Wednesday, 5 February 2014

Soldering SMT components by hand

A few people have asked about how we soldered up our working board game sections a short while back. Specifically, they asked about our pick-n-place machine and reflow oven (as it was assumed that this is the only way to successfully use SMT components in a homebrew/hobby project).

The truth is, we hand-soldered all of the components on the board.
Although some people have had success with tiny 0806 sized components (we've soldered a few in the past) generally we try to stick to 1206 sizes for resistors and capacitors, SOIC for ICs and microcontrollers and SOT-23 (or SC59) for other silicons; these sizes are perfectly manageable (if not a little fiddly) for soldering by hand.

If you're working with home-made boards with no solder resist layer (as we do) you might find tinning the board with some plating powder helps with getting the solder to flow just where you want it to. We used to do this with every SMT board, but over time, just got into the habit of brightening up the copper with a bit of wire wool and soldering the entire board in one go, before it has time to tarnish.

The first trick with hand-soldering SMT components is flux. Lots of it. Get it all over the pads, inbetween the pads and everywhere. We use a non-clean, non-corrosive flux dispensing pen from Maplin. It's quite expensive, but ours has been going for a few years now and we use flux on everything so it does last a good long while.

Our next secret weapon for hand-soldering tiny components is solder paste. Put your 0.2mm reels of solder and your super-fine iron tips away: a regular pointy iron tip and solder paste is all you need!

You can get solder paste easily from eBay, but be careful you don't end up with a tub of solid flux-jelly (it's often also described as soldering paste). The stuff to look out for is a silvery grey colour - if it's an off-white or orangey-yellow colour, it's flux. Solder paste is a mix of solder and flux combined. You can buy both leaded and lead-free versions of solder paste. For one-offs and prototypes, leaded solder paste is ok. We try to use lead free where possible.

Beware of misleading adverts like this one:

The description clearly states it is lead-free - and then also lists the contents as Sn63/Pb37!
For those of you who never listened during chemistry at school (and don't play along to the science rounds during Pointless on BBC1) Sn is the chemical symbol for tin, and Pb is the chemical symbol for..... lead.
So here's some amazing stuff: lead-free solder paste, with 37% lead. Excellent!

Make sure all the pads (and if you're using a homebrew board, between the traces) get a good coating of flux. If you're using professionally made PCBs, you can apply the solder paste quickly and easily as a "slug-trail" across all the pads

If you're using homebrew boards, it's very easy to create solder-bridges across the tiny gaps between the SMT pads (or even across traces, if you've routed any between the pads of your components. For homebrew boards, we tent to put the paste only where we want it to end up

Here you can see we use a small jewellery screwdriver to deposit tiny dots of paste, just onto the pads we want to solder, working just a few components at a time

You can use a stencil to apply the paste onto just the pads (some people do this, even for professionally made PCBs) but for hand-soldering, it increases the chance of you resting your hand (or arm) into some paste and smearing it across the rest of the board! We tend to work on three or four components at a time.

Using some small nosed tweezers, carefully place the component into the solder paste on the board. This is the really time consuming bit. One twitch of a shaky hand, and you can easily spread the solder paste from the pads onto the surrounding traces (if you do this, remove the component, clean up the paste, reapply flux and start again!)

With the component in place, it's ready to start soldering.

Hold the component down using the tip of the tweezers. This is important. In a reflow oven, all of the solder melts at pretty much the same time, and the surface tension in the liquid solder can realign badly placed components, and they end up perfectly in the middle of the pads. When hand-soldering, we're melting the solder one pad at a time - which will try to pull the component towards the molten solder. Always make sure the component is held firmly before soldering.

With the component in place, and held down by the tweezers, touch the iron between the pads and any surrounding traces. This performs two tasks - firstly it melts the solder and gets the component to stick, but it also clears away any excess solder paste from between the pads/traces. If you've plenty of flux on the board, the excess solder will simply be burnt away.

A regular sized iron tip will be fine, even though the components are relatively small. In fact, using a tiny SMT-specific tip might actually be more difficult - a simple, conical tip that is wide enough to heat both traces as you run it between them is going to be much more efficient than a tiny little tip which can only heat one trace at a time.

For our homebrew boards, we use 0.5mm traces (it helps when etching in "old" ferric chloride or when etching times are prolonged) and occasionally go as low as 0.38mm when running traces between SMT pads. Even with relatively chunky traces, we can use this method to successfully solder 1206 resistors over the top of traces without any bridging.

Once you've done a few small components, it's just a case of getting into a rhythm - but take your time on each one!

With practice, we found that a board like the one above, with 12 x sot-23 and 14 x 1206 resistors can be soldered up by hand in about 20-30 minutes. Obviously this is nowhere near production speeds - but it does prove that SMT components can be used for home-made, one off prototypes.

We've got quite a few boards to solder up over the next few nights, so we're going to be perfecting our hand-soldering technique, using this very method!

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