New solder paste stencils from OSHstencils are awesome
I recently made a couple of small batches of pre-production prototype samples for a customer. I was not convinced laser cut Kapton tape stencils would be long-lived enough to handle hand assembly of multiple units. I was resigned to ordering multiple stencils to cover this. Around this time, much to my convenience, OSHstencils launched low-cost stainless stencils. I decided to give them a try.
And they are good.
The website ordering process was a smooth and simple as ever, and I got notification my stencils were shipped in 24 hours! A week or so later, they landed in the UK. Not bad for USPS transport by land and air! Cost is a little more than Kapton/Polyimide (actually it’s double) but definitely worth it for the confidence. I expect that these will last for dozens of units, and they will survive a little rough handling without stretching out of shape and ruining the alignment.
As you can see from the image, they are frame-less and are intended to be used ‘by hand’ by simply taping them down over the PCB. You can see some smearing on them in the image – that’s just me lazily wiping them over with a soft cloth and IPA (Isopropyl Alcohol.)
When delivered they were perfectly shiny, with good sharp corners. They are still sharp now after 20 or 30 uses.
Prototype PCB Jig
Every PCB is a different shape and size, and for these two prototype build jobs, one was pretty small and rectangular, and the other circular. I made a quick (and not great quality) image of the PCB in a simple laser cut jig:
You can see the PCB jig is just taped to a bit of MDF for handling, and the PCB being pasted is simple taped in place over one corner. It doesn’t take much, but as you might be able to make out, there isn’t too much PCB area to tape over – so one corner it is! It just doesn’t need to slide around when you’re trying to align the stencil.
Small PCBs will hang on to the stencil from below, due to the stickiness of the solder paste, so you need to make sure they’re taped down or you will smear the paste trying to get it off. Ideally, you would have a frame, and a gap between the stencil and the PCB, so the pressure of the squeegee will push the stencil into contact with the PCB as the solder paste passes by, then spring back out of the way, allowing the stencil to be flipped out of the way to remove the PCB at the end of the process. This tape-it-down method is a little harder to control the smearing and PCB to stencil adherence, so a little thought about removal is needed.
Aligning the stencil to the PCB was much easier than Kapton, as it is obviously rigid (whilst still being a little flexible to accommodate a non perfectly planar jig) It was simply a matter of rotating and sliding the stencil into position, then fixing on one side with a single piece of tape. Using a single piece of tape on one side of the stencil, if you ensure it is no wider than the stencil, you can make a rudimentary hinge, which will allow you to flip the stencil up when you’re done.
The PCB is slid in below, and taped in place, then the a second piece of tape is added to to the stencil to lock it in place. Apply solder paste, squeegee, remove the second piece of tape, and in one swift movement, flip the stencil up and swing it over, using the first piece of tape as a hinge. This way, the stencil is removed from the PCB without sliding, and ripping off the paste or smearing it. As long as you keep the temperature in the room below about 22 degrees C, the paste shouldn’t smear too much when you do this, and even small parts will have a reasonably well placed paste blob, which isn’t too thick. You can wipe the stencil over with a IPA-soaked cloth or Q-tip to remove excess solder paste from the underside before you insert the next PCB and repeat. If the alignment moves too much, you’ll need to remove the hinge, and re-align, but for rectangular PCBs, I didn’t need to do this so much, once I found a good place to tape them down to.
I’ll definitely be using these again for future prototype builds and R&D projects!