Making my own Solder Stencils

I ran into some issues soldering the 0402 varactor diodes for the NLTL pulser. Essentially, I’m finding a lot of SMT packages that have pads only on the very bottom. QFN/MLP style packages often have pads that wrap around the sides, making it possible to manually heat up and let solder wick underneath, but there are a number of new packages with contacts only on the underneath - not to mention die attach/thermal pads and we haven’t even started on BGAs.

So what to do? Well, reflow with solder paste is the obvious solution, but manually applying paste, which works ok for 2-lead packages, often fails with this kind of stuff because it’s far too easy to apply too much, leading to invisible bridges and there’s no chance of rework. So…the big boys make stencils. You can get stencils made, IIRC OHS Park among others will do them relatively cheaply, but at that point you’re starting down that slippery slope of sending things off and waiting to get them back, and paying along the way. That slope leads to assembly houses and large minimums, which is not the thing you want when you’re not sure if something’s going to work.

Enter semi-disposable stencils cut in-house. Combined with etching your own PCBs, this means you could design and build a board in one day, assuming you’ve already got the parts. But you’re probably not cutting stainless…

Turns out overhead transparencies are just the right thickness (3-4mil) for most pad footprints, and can be cut with x-acto knives and their motorized equivalent, the craft cutter. This guy did just that a couple years ago with a Cricut. Well, I had a Zing which is a similar cutter from KNK. The software was incompatible, but I knew the Zing would take a PDF - so I just took my Gerber, exported it to PDF, 1:1 and did a test. So far so good, everything lines up. I want to re-do the board to include both NLTLs, amplifiers, and filters to put everything except the OXCO on the same board. Then I plan to cut a stencil, double check everything and bake it. Fingers crossed!

First transparency cut with the Zing