Homebrew PCBs

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