Tools of the trade
I noticed recently that I’m spending too much time building boards – it seems that my trusty air rework station just can’t work any harder. With ski season just a week away, I was looking at ways to minimize any work-related activity to save energy for skiing. As luck would have it, weather this weekend was extremely unpleasant – perfect time to stay home and upgrade my production to small scale plastic stencil/toaster oven process pioneered by Sparkfun and improved by many others. Below is my experience with it together with some pictures – enjoy!
In addition to typical household equipment – old decommissioned toaster oven, putty knife, paper towel, rubbing alcohol, and pair of good tweezers the process calls for two special items, which would have to be ordered in advance. One of it is a solder paste, another is a stencil to apply solder paste over PCB pads.
A Gerber for a stencil can be made from tCream layer in Eagle. This layer is generated automatically and works as-is with no modifications in most cases. However, if board contains packages with power pads, default footprint needs to be modified. Screaming Circuits has an article about proper QFN power pad solder paste pattern.
The stencil itself can be made from several different materials. Large scale production shops use stainless steel. Stainless steel stencil can be cut very precisely and work for many application cycles; it is, however, expensive. Another popular material is Mylar; it is not very durable but cheap. Kapton is more sturdy than Mylar (NASA used it as a micrometeoroid protection layer in space suits for Moon mission and Skylab project) and cost is roughly the same as Mylar – I prefer this material for my stencils.