Monday, 13 July 2015

Jig for SMT reflow soldering

After we got our PCBs and made a CNC pick-n-place machine from bits of junk, and created a solder-paste stencil from mylar, we were ready to start assembling our PCBs.

Except it's a multi-part operation. And one part was still a bit fiddly. Sure, getting the paste onto the boards was easy enough. And placing the components using our junk machine was quite painless (although quite a bit slower than we originally anticipated, thanks in part to the - relatively slow - speed of the geared scanner motor).

We even went crazy and bought a hot-air rework device to make soldering a bit easier (rather than have to hold each component exactly in place, we can blast it with hot air and let the paste pull the component into the correct position)

All this is fine for our surface mount components, that sit on top of the board. But we've also some hall sensors that not only sit on the board, but project out from the sides. And it's important that we place these so that their sensor parts are about 35mm apart (a few mm either side isn't critical).

Keeping the sensors raised while soldering the legs - particularly with a large hot air nozzle - while also ensuring they stayed the correct distance away from the PCB edge was proving tricky - so we decided to make a jig, to hold the board and the hall sensors during soldering.

In the fullness of time, we'd like to use a CNC mill and create the jig from some aluminium or similar material, but for now a few sheets of laser-cut mdf will suffice - at least to demonstrate whether the idea is worth pursuing or not.

The first - lower - layer contains slots for our PCBs to fit. This is cut from 2mm mdf (the boards are 1.6mm thick) and in the bottom of each slot we put some thick card so that the PCB is raised slightly. This means that the surface of the PCB is perfectly flush with the top of the mdf.

(the small rounded bits at the base of each pcb slot are to allow it to be easily lifted from the jig once soldered)

The top layer includes the pcb slots plus shapes that the hall sensors can be dropped into - ensuring that they are perfectly aligned with the connectors along the edges of each PCB, ready for hot-air soldering.

(a quick dry fit to make sure everything lines up)

Normally we'd just use some double-sided tape to stick our mdf pieces together, but since we're going to be subjecting this to some serious heat, it seemed sensible to use a generous amount of PVA to hold all the parts together. If the design proves successful, we'll probably invest in a slab of (soft) aluminium and have a go at routing the design on our CNC machine.

The final jig in use:

After trying the jig with a single board, we were very quickly soldering our PCBs in batches, up to ten at a time.

In practice, manually placing components with tweezers turned out to be quicker than using our junk-built cnc (and even worked out quicker/easier than manually placing using the vacuum pen). But the jig has proved invaluable for getting the hall sensors lined up and soldered in place quickly and easily.

Now all that remains is to embed them into the mdf panels that make up our electronic board game sections, and connect to one overall master controller. That's going to require a trip to the workshop, so for now, we'll just carry on soldering more of these PCBs up, until we can get down there!