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How To Solder Surface Mount Devices

Many electronics enthusiasts hesitate to build projects involving surface-mount devices (SMDs) because they're daunted by the prospect of soldering such tiny parts to a PC board. But it can be done. Jim Rowe shows us how...

By Jim Rowe

IT’S TRUE THAT SMDs are not really intended for manual assembly. They’re designed for automated pick-and-place machines and reflow soldering ovens.

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Fig.1: a selection of common SMD components, shown here twice full size (if we showed them normal size they'd be hard to see in some cases!)

The problem is that more and more ICs and other components are becoming available only in SMD form. As technology marches on, it’s becoming necessary for everyone to get used to working with SMDs.

You may already be familiar with the simpler SMDs like resistors, capacitors, diodes and transistors. Some of these are shown in Fig.1. Note that they’re all shown twice actual size, for clarity.

We’ve used these in various projects published in the last few years, and shown how they can be soldered onto a PC board: use a soldering iron with a fine conical or ‘flattened conical’ tip and very fine (0.71mm OD) resin-cored wire solder.

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Close-up view of a 44-lead MQFP device with 0.8mm pitch (lead spacing), after being reflow soldered using a low cost snack oven.

Figs.2 & 3 show how this is done. The basic idea is to hold the chip or device in place while you tack-solder one or two of its leads to hold it in position. This then allows you to solder all of the leads to their pads in the usual way.

It needs to be done carefully and fairly quickly, so you don’t damage either the SMD or the PC pads by overheating. You also need to make sure you don’t apply too much solder, which can cause fine solder "bridges" to short between pads or tracks.

If you do get solder bridges, they can be removed by applying the end of some fine de-soldering braid to the top of the "bridge" and briefly applying the tip of your soldering iron to the top of the braid, so the end of the braid heats up to the solder’s melting point and ‘sucks up’ the excess solder by capillary action.

OK, so what is the real problem with SMDs?

Um, it’s the large SMDs with umpteen dozen closely spaced pins.

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