Tensioning a stencil flat - Page 1
Forum section: electronics / general

author: jmw
author_rank: Frequent Contributor
timestamp: June 16, 2020, 05:32:57 pm
content: I was assembling a board this weekend and it was big enough (> 100 mm on a side) that I couldn't rely on the stencil to lay truly flat on its own. Sloppy paste job and lots of rework ensued.Right now my process is use chipboard, same thickness as the PCB, and cut out the board outline, and tape the stencil down. Does anyone have any hacks to get stencils tensioned flat and clamped to the board? I don't do it often enough to justify buying a stencil printer (seems like the price of entry is about $1000 for a manual one, and they take up a lot of space), and framed stencils add big shipping costs. Are there any process improvements I can do that are less expensive?

author: IconicPCB
author_rank: Super Contributor
timestamp: June 16, 2020, 07:39:14 pm
content: WASH the uderside of stencil to improve gasketing

author: Cerebus
author_rank: Super Contributor
timestamp: June 16, 2020, 08:36:00 pm
content: Make a vacuum frame. Make a box 3-4 inches deep, as big or small as you like on top and bottom, any old bit of timber on 4 sides, bit of hardboard on the bottom, bit of perforated hardboard on the top - glue the whole lot together. Drill a hole in the side that will take the hose from your domestic vacuum cleaner, plug it in, turn it on and SUCK. If you go big enough, put a few small full height blocks between the front and back to stop the lower internal pressure bowing the front and back in.You'd be surprised how effective something like this is at clamping something down and pulling things together. Any area not covered with your work obviously needs covering to preserve the vacuum, but a piece of paper is enough. I've used table sized home cobbled vacuum frames like this to print down large bits of photolithography and seen similar used to print down screen printing frames large enough to make the adverts that go on the sides of buses.

author: E-Design
author_rank: Regular Contributor
timestamp: June 16, 2020, 09:43:20 pm
content: Really 100mm ? thats not very big at all! You must be doing something wrong if you had a bad stencil result. Are you saying you dont have a flat stencil? or cant keep it flat while pasting? You said tape the stencil down and I think thats not what you want.Tape down the board and practice holding the stencil in place without letting it move while you wipe the paste... then lift up stencil as vertical as possible for a clean release.I'm sure there are some good videos about the technique. I've done 6" x 10" boards by hand with great results - just practice.

author: jmelson
author_rank: Super Contributor
timestamp: June 16, 2020, 09:55:46 pm
content: Quote from: E-Design on June 16, 2020, 09:43:20 pmRight, I've done up to 200 x 300 mm boards with an unframed stencil, and it worked quite well.Jon

author: SMTech
author_rank: Frequent Contributor
timestamp: June 17, 2020, 10:49:04 am
content: Quote from: jmelson on June 16, 2020, 09:55:46 pm200*300, I'd rather shoot myself, yuk. Although I'd tape before I held it in place, that's crazy talk. 100mm shouldn't present any issues tho'Make up a frame or cut an old one out of its frame, stick card to the edges of your stencil using tape, now use masking tape or other non-stretchy tape and pull it tight on the frame from the corners going from your new card border to the frame.If you can get your freebie stencil to come a decent amount bigger than your PCB then you can get a bit more inventive and mimic some real tension methods. One of our tensioning frames essentially holds the outside edges and then pushes outwards just inside of that, which is in principle fairly simple.

author: jmelson
author_rank: Super Contributor
timestamp: June 17, 2020, 07:03:04 pm
content: Quote from: SMTech on June 17, 2020, 10:49:04 amWell, it was actually a board area of 178 x 280 mm, and probably an active stencil area of 150 x 250 mm or so.I really didn't find it much of a problem. And, now that I have MUCH better solder paste to use, it shouldn't be any problem at all.Jon

author: jmw
author_rank: Frequent Contributor
timestamp: June 17, 2020, 07:20:46 pm
content: The stencil does have a bit of spring to it so that even taping it on all four sides means it's not resting all the way flat. I could probably try some stronger tape (like gaffer tape) to keep it it stretched first.How are people doing the "hold the stencil down in place" idea? I have five fingers to apply pressure and that's not all that uniform...

author: olkipukki
author_rank: Frequent Contributor
timestamp: June 18, 2020, 02:20:02 pm
content: Quote from: jmw on June 16, 2020, 05:32:57 pmHow much are youhappyready to spend to solve about?

author: AlanS
author_rank: Unknown
timestamp: June 28, 2020, 07:38:13 am
content: Quote from: jmw on June 17, 2020, 07:20:46 pmMy boards are 100 x 75mm.* I surround the PCB with off-cast boards from previous projects. I leave one gap so that I can get to one side after stenciling and lift it out.* I get stencils at about 15 x 15 cm from JLCPCB.* I then tape it on the side where I want the most accuracy - using masking tape. This is the end from which I wipe the solder.* I make sure the paste is to temperature at 20-25C and mixed through, this will take time.* I clean each board and let it dry.* I then place the paste on my credit card (all expense spared); and push it down onto the stencil at the preferred end of the board.* Carefully drag the paste at 45 deg. until well past the last hole. Try hard to do this on one pass - it makes a big difference.* Putting in 3 fiducials on the board and having them etched through on the stencil will help loads in making sure your paste goes where it needs to.* When lifting out each board, bring the stencil back in one motion and have something to hold the stencil off the work surface at the very edge so as not to smudge any leftover solder paste.* And then as mentioned above practice. It's easier and cheaper to put down paste and then clean it off than have to do a lot of re-work post solder reflow.Go for it.