Maddell is a company we frequently bought products from, they have a
station you can buy for using a hot air gun for BGA rework
http://www.madelltech.com/m3-5.html
They also sell just the PCB holder listed here
http://www.madelltech.com/m3.html
I strongly suggest a controlled environment where you can monitor
termerature and adjust accordingly to a set profile, usually
microcontroller or PC controlled.
The reason I suggest 5 thermocouples is that especially when using a
hot air gun, the middle of the part will heat up faster then the
sides. when this happens the part can buckle and warp, all of the
middle solder balls will reflow, while the outside solder balls will
not. and what happens is that you ramp up the temperature so that the
outsides reflow, and then the middle goes too far down and you have a
short, or shorted solder balls.
Also, the reason for one at each corner, is because of differences in
layers, traces, etc, each corner will heat up differently. what will
happen is one corner will reflow, while another will not be at the
correct temperature yet, then the part becomes slanted, and further
reflowing will cause a short. Or the part will buckle and warp.
You can get away with not wasting a board or a part, by using the
kapton tape to tape the thermal couples to the bottom of the board,
and you can get away with only 3 thermocouples as with experience and
with a good base profile, the middle temp should not be far off from
the corners. And then by starting with a conservative profile, you
can bring the temps up in successive runs until you find the profile
that works. however you can only run a board so many times before it
starts to warp, burn, or peal. that is why preheating the boards is
important, and a proper cooldown is needed.
-Steph
ew_PCBs@yahoogroups.com, "electronut" <electronut@...> wrote:
>
> Steph,
>
> Thanks for the excellent advice. It's really usefull for a hobbist
> like me (or others on this forum) to talk to someone who has done
it
> professionally.
> The suggestion to drill holes in a trial board and put
> thermocouples in them is very interesting and I haven't seen it
> anywhere else.
> I'm thinking of converting a toaster oven to a "reflow oven".
There
> are several sites on the Web that describe how to do that. A
MAX6675
> can convert the temp into a digital signal that can be fed to a
> small microcontroller or to a PC.
> Probably, I'll do just one thermocouple for starters unless you
> really think I should do all 5.
>
> But I hadn't yet considered that fact that the board might warp
> slightly which would stress the BGA. Thanks for pointing that out
> :-).
> So I thought a jig like this might be useful when soldering with
> hot air:
>
http://aoyue.de/en/Aoyue_328_Working_Platform_smd_Rework_ESD_Tool.htm
> What do you think ?
>
> Any other thoughts on the toaster "reflow oven" ?
>
>
>
> --- In Homebrew_PCBs@yahoogroups.com, "Stephanie Ann Thompson"
> <setlahs@> wrote:
> >
> > 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
Show quoted textHide quoted text
> > 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
>