On Thu, 12 Feb 2015 15:51:48 -0800, mike wrote:
I suspected that one should use the lead-free profiles. ???
Yes, if the oven temp controller had the ability to follow a
temperature profile. I'm not even trying to do that. Buy
coincidence, just shoving in the board into a pre-heated oven, and
letting it cool down seems to follow somewhat of a reasonable ramp,
soak, and cool profile. However, that's pure luck, not intent. My
point is that for reflow (i.e. repair) it does not seem to be that
critical. Once I understood what was happening, I soon discovered
what was the causing most of the reflow failures:
- Too cold during soak.
- Too short a soak time.
Everyone was too worried about destroying parts by over heating, so
they went for the lowest possible temperature and the shortest
possible soak time. That's not going to work as I proved to myself by
trying some of those recommendations. What I found was that I could
soak for quite a long time as long as I didn't get close to the
melting point of the solder paste used to solder the non-BGA parts. If
I could keep the temperature at about 220C, I could leave the board in
the oven nearly forever and it would come out ok. However, there is a
problem. Because I didn't have a proper temperature controller, doing
that would cause the temperature to slowly climb. The result is that
the soak temperature is not constant but rather an increasing ramp. I
can live with that as long as I target the end of the ramp at less
than about 220C. If you want to do it right, go thee unto eBay and
spend about $70 on a programmable oven controller.
I've been afraid to put a whole motherboard in a toaster oven, assuming
I could find one big enough. There's lots of plastic stuff/connectors
etc on the board.
Presumably, you could find a sacrificial motherboard to test. Shove
it into the oven at the highest expected temperature for a reasonable
time and see what melts or burns. Thermoplastic parts will melt.
Thermosetting plastic parts will not. You'll find most connectors
made from thermosetting plastics because the original motherboards are
soldered in a IR oven, which uniformly heats everything to the same
temperature. However, if you want to be sure, LOOSLY wrap the
connectors in aluminum foil. You want some air gap between the foil
and the connector so that air acts as an insulator.
Most toaster ovens have exposed elements.
Really? It thought the UL banned that in the 1950's. My Black &
Decker has two glass tubes with heating elements inside.
You get lots of infrared and
a variable amount of convection heating.
I think you'll find that it's almost all IR with a tiny amount of red
in the visible spectrum from the red glow. Convection implied air
flow. Unless you buy a convection oven, the IR (heat) is very
unequally distributed around the oven. I proved that to myself when I
ran my thermocouple probe around the oven to see how bad it was. My
guess(tm) is about 40C from the hottest near the center to the coldest
near the lower corners.
That scares me too.
Note my domain name: LearnByDestroying.com
You haven't learned anything until you've destroyed it and then had to
make it work.
Thought about
putting reflective foil everywhere but the chip and count on the IR to
do it. But it's completely uncontrolled.
Yep, but as I found, it works well without the proper controls. Sure,
I would like an overpriced oven with the proper profiles, but lacking
the financial justification, I used what I had and it worked. Foil
works if leave an air gap.
My current tooling consists of a bottom heater made from an old TIVO
case. I cut a 2"x2" hole in the top for the BGA chip area to sit on
and drilled a hole in the side for a power-controlled heat gun. Welded some
baffles inside to guide the air from the heat gun to the square hole.
I'll make it simple for you. Hot air does not transfer enough heat by
convection. Close in, radiation is a far more efficient heat transfer
mechanism. Loose the hot air gun, hair dryer, or flame thrower, and
switch to something that resembles an IR source, such as a heater.
Top gets heated with a commercial SMT rework heat gun with precise air
temperature control.
Again, lose the hot air gun. I have one of those. It will work with
a nozzle that is made for the BGA chip. I only have one size (forgot
which one) and that works because *ALL* the hot air lands on the BGA
top. That's not the case with just blowing hot air in the direction
of the BGA. When I tired that, I found that it took me about 30
minutes to heat the BGA top to 220C. I could do the same with the
proper nozzle in about 20 minutes, or with an IR heater in about 10
minutes. (The times are from memory and may be off somewhat).
My air compressor is running wide open and I'm taking all the
air flow I can get.
Your air compressor is cooling the chip. Try the same wattage heater
with slow or fast air. You'll find that the final temperature of the
slow moving air is much higher than the fast air.
I know what package temperatures I want and have some idea from the
thermocouples.
What I don't know is what air temperature I should be using from the
heat guns.
It doesn't matter. Monitor the BGA case temperature and do whatever
it takes with the hot air gun to make it happen.
I was hoping to avoid a bunch of experiments to determine it
empirically.
Man did not discover how to use fire without someone getting their
fingers burned. I could work out the thermodynamic concepts, but with
your uncontrolled setup, it's unlikely to be achievable or
reproducible. It think you could forget about putting your finger in
the fire, but a thermometer or thermocouple would work nicely. One
measurement is worth a whole bunch of guessing.
Would be nice to have a target temperature to start
from.
I gave you that in my previous rant. Somewhere between where the BGA
solder reflows, and where the solder paste starts to soften. My
guess(tm) is between 200C and 225C.
This site is interesting:
https://www.youtube.com/watch?v=c_Qt5CtUlqY
Stencils and solder paste work very will with BGA chips. Good video,
but I wouldn't try that with a 400+ pad BGA chip.
I hava a Maxtra 852D+ rework station.
I have exactly the same soldering station, except it says SAIKE
instead of Maxtra. Not great, but good enough and the price was
right.
It's capable of 550W, but the air flow isn't high enough
to keep the heater duty cycle high. I have very little
actual experience with the unit. And my thermocouples
read
the air temperature 30C lower than the digital readout.
Very small changes in position make radical differences in the
measured temperatures.
Yep. The thermocouple running the display is near the nozzle end in
the hand piece. By the time the air has gone to the nozzle, it has
cooled down quite a bit.
I seek less ambiguity ;-)
There is no one answer because everyone's setup is radically
different. I just want to fix my backlog of dead boards and move on
to something more interesting. Good luck.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558