I just completed my first “skillet job.” Inspired by the good work published by the guys here at Sparkfun, I decided to go for it with a Mylar stencil and electric skillet.
My project is a brand new product that includes plenty of 0603’s, .025" fine pitch SSOP’s and even a QFN-16. A 5mm sq chip scale device (gulp!). Doing this with more conventional means, just wasn’t going to cut it, and so…
I went to Target and found a Rival electric griddle that was cheap enough.
http://www.amazon.com/Rival-Electric-Gr … ome-garden
Better yet, the cook surface is separate from the heating element. My hope was that this type of unit would heat more evenly than one with the heater in direct contact with the cooking surface. Although there is still some temperature gradient across the surface, it worked fine and was very flat, so the board made good contact.
I used a .003" thick Mylar stencil and solder paste scrounged from a local board house. It seems that they throw away excess paste instead of returning it to the container, so getting a friend to bring some home from work was easy. I also noted that it had a stiffer consistency than the stuff I got in a syringe from Digikey. This makes me think that the stuff board houses use for stenciling may be formulated differently in order to stay put and not sag. The Mylar stencil is laser cut. This means that there is a melted bead on all the edges where the laser melts through the plastic. This may help seal the edge against the bare pad, so excess material does not squinge under the stencil and spread out–a problem that I’ve heard can happen with metal stencils.
I had total success on my first try! One swipe with a 30 degree rake using a wide putty knife and only moderate pressure was all it took to lay down a smooth, even layer. I carefully peeled back the stencil and proceeded to load the parts on the board.
I then placed the board on the griddle and brought the griddle up to 150C and held it there while the board caught up. A cheap thermopile (non-contact) type thermometer similar to this:
http://www.radioshack.com/product/index … age=search
helped me on an earlier calibration run and to double-check as I went. Then a quick trip up to 200C and presto–my board was ready to serve! The only thing I would change would be to figure out how to get the board cooled down faster than leaving it on the griddle while it cooled at its own very leisurely pace. I would consider a small fan from above (or below) or a spatchelor to ease the board off the surface. Careful! Don’t bump it while still molten!
Here are a few pictures of the solder paste and the soldered result. Sweet!
Note some of the unpopulated 0603 part placements for relative scale.
Best,
Q
http://www.iftrontech.com/images/392_Fi … inch_1.jpg
Above: .025" pitch SSOP part.
http://www.iftrontech.com/images/392_QFN-16_5mm_sq_.jpg
Tiny leadless QFN-16
http://www.iftrontech.com/images/392_Fi … inch_2.jpg
Results speak for themselves! Those are 8 mil lines and 15 mil vias.