Surface Mount Assembly
Let's assemble boards in volume!
Solder Paste Stencils[edit]
The laser cutter can cut Mylar or Kapton sheets into stencils for spreading solder paste onto a board. 4-5 mil thickness is pretty common, but for really fine pitch parts you want a thinner stencil. Adafruit says they use 2 mil Kapton from McMaster but Mylar (3M PP2200 4 mil transparency film) is about 100x cheaper, but melts instead of burns, leaving a slightly worse result, and has the potential for the cut pieces to stick back to themselves.
In KiCAD, you export a DXF of the solder paste layer using the File > Plot tool:
Import that into RDWorks, then do a kerf compensation to shrink all the pads. The finer the pitch component, the more kerf compensation you need in order to leave a reasonable amount of material between two adjacent pads. Adafruit recommends 2 mil.
Either cutting or scanning will work. If you use the scan mode like Adafruit or Felix Rusu, there is no chad left. Juan Chong uses two sheets of Mylar, and only cuts through the top sheet, so that the chads stick to the bottom sheet.
| Material | Thickness | Cut Style | Speed (mm/s) | Scan Interval | Min Power | Max Power |
|---|---|---|---|---|---|---|
| Kapton | 5 mil | Scan | 200 | 0.05mm | 20 | 20 |
Once your stencil is cut, line it up on the board, tape it down and use a credit card or flexible metal paint scraper to spread the paste across the stencil. Read Sparkfun's tutorial. If you're doing multiple boards, you can cut the outline of your board in 1/16" acrylic, line up the stencil and then tape the stencil to the acrylic on one side, so that you don't have to line up the stencil every time.
Pick and Place[edit]
We have a Liteplacer! Roy was working on installing the software. If you're only making a couple boards, placing the components by hand with tweezers is faster.
Infrared Reflow Oven[edit]
We also have two reflow ovens! One of them has aftermarket firmware flashed to it. You can do custom temperature curves!