Killer PCBs

Step 11 Solder Paste Stencil

If you've gotten this far, you must have pulled a few late nights and consumed copious quantities of carbonated (and caffeinated) beverages. If so, grab one of those soda cans and cut the top and bottom off with a pair of scissors or light shears. Try to cut it neatly without ragged edges. Next cut lengthwise to produce a strip of aluminum about 8" by 4". This strip will be very curly, but you can't "unbend" it without making wrinkles. For the following steps, you can either deal with the curl, or you can anneal the metal by heating it in a toaster oven to about 450 degrees F then let it cool slowly back down. Next you must sand both the inside and outside of the can with 220 to 320 grit wet/dry sandpaper to remove the paint (note that there is clear paint on the inside of the can--otherwise the soda would eat through it). You can use some heavy-duty paint stripper for this, but sandpaper seems a bit greener. Once you've gotten all the paint off, scrub well with SoftScrub with Bleach to remove every trace of oil. Next laminate photoresist on both sides just like you did with the PCB. Make your artwork using the tcream layer as you did before, but don't make negatives this time--the output of Eagle is already a negative! Be sure to make two transparencies each for front and back (and be sure to mirror the front so the toner will be right up against the resist). Develop as before and etch in diluted HCl. I diluted the HCl to about 50% (pour acid into water, not the other way around). Etching will start slowly until the oxide layer has been removed, then speed up significantly. Don't etch too fast, or the board will heat up and the resist will come off. If you leave the resist in place you'll have a ~5 mil stencil, or you can strip the resist to get a ~2 mil stencil--but beware, NaOH will attack the aluminum with just a bit less tenacity than the HCl (depending on concentrations). Next use your failed boards (you should have a few of these by now) to rig up a solder paste jig. Apply the solder paste with a paint scraper, or similar, place your parts, and get on with the reflowing...

Comments

[jeff-o]

Hmmm, I wonder if you could use galvanic (aka electrolysis) etching for this step...

[arirang777]

double WHOA!! I don't really need to do a PCB, but this instructable is pushing me to do it just for the heck of it. Thanks for the clear pics!!

[incoherent]

Hey, thanks for the vote of confidence. It can be surprisingly difficult to get clear macro pics--I still need to make a ring light, but they're working out pretty well just with my wife's stock Nikon D60.