Saturday, 25 October 2014

Soldering SMT components using a toaster oven

We recently had some "professionally made" PCBs sent over from
Their service was great - super-fast, made and delivered in under a week - and the website was really impressive, keeping customers up-to-date with every step of the manufacturing process

Pricing was pretty impressive too - a massive 200mmx100mm pcb cost about £8 (when ordered in small volume) and went as low as £2.20 each in bulk. The cheapest we'd been quoted for a fast-turnaround, UK-based PCB was £41 each!

A week later and DHL dropped off a parcel with our ten soon-to-be board game sections

It took a number of days to actually clear some time to do anything with them, and when we did, we thought we'd try using an SMT reflow oven to solder all the components in one go (rather than tacking each one down, one-at-a-time with a soldering iron).

A board with solder mask is much easier to work with than one of our "raw" home-etched boards. It's because of the soldermask we can even contemplate something like this - since any excess solder paste will be drawn to the pads of the components (on a homebrew board, baking it would probably create solder-bridges across the tracks, and nasty burnt board, where the copper has been etched and the exposed paper base remains unsealed).

Placing the solder paste and components by hand (using some horrible chunky tweezers) took about 30 minutes.

Completing this process proved more difficult than we'd first expected. When hand-soldering SMT components with a soldering iron, we tend to work in groups of six or eight components at a time - applying the paste and sticking them down in small clumps. This means that when we come to put more components on the board, we can freely spin it around, and it doesn't matter if we touch - or even lean - on the previously placed components.

With wet solder paste all over the board, it took a lot of effort (and sometimes some re-aligning) to get all the components correctly placed on the board at the same time.

The SMT reflow oven at BuildBrighton is little more than a bog-standard toaster oven, with a controller on the mains plug. The controller ramps up the temperature in a controlled manner, using a rather crude "PWM approach" on the main plug. The oven is set up at full power, full timer, and the controller on the plug literally plugs and unplugs the oven from the mains, to maintain the heating/cooling profile. It's a crude approach, but one that a lot of people are comfortable with, so we went with it too!

It took about eight minutes in the over before the solder started to reflow.
As it did, the hall sensors started to float around on top of the pcb, as the molten solder pulled on each leg, dragging them onto the pads. After leaving for a few minutes to cool down, we took out the pcb to inspect it.

All the hall sensors were soldered well, but the microcontroller had a massive amount of excess solder on it, bridging many of the pins.

Luckily, CNC Paul is quite the whizz with desoldering braid. Quite a few of the BuildBrighton members have said they have little success with the braid, so we were in for a masterclass in how to use it properly, to clean up all the excess solder on this little lot.

True to form, CNC Paul took no time at all, and made a really good job of it!

The whole board just had a quick wipe down with some isopropyl alcohol to clean up any excess flux, and is now ready for programming.

On the whole, we're pretty non-plussed about cooking the boards in a toaster oven. Having to place all the components while carefully avoiding previously placed components is tricky, when all of the solder paste is wet. Our stick-em-down-with-an-iron approach at least ensures that the components are properly stuck down as we go!

To properly solder the microcontroller, we should really have used a stencil to get just the right amount of paste on each pad, rather than a squiggly line of paste across all of the pads.

The nice thing with our soldering-iron approach is that it makes laying down the solder paste on multi-pad components really easy - just place a line of paste, then "rake out" the unwanted paste from between the pads. Using the right conical tip on the iron means that, almost as a by-product of this action, the pads above and below the tip get soldered, while any possible bridging material is removed at the same time.

"Cooking" our board was an interesting experiment, but not really much quicker or easier than hand-soldering with an iron - especially if you consider the amount of "clean up" work required afterwards.
Never mind. It's another technique to have learned, and something we at least know we can do, if we want/need to. We've another 9 boards now to solder up - the only question really is, how?