Mike Young wrote:
"clifto" wrote in message
...

Tim Wescott wrote:

Mike Young wrote:

(Yell if a quick primer on SMT
construction will be useful. I reflow boards in a skillet on the
kitchen
range.)


I am aware of toaster oven reflow, but I haven't seen skillet reflow.
If you have the time and inclination to prepare a little web site --
particularly if you could do pictures -- that would be cool.


What's to figure out? A little olive oil, fresh-ground pepper...

Tune in Thursday when Emeril demonstrates reflow in a boil-in bag,
with pesto.


That's not too far off. We're all somewhat mislead by the assumed
complexity. After all that we've read and thought, it's not easy to
accept on faith that reflowing doesn't have to require carefully
controlled equipment and expensive stencils. For now, consider only that
hand soldering is much more stressful to the parts than almost any other
(mis)treatment. Uneven heating; uncertain temperature rise; solder
bridging and subsequent rework and more heat stress; ... on and on. On
that basis alone, stir-frying is already a huge improvement. At its
best, it isn't very far from a more stringently controlled process.

A few pictures of it working will help make the point. I had every
intention at the onset of taking a few, and plotting temperatures taken
with an IR thermometer. After doing the first board, though, the whole
process seemed as obvious as noting that a rock will indeed drive nails.
Or something like that. Bear with me.

The temperature profile is likely the major concern in your mind. Here's
an example of a profile:
http://www.stencilsunlimited.com/solder_products.php. Click on the link
for "SynTECH Technical Data Sheet". The profile is on page 2.
(Incidentally, their prices are among the lowest I found for small
quantities. Less than $25 for a 35g syringe shipped. I use their 63/37
paste.)

There are two important control points: flux activation at 140^C, and
reflow at 220^C. Flux activation is easily recognizable by the gray crud
that forms on the paste. Reflow is also obvious from visual cues: the
paste turns silvery and looks like molten solder. All that's needed now
is to control the time.

Medium flame on my kitchen range is just about right for my thick
bottomed aluminum skillet. Flux activation "happens" in a little less
than 2 minutes. Reflow starts about 2 minutes later. When the last bit
of paste turns silvery, cover the skillet, count to ten slowly, and then
turn off the flame. Let it cool undisturbed.

The rest of it is putting on the paste and populating the board. You
don't need a stencil to do that. Just draw a neat bead along the pads.
The parts center themselves when it reflows. Compare that to the
contortions of holding an SOT23 in place to solder by hand.

There is a difference in how you plan your work. Since you can reflow
only once without special pastes and finer control, everything has to go
on in the 6 hours the paste remains workable. The NorCal 2030 probably
shouldn't be your first project.

Anyway, stir frying works well and easily with very few critical
details. Try it on a scrap board first to "calibrate" the stove. Within
reason, a little too hot is better than not hot enough, and a little too
much paste is better than not enough.

Do you just sit the board on the bottom of the pan, or do you space it
up? Can you do double-sided boards? What else didn't I ask?

I've seen the web sites showing how to do toaster-oven reflow -- have
you tried that?

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com