LIB3 is an open source hardware start-up from upstate New York. Thus far, the team has made some interesting products such as the piLED kit. However, they have big dreams for the future. LIB3 plans to become a contract assembly house specifically targeting low volume makers. To do this they have to build their own tools. LIB3’s latest project is a solder paste dispenser for surface mount components. Traditionally solder paste is applied with stencils made of stainless steel. In more recent years laser cut kapton has become a favorite for low volume production.
Both of these systems require a stencil to be made up. LIB3 took a different approach, and modified an old CNC glue dispenser for paste. The team got their hands on an 1991 vintage X/Y glue dispensing system. X/Y systems in this era were big, heavy affairs with powerful motors. LIB3 removed all the control electronics and built their own system from scratch. New features include direct computer control, and a vision system.
When looking for a way to make his own stencils he considered two options: plastic and aluminum. He produced both (more about the plastic stencil and his reflow process is discussed in this post). Plastic is a bit easier to work with since it lays flat. But it proves to be too thick. After applying paste with a squeegee there’s way too much solder on the pads. Aluminum beverage can walls are much thinner, depositing less paste.
We’ve seen soda cans used in the past, but they were produced through an etching process. [Simon] cut these holes using a CNC mill. This required a bit of futzing to figure out the right settings. For instance, he used Altium to produce CAM files from his circuit design. But the program is set up to mill the outside of traces, resulting in openings that are too large. He fixed this by setting the pasted expansion rule in the program to a negative value. The other advantage to using a mill is that he can cut precision tooling holes to ensure proper alignment. You can see them in the upper corners of this image.
There’s some really cool stuff to find if you wander around a Michaels craft shop or Hobby Lobby long enough. Recently, [Ben] picked up a craft cutter – a small vinyl cutter-like device meant for scrapbooking and other crafty endevours. He’s using this machine to create solder paste stencils that are better than any laser cut stencil he’s used before.
Like a build we’ve seen before, [Ben] is using a desktop-sized vinyl cutter, the Sihouette Portrait, with 4 mil Mylar. After converting the relevent layers of a Gerber file into .SVG files, [Ben] loaded up Robocut to cut very, very small holes in his solder stencil. The results are great; much better than a laser would cut Mylar, and good enough to apply paste to a few hundred boards at least.
While [Ben] is using his stencils to apply solder paste, we’re wondering if a similar process could be used to apply a UV-curing solder mask to home-fabbed boards. That would allow for some very professional-looking boards to be produced with a turnaround time of just a few hours.
Even if you’ve overcome your fear diddling about with tiny SMD components, applying solder paste – especially if you’re populating more than one board at a time – is still a chore. The pros use very expensive laser cut stainless steel solder paste stencils, something still a bit out of reach to the casual hobbyist. [Felix] solved this problem by making his own solder paste stencils very cheaply using empty soda cans.
The process begins just like any other home etching tutorial by lightly sanding the un-bent aluminum can and applying the etch resist via the toner transfer method. Etching is done with off-the-shelf HCl and hydrogen peroxide, resulting in an amazingly clean stencil comparable in quality with a professional stencil.
Sure, going through a dozen-step process to make a solder paste stencil may not be as convienent as [Cnlohr]’s toothpick and tweezers method, but [Felix]’ method is just about up to par with extraordinarily expensive laser cut stainless steel stencils. Not bad for something that came from the recycling bin.
In the video after the break [Cnlohr] starts by dispensing a glob of solder pasted from its storage container. He mentions that as long as you store the stuff in the refrigerator it’s rather easy to work with. Because most of his projects are single boards it’s not worth it to have a solder stencil produced. Instead he picks up a bit of the solder glob on the end of a toothpick and applies it to each pad.
This isn’t really as bad as it sounds. The fine pitch TQFP footprints can just be dragged with a bit of the paste. After this application — which took around seven minutes — he grabs some tweezers (not the vacuum type) and begins placing each component. If he missed some paste he’ll discover it in this step and add where necessary. The last step is a trip through his toaster oven.
If you’re making your own boards with SMD parts, you might want to get a solder paste stencil. Usually made of laser-cut mylar or extremely thin steel, these stencils allow you to squeegee solder paste onto your board’s pads and make assembly a whole lot easier. [Rochey] needed a stencil for a board he was working on, and lacking a laser cutter he turned to what he had available – a few bits of plastic and a CNC machine.
[Rochey] began making his stencils out of laminating pouches and an xacto knife. This worked well, but it was time-consuming, and a bit fiddly when cutting 1 mm square holes. To speed up the process, [Rochey] put one of these laminating pouches on his CNC machine, exported the ‘Top Cream’ layer in Eagle to the CNC software of his choice, and had his machine attack the plastic with a 1 mm drill bit.
To [Rochey]’s surprise, everything went as planned; in five minutes, he had a stencil with perfectly accurate holes that masked off everything but the SMD pads.
We’re kind of surprised we haven’t covered this concept before since it only uses techniques that are commonly avaialable for home PCB fabrication. [Ray] made this solder paste stencil out of a sheet of copper using the same etching techniques you would for a circuit board. He designed and printed a resist pattern, with toner everywhere except the places where there should be holes in the stencil. He transferred the toner to the copper using an iron.
The difference here should be obvious; this a thin copper sheet with no substrate. Because of that, you must protect the copper surface before etching. he covered the entire thing, both sides, in packing tape. After that it’s into the Cupric Chloride bath to dissolve the exposed parts. Once the tape and toner has been removed you can scree a precise amount of solder paste onto your boards.
This isn’t for everyone, but if you’re assembling many boards it’s not a bad approach. If the stencil is no longer used it can be recycled, but we do wonder how corrosion on the copper will affect the stencil’s performance.