Homebrew PCBs

Sure thing. :)
The machines I used to use were Summit 1200 and 1800 BGA stations. 
They had a fine adjustment for placement, and also used a prism 
system. Some really cool stuff. The balls were really tiny, like 1/3 
mil or smaller. anything larger became really easy as the solder 
balls were further apart.
The best thing to do before attempting to do a part, is to use a 
dummy board. Basically an exact replica of the board you wanna do, 
minus the parts. for the most accuracy drill holes where the part 
will be placed. one hole at each corner roughly 3-5mm away from the 
edge, and one exactly in the middle. Place a thermalcouple in each 
hole you drilled, and glue it in with high temp epoxy, or use some of 
that high temp tape if you wanna save the thermal couples. sorry i 
forget what it was called, but it was expensive, and looked this 
weird orange color. You will need a temp logger to log all of the 
temps at once, and graph them.
Next look at the manufacturer specs. if they're good, they'll list 
the reflow profile in the device specs
now you're ready to test a profile. it will be a rough profile 
because you won't have a part on the board. The profile will change 
once you add a part. if the manufacturer gave you a profile for the 
board, then start out with that.
Now lets hope the machine you get will have some temperature 
controll, like control over temperature ramping and hold times. I've 
looked around and the cheapest way for that is to use a reflow oven. 
I haven't seen any hot air wands that support temperature profiles.
The best trick for this is to buy an oven and outfit it with a 
controller. That is unless you want to spend 5000$ on a standalone 
station.
Preheating the board is a very important step. Preheating the board 
too long is a waste of time, though rarely will it ever damage the 
board. Preheating it too short then the board won't have enough heat 
soaked in it, and will require the profile to be much hotter. too hot 
and it will scorch the board and warp it.
also, it's best to cycle the board at a low temp, like 150C for 5 -10 
mins, and let it cool a bit or so for 5 mins before starting the 
profile. Thiw will remove water vapor from the layers and prevent 
bubbling/pealing.
Next is ramping the temperature up. instead of shocking the board 
with 400C or so, you want to ramp up the temperature gradually, 
usually a period of 60-120 seconds. Most places will tell you not to 
ramp the board faster then 3-6C a second. Any faster and the board 
will warp.After ramping up, it's good to have another preheat at the 
much higher temp, but still below the reflow point, for about 30 
seconds or so. this lets the board temp stabalize. then finally you 
ramp up the temp again, this time really quickly and up to the reflow 
temperate (actual temp on the sensor should be around 10-15C higher 
the the reflow point of the solder. the reflow time will vary from 30-
60 seconds. this part takes trial and error.
after that bring the temp down, and then ramp the temperature down 
using the same gradian as before, this will prevent warping and 
cracking.

Umm and there you have it. It's better yet if you have a part on the 
dummy board. Preferably a bad part.
If you go the way of using a hot air gun, USE A HOT PLATE to preheat 
the board.
As for placing the part, good luck. you'll need to find a way to do 
it by hand unless you're a mechanical engineer and rig up your own 
pick and place.
before placing the part down, make sure the PCB is really clean. Buy 
some really good flux paste (the one we used came in syringes, was 
like 16$ each, had a high viscosity, was very sticky. This kept the 
part from moving around. The pads need to be pretinned for best 
effectiveness, but have ZERO excess solder on them, it needs to be 
flat or the part will slide down. the amound of flux on them should 
be just enough to get the part to stick. really, not a whole lot 
needed.

Hope this helps.
-Steph
--- In Homebrew_PCBs@yahoogroups.com, "electronut" <electronut@...> 
wrote:
>
>  Stephanie, the parts I'm looking at are about BGAs with 400-500 
> balls.
>  You seem to know a lot more than me about the process. Any tips 
you 
> can share are much appreciated.
> 
>  timbomcnuckle: what king of PCBs have you done with your reflow 
> oven ? What kind of parts have you done ? Have you done BGAs, or 
> 0.5mm TQFP ? Did you do two layers PCBs ?
>  Please share your successes.
> 
> 
> 
> --- In Homebrew_PCBs@yahoogroups.com, "Stephanie Ann Thompson" 
> <setlahs@> wrote:
> >
> > I don't believe a simple hot air gun setup will work for a BGA 
> part 
> > with 500 pads. You need to preheat the board, and use a bottom 
> heater 
> > to ramp up the heat as well as a top heater. And with more 
> products 
> > going Lead Free, it will take a lot more heat and a lot more 
> complex 
> > heating profile to properly reflow the parts.
> > I had worked on a BGA machine before, the big expensive automatic 
> > type... Even then it would take a LONG time to properly create a 
> > reflow profile to get maybe 95% success rate. (of course we were 
> > working with like 1000+ pads) I can give you some tips and 
> pointers, 
> > but for only doing 1-2 a year, it can get pretty expensive 
> > (especially with FPGA's) and frustrating when you fail at 
> reflowing 
> > the part.
> > --- In Homebrew_PCBs@yahoogroups.com, "timbomcnuckle" <jaydag71@> 
> > wrote:
> > >
> > >  hey!, I haven't used a hot-air station yet but I have done the
> > > over-reflow a bunch fo times and I would take the time 
> investment 
> > into
> > > making your own rather than the alternative- especially if you 
> are
> > > only oding a few boards per year! Elektor did a great version 
of 
> the
> > > taoster oven re-flow controlled with an AVR I think, lcd 
display,
> > > programmable curves, precision control (within reason),, I 
think 
> the
> > > only complaint was lack of insulation, but you can always stuff 
> with
> > > rockwool or something.
> > > 
> > >  I have used old style black&decker toaster-oven without any
> > > modification a few times but I got a better one now that I 
still 
> > need
> > > to finish. gluk.
> > >
>