Tuesday, 31 May 2016

Etching PCBs - populating with components

Once you've got your PCB etched, with shiny exposed copper traces and drilled, ready to accept components, strictly speaking, you're done.

While the board manufacturing process is finished, to make a complete working device, you need to populate the board. And there are a couple of tricks we use when soldering to our homebrew boards.

The first thing to remember is that homebrew boards rarely have a solder resist (you can buy iron-on green sheets, but we've found them to be more hassle than is worthwhile - and they can be relatively expensive). So you need to be careful when soldering your boards.

If you've designed your boards according to our guidelines, you'll also likely have bits of exposed copper between traces. This makes it easier to create accidental bridges between tracks and traces, so care should be taken when soldering.

In the past, we've tried pre-tinning our homebrew PCBs. This simply involves creating a solution into which you leave the bare copper board and after an hour or so, the copper is magically (ok, chemically) coated in tin. This process helps protect the tracks (exposed copper tarnishes easily, while tinned copper remains shiny and bright for longer) but rarely have we found the process to be worth the extra cost, hassle and time.

Tinning power is sulphur-based, so it smells a bit like rotten eggs too. So we don't bother with it any more - with the correct soldering approach, it shouldn't be necessary.

The trick to soldering homebrew PCBs is flux. And plenty of it.

A no-clean rosin flux pen might seem a little extravagant at around a tenner a go. But they make applying flux really easy, and last literally years. Well worth the investment. As Steve might say "quality remains once the price has been forgotten". This is one of those times when it's well worth spending a little extra to make things much easier in the long run.

If you're soldering SMT parts, draw a line of flux across all the pads, in one motion - don't bother trying to pick out each pad or pin; a single sweep of flux should be plenty.

Same with solder paste.
You can try soldering each individual pin on an SMT component, but you're more likely to create bridges across the pins, and make a big blobby solder mess of everything! So our technique is to draw a line of solder paste across the pads.

Messing about creating stencils and applying paste to each individual pad can be very time-consuming.

Far easier, to just draw a single line of solder paste across all the pads

If necessary, you can hold things in place with a blob of blue (or white) tack while soldering. We much prefer this approach than using those clumsy "helping hands"
Blue-tac allows you to easily position and re-position components and wire, and angle them to make soldering easier. It can do everything the helping hands do, but without the extra hassle of fiddling about with lots of little wing-nuts and tightening screws.

It's also great for holding components on the board while soldering the pins.

With the component held firmly, and a squiggle of paste across all the pads, the trick is not to heat the legs of the component to make the solder melt and stick to them - more to "rake out" the excess solder from between the legs. In doing this, the iron tip will automatically solder the legs to the pads on the PCB, while - at the same time - removing any excess solder which might otherwise have caused a difficult-to-remove bridge between the two pins.

So there you go.
That's how we go about making up our PCBs.
It's not the "perfect" way of making the "perfect" board. We often cut corners and don't always use the cheapest nor optimal method. But after a good few years of experience, it's a balance between how quickly we want to get a result, and how much in time/materials we're willing to invest to get the result we're after.

Of course, if we want beautiful, professional looking boards, we could invest the time and effort in tinning and/or applying solder resist. We could create stencils for applying the solder paste and put a bit more effort into the final, finished result.

We could use cheaper materials, cheaper transfer paper, cheaper toner, cheaper etchant, a bubble tank to speed up etching, hot air or a skillet for solder reworking, etc. etc.

But to get a working PCB, we've found this combination of approaches gives the best compromise between speed, cost and reliability. Of course, we'd love to hear your experiences too.....