Jeff Liebermann wrote:
On Thu, 07 Jul 2011 09:21:16 -0700, mike wrote:

I bought a Thinkpad X31.

I have one on the bench as I write. Nice machine with a few design
defects. The plastic case is really flimsy and easy to crack along
the sides. I have yet to see an X30, X31, or X32 that does NOT have a
crack in the case somewhere. Plastic epoxy will fix it if you don't
care what the result looks like. Mine resembles Frankenstein:
http://802.11junk.com/jeffl/crud/Thinkpad-X31.jpg
X30 on the left, X31 on the right.

Fails video memory test and the image looks like
it's in 8-bit color mode.
Pushing hard on the motherboard fixes the image.
Lots of googling uncovers many people who've reflowed
the nVidea BGA chip by means varying from oven to hot
air to a blowtorch.

The X31 uses an ATI Radeo Mobile 7000 video chip.
http://www.thinkwiki.org/wiki/Category:X31

Before you blame the video chip, carefully inspect the video ribbon
cable. I've had failures with that cable that looked much like a
video chip problem. They're available on eBay for about $7.

The bad news about this one is that the video BGA
and a RAM BGA are sitting on a carrier that's looks
like it's made out of circuit board material and is also
BGA'd to the mother board.

Yep, that's the problem. Hit it with too much heat and it will melt
or burn (I forgot which). I monitor the heat with an IR thermometer.
Not hugely accurate because it's mostly measuring the temperature of
the hot air gun tip, but good enough. It really doesn't take much
heat or time to reflow the solder with the "skirt" of PCB material
around the BGA. Work fast. This should give you an idea of what to
expect:
http://www.youtube.com/watch?v=NJlgPbELL0E
(Drivel: Not the giant blob of Arctic Silver on top of the CPU to the
right. That's about 100 times too much. Oh well).

Incidentally, I just hot air reflowed and fixed 4 out of 6 HP
JetDirect J4169A cards, which have the same style BGA chip.

I'm not optimistic about getting the heat thru the chips
and the carrier to the motherboard without burning up something.

Rosin flux really helps. It largely prevents oxidation (cold solder
joint). It conducts the heat to odd places. It explodes if you get
it too hot.

In my limited experience, a real hot air desoldering station is a
must. You don't really need all the BGA nozzles as a small 3mm nozzle
is usually sufficient. I bought mine on eBay for about $80 with 4
nozzles. I suggest you practice on an old PCB before trying the real
thing. Getting the temperature right and how fast you move the
nozzle, require some practice.

The thing works well enough to surf the web or control stuff.
I'd hate to brick it. Certainly not worth a replacement motherboard
that has or will soon have the same problem.

It's worth saving.

Anybody got experience reflowing this dual-stack kind of thing?

Yep. My success rate was about 2 out of 5 motherboards with that
configuration. Can't win them all. You have a better chance than
most because you can sorta tell where to reflow.

A few hints:
1. Don't push down on the "skirt" while hot. That will cause the
solder balls to migrate across adjacent pads.
2. Use flux even if it makes a mess. I messy working computer is
better than a clean brick.
3. Build a heat shield out of aluminum foil. The hot air will melt
adjacent plastic quite easily.
4. Give it time to cool down.

Good luck.

Thanks for the inputs.
Flux arrived.
I practiced on a TiVo board. Had to get the air temp over 350C to get
enough heat to desolder a BGA.
I then took a shot at the laptop.
The intermediate board curled up on the corner when I heated it. I
feared I'd lost connection on that corner. Probably shoulda dropped the
temp cause of the thinner intermediate board.

When I tried to run it, I got no video. My heart sank...
I didn't think I needed to plug in the boards for wifi, firewire
and modem, but I took a shot and reassembled the whole thing.
IT WORKS.

Got any suggestions for a blower for a hot air gun.
This Leister is a 600W element and temperature regulator
at the end of a 10-foot air hose. They ran the power wire
down the center of the air hose and it provides a lot of back pressure.
It's designed to run off shop compressed air. My 3/4HP compressor
won't supply as much flow as I like. I haven't found anything that
can give me more volume against the back pressure. I even tried a 14"
blower designed for blow-up advertising devices. No luck. Back pressure
just stalls the impeller and the flow drops to nearly nothing.
Can't really justify a bigger air compressor and a new circuit to run it
for this.

Does it make sense to put fans in series to get better flow under
pressure? I gots lotsa fans.

Ideas?
Thanks,mike