Thought I'd share a couple images of my first attempt at making a solder paste stencil.
Images in this album...
I have seen people do this with cutting and straightening old aluminium cans, but since I don't drink much from cans I went and bought some disposable aluminium trays from the local cheap shop, about $1 US each.
I cut out a flat section of the tray large enough and used 800 grit wet sand paper and a spritzer bottle of water to quickly sand one side in case there was some coating, then I cleaned it with methylated sprits and applied the dry film.? For film application I did it exactly as I do for PCB material, use the spritzer water bottle and squeegee the film on then cool iron, I documented this some time ago in my tips n tricks (), since then Big Clive has made some videos showing basically the same process if you youtube it, I think he uses a fancy piezo mister, I just use a dollar store water spritzer bottle.
The artwork was exported as a gerber from DipTrace and run through my script?
to generate a positive PDF (pads opaque) and exposure was exactly as for PCB (22 seconds in my case), developing etc also as for PCB.
The back side of the sheet was covered in clear packing tape and the artwork touched up with etch resist pen.
Etching was done with HCl and H2O2, no specific ratio I just covered the sheet in HCl and added "a dash" of H2O2 from my rather old 35% bottle (which is probably nowhere even remotely 35% any more).
Fuming was copious so as well I was outside.? Etching was fairly rapid, but not as quick or vigorous as I had expected, it was difficult to judge how far through it was, I think next time I would etch it upside down with the resist side at the bottom I think that would make it easier to judge done-ness.? Ad-hoc agitation was provided by manual sloshing of the container.? No time was recorded but I estimate it took about 7-10 minutes.??
On removing the etch which looked good from the top it was evident that a fair undercut had occurred, and that some areas were better than others, because it's undercut and the resist is quite dark, this isn't so apparent from the top side during etching.
Removing the packing tape was quite easy, I had been concerned it might stick too strongly but it doesn't seem to be the case, next time I would be more careful not to have tape wrap over the edge, but still leave a little bit to make grabbing it to peel off easier.
An acetone bath was used to strip the resist.
The result clearly has room for improvement, I'm not sure that reducing the arpertures further than the DipTrace default is necessary bu more careful exposure and and being quicker to remove it from the etch are needed.? I have not attempted to use the stencil (mainly because I don't have any solderpaste on hand currently) but I think at least 50% of the QFN patterns are usable and possibly most of them would work without too much rework after reflow, I think all the passives (mostly 0603) would be fine on all the patterns.
