Computing stuff tied to the physical world

Soldering a TSSOP chip by hand

Building circuits with small chips may seem daunting at first, but it really isn’t very hard. The trick is to mount those small components on a “breakout PCB” first, after which they can readily be handled, used, and re-used on a breadboard.

Here is the LPC812 in a TSSOP-16 package:

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Those pins are 0.65 mm apart, i.e. four times as dense as those 0.1″ breadboard holes.

It all comes down to using the proper tools, and a bit of patience:

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From left to right:

  • the breakout PCB, which must match the chip itself, of course
  • below it, that tiny LPC812 µC chip
  • next, solder wick – this can be used to remove solder and solder-bridges
  • at the top right: a fine-tipped temperature-controlled soldering iron
  • below: tweezers, preferably bent as shown here
  • lastly: soldering flux – a cleaning fluid in a small marker-style dispenser

For the soldering iron, pick one with a very fine tip – this one has a 0.4 mm round tip, but even the larger tips can be used with a bit of practice. Note that it is not essential that the tip be narrower than the distance between the pins, but it does make things a bit easier.

The main ingredient

That would be the solder, of course!

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Two key details: 1) the solder must have a flux core (leaded solder flows a bit better than unleaded, but the choice is up to you), and 2) use the thinnest diameter you can find, at most 0.5 or 0.6 mm. The reason is that the amount of solder applied is crucial – the thinner the wire, the easier it is to control how much you apply.

A short piece, wound on a small plastic or foam core makes it easy to handle.

Preparations

Make sure you have the above tools and materials ready, on a clean surface (preferably an antistatic mat). That chip can easily be dropped – without clutter, it’s easier to find it again.

Make sure you’re sitting comfortably. This will take 5 to 20 minutes of your concentration.

Apply solder flux

Shake the flux pen, remove the cap, and spread a drop of flux over all the pin contacts. It’ll evaporate when heated, dissolving any oxides and other residues from PCB production.

This can be repeated even midway through the process, if you find that the solder isn’t flowing nicely anymore. Also use a sponge or braid to wipe the soldering tip clean.

First pin

This is the most important step: getting a single pin of the chip soldered, in exactly the right place. Also don’t forget to check the orientation, so pin 1 lines up as intended.

To start, apply a tiny, tiny, TINY amount of solder to the first pad (any corner pad will do):

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Then, holding the chip with the tweezers, melt the solder next to that first pin (pin 16 in this example) and hold the chip steady. Fluidity and capillary forces will do the rest:

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If you can’t clearly see what you’re doing: use better lighting, a magnifying glass, or both!

Alignment

With one pin soldered, it’s still possible to align things. This is essential – the chip has to be placed perfectly on the pads at this point. Using the tweezers, you can bend the fixed pin slightly, if that helps get all the other pins spot on. If the first pin is too far off, re-apply heat to melt the solder, and try again. Here’s what a properly placed chip looks like:

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If you look closely, you’ll see that two pins have already been soldered (accidentally). That’s fine, as long as all the pins are right on top of the pads, all is well.

Do not proceed past this point until things are just right. Corrections later are impossible.

Tacking it down

Now, all we have to do is pick another corner pin. and heat it up with the soldering iron while applying a small amount of solder:

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Here’s the result:

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That’s it. The hard part is over – really.

The remaining pins

It’s easiest (if you’re right-handed), to work upwards, so that the hot soldering iron is slightly above the pad you’re about to solder, heating up only unsoldered pads. That way, all the pins already done won’t risk getting heated up (and messed up) again:

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All the right-side pads are done. With a bit of practice, you’ll notice that you don’t have to apply solder to every pin – there is often enough left on the iron to melt and “reflow” what’s already on the pads. For this to work, the pads and the pins have to be really flat, i.e. not bent in any way.

Oops, too much solder!

But hey, it doesn’t always work out as we’d like:

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No worries, fixing this is easy. Take the solder wick and cut off any old piece: keep in mind that solder wick can only be used once! Then press the fresh wick on top of the pins and put the soldering iron op top to heat it up:

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It’s best to hold the solder wick by its plastic container, as the wick itself will get very hot. What happens next is that the wick will soak up the solder underneath, again due to capillary forces. As a result, those joints will end up almost completely solder-free:

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Then just continue as before, starting again with the same pin, and applying a little less solder this time. Rinse and repeat as needed – most chips can support quite a bit of soldering and de-soldering before causing any problems. But not indefinitely, of course.

The other risk with constant heating, is that at some point the thin copper traces from the PCB can come loose. This can be hard to fix, so don’t overdo it with the heat.

Speaking of heat: set your soldering iron to no more than about 320°C for leaded solder, and 360°C for unleaded solder. Lower is ok, but then it may take a little more time to melt.

Final result

Here’s the second row of pins, all soldered nicely and cleanly:

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It might seem like a lot of work, and initially you’re bound to reach for the solder wick quite a bit, but with some practice you will be able to confidently solder these chips without ever having to fix up any pins or pads.

This was one of the hardest examples by the way. The “SOIC” type chips are somewhat larger, with a 1.27 mm pin distance. There is in fact a version of the LPC812 in an SOIC-20 package, but it seems to have a few less features than the newer TSSOP-16 (or TSSOP-20) package used here. The technique is the same either way.

The payback

There is a very good reason to have a go at it and master this skill: more and more new chips come out only in SOIC, TQFP, TSSOP, or smaller packages. By learning to solder them, you’ll gain access to a huge range of fascinating sensors and microcontroller chips.

It takes some practice and patience – but anyone with a reasonably steady hand and good eyesight (aided or unaided) can do it. Welcome to the world of miniaturisation!

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