Tal Bejerano - AMC wrote:
>
> well... I thought we do some regular soldering work here..not a NASA
> experiments :-)
> I'm sure there is a simple solution ...


There IS a very easy (and cheap!) way to do this,
which we discovered thanks to Alexandre G and the
temp curve info he so kindly supplied.

We use a typical cheap toaster oven with no fans,
and position the boards about 1" over the
elements, on the standard slide-out rack that
comes with the oven. This solders about a 2" max
"depth" area of the board as it has to be over
the bar element. For larger areas you need a fan
oven which costs a bit more.

The next step is to calibrate the oven dial
to 150'C, I did this by heating a large ceramic
block (a 30W resistor) over the element and
measuring it's temp, found that to get a good
constant 150'C above the element required the
oven dial to be on 160, which was marked with
a pen. This is our pre-heat temp.

If you don't have an IR thermometer ($40 ?) you
can get a 200'C glass lab thermometer ordered from
your pharmacist, probably about $12.

Soldering was incredibly easy. We put the boards
in, set oven to the pre-heat 150'C mark, and
wait 2 minutes with a timer or stopwatch.

Then, crank the oven up full, (elements full on)
and simply watch the solder paste with a bright
light through the oven door. This takes about
15 or so seconds for all the paste to melt and
go fully "wetted".

At this point just open the oven door, turn it
off, and the rack the PCBs are on is *gently*
slid out a few inches to cool.

It gives 100% perfect SMD soldering with the
absolute minimum heat needed and possibly the
quickest cool down and turn-around time.
Boards can be lifted out with pliers and the
cycle repeated within about 20 seconds of the
door opening.

A fan forced oven may be better for larger boards
but might take longer getting from 150'C up to
the wetted heat which might stress components.
For tiny boards a cheap bar-element oven works
very well with no expensive timers, but you do
have to watch it!
:o)
-Roman

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