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
> 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