Can anybody comment on the toaster oven approach?
Is it practical for things like this? How much does a solder mask cost? How
much other stuff do I need? Does the solder paste need to be refrigerated
and other quirks like that?
What are the chances of a newbie getting a 44-QFN right on the first try?
If you have a toaster that you have modified to retain heat better and
has an actual reflow controller, the toaster approach works /really/
well. The toaster I built follows the JEDEC-standard reflow profile
pretty much to the letter, other than taking longer to cool down than it
should (due to not having any sort of fan to move air). So as far as the
solder and components are concerned, it's basically no different than a
decent commercial reflow oven.
If you (or anyone) goes the toaster route, I really recommend getting a
conversion kit -- I used the Controleo2 (from http://whizoo.com/) --
they have a v3 now with a graphical touchscreen. You want the kit
(rather than just the controller) because a normal toaster oven just
leaks too much heat, and won't heat up fast enough, so you need to do a
bunch of other things to one to actually be able to hit your curves.
(They have their whole build guide at http://whizoo.com/reflowoven, so
you can see what you have to do. I built mine over a weekend. I believe
their current kit comes with everything you need but the toaster (their
previous kit required the addition of sealant, a tray, and the
insulation blanket)).
It's basically fire-and-forget -- populate board, stick in oven, press
'start', come back in a bit...
Stencils are pretty cheap (and fast!). I don't remember the exact price
off the top of my head, but I did a not-small board recently (60x110mm),
and the stencil (via OSHStencils) was less than $10, and got to me in
two days, and they're pretty much the same stencils any assembly house
would use.
The main problem with solder paste is that the flux degrades over time
(and you /really/ want your flux). Refrigeration slows that down, but
there's limits. You can, though, buy your solder paste in small
quantities (I think a 15g syringe, which will do a pretty decent number
of boards, is ~$15), so having it eventually go bad isn't that big a
deal. People have reported getting ok results with years-old paste,
though -- I suspect the results will be at least partially dependent on
the details of your board design (how fine-pitch the footprints are, and
such).
Odds of getting a 44-QFN right on the first try are pretty good. Neither
the chip nor the solder paste need be perfectly aligned for things to
work, at least if you're using a PCB with a proper soldermask. At that
pitch, you could probably even just smear some solder paste over the
pads, place the chip, and have success (though I still suggest a
stencil). The biggest problem source (for me, at least) is getting /too
much/ solder paste down (and ending up with bridge pins because there's
just no other place for the solder to go), thus the use of a stencil...
-j
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