these work, firstly remove as much solder as possible with solder wick. Then gently lift each end of the device in turn with the tip of a craft knife whilst melting the solder on as many legs as possible at that end. Work very gently and patiently to avoid lifting tracks from the board.

Repairing tracks

Minor damage such as broken of lifted tracks can be repaired with care using fine copper wire. A single strand from a piece of stranded flexible wire will do. It's usually easiest to take a strand which is too long and solder one end (don't forget the flux) before bending it to shape and soldering it to the other side of the break. Trim it only after soldering both ends.

External links

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