AVR-Chat

--- In AVR-Chat@yahoogroups.com, Kathy Quinlan <kaqdialup@...> wrote:

> ... solder ... a 40 lead QFN with
> centre ground pad ... 6mm Sq with
> 0.5mm pad centres ...

It's not a good idea to heat the PCB from underneath.  Even heat from
above takes the material of the PCB above its glass transition
temperature.  I have had lots of success with a carefully chosen and
slightly modified toaster oven.  I have soldered parts down to 0.5 mm
centres, although not with the pad underneath, but I can't see that
presenting a problem.  I've soldered high-power LEDs where the pad is
completely hidden underneath the body of the part.

I found a toaster oven with a large cavity and four heaters, two above
the food grid and two below, for $30 US in a chain drug store.  There
is no thermostat.  I rewired the bake / toast / broil switch so that
on bake the heaters above and below are in series and on broil the
upper heaters only are on.  I preheat the oven for five minutes on
bake.  Then I put in the PCB(s) and give it another five minutes on
bake.  Then I turn it to broil and watch very carefully through the
glass front.  I have some temperature tell-tale material and I put a
chip of it on the PCB and watch for the colour change.  But, you can
also look for the joints to melt and turn silvery.  After all have
melted, count to ten to make sure, turn the heaters off and open the
door.  If the PCB is not so small as to be in danger of dropping
through the grid, during the last few seconds I tap the sides of the
oven with my fingertips.  This jostles the smaller parts into place,
particularly 0603s, SOT23s and those devilish TinyLogic SC70s.

I think the two heaters on the top are important for all but the
smallest PCBs.  I have recently done a small run of 3.6 inch (90 mm)
square PCBs and, not to boast, they look as if they came off a re-flow
assembly line.  I plan, actually, to move the bottom heaters to the
top too and add a controller, but what I have works well enough that I
don't seem to get around to this.

Kathy, if you can find a toaster oven with two heaters at the top I
think you should try some variation of my method.

Graham.

P.S. I know I'm too far from Kathy to help her out directly, but for
anyone in the US I am not fully loaded right now and have general
embedded systems capacity available.