Working with solder paste stencils can be a real faff, they rarely sit flat and move around when you so much as breath on them. [Unexpected Maker] airs his frustrations, and comes up with a simple solution, he simply makes a 3D-printed jig to align the PCB panel and applies his shop vacuum cleaner and hey presto!
If you’re ever been tempted to switch from frameless to framed solder stencils, then you’ll notice they can be rather awkward to work with. The usual online vendors have plenty of listings for stencil frame holders, but they do all seem to us, exactly the same, and more suited to stencilling T-shirts, than working with tiny PCB footprints.
The problem with unframed stencils is one of clamping and registration to the PCB, which framed stencils fix, when used with a jig that can dial in the rotation and translation errors.
But problem with those is, unless you have a perfectly flat support region all round the PCB, the weight of the frame tends to make the stencil bow up over the PCB, causing parts of it to lift away from the solder lands. This results in paste not being pushed into the places you want it, and instead it sticks to the stencil apertures and comes away when you lift it up. Most irritating.
You can try offset it by taping spare PCBs of the same thickness all around, but this is not always terribly successful in this scribe’s extensive experience doing this job by hand. [Unexpected Maker] solves this bowing issue by making a 3D printed jig that bolts to the stencil holder, takes a custom top plate with holes in, which in turns allows a vacuum to be applied from below. This sucks the PCB down to the jig, keeping it flat (in case it is also warped) and also pulls the stencil plate directly down to the PCB, making it also lie perfectly flat.
The second benefit of this setup is that you can now use frameless stencils on the same jig, as the vacuum is strong enough to keep it in place, but not so strong you can’t make minor alignment adjustments prior to pasting.
This simple idea helps make a common PCB prototyping task, suck a whole lot less.
Thanks [Foaly] for this sucky tip!