New subject: Soldering BGAs, was Re: R.I.P. for D.I.Y.

25 Apr 2002

...
 What I do for BGA protos is this:

1. Solder the BGA(s) first!
This is for a couple of reasons, its easier to inspect the soldering, and if
you mess up the BGA soldering you can start with a fresh board and not have to
...
 remove the BGA part. (Though I have removed and
resoldered BGAs with pretty
good luck)

2. Build a "frame" around the BGA by soldering pieces of scrap circuit board
material on all 4 sides of the BGA, leaving a little (5 mill) clearance on
each side. I do this with pieces of paper as a shim that I later remove. This
is the most critical part, the BGA part must be located in the right position
on the PC by the frame. The 5 mill clearanec on the side is so the BGA can
"float and settle" when the balls melt -surface tension will bring it into
perfect alignment with the PCB pads. I do the frame alignment by setting the
BGA on edge (vertical) against each of my frame pieces and checking the
ball/PCB pad alignment. 
So do you lay out the PCB with areas on the four sides to which you can solder
your "frame"?

...
 3. Put _A LOT_ of rosin core flux on the PC BGA pads.
You dont need any solder
...
 with the common solder ball type BGAs they have plenty
in the balls.

4. Heat from behind with a heat gun. I use a standard $59 Granger heat gun.
This requires some pratice to avoid overheating the board. You can pratice by
soldering small SMT parts to scrap or used PCBs. Moving the heat gun around in
...
 a small circular pattern will help avoid PCB damage.
When the Rosin flux
really begins to smoke, the temperature is just about right. You can see when
the BGA solder balls melt, because the BGA chip will sink about 1/2 mm. When
you see it sink, Your done! 
When you say "behind", I assume you mean the underside of the PCB, not the
top of the BGA? Does the board want to be laying flat so that gravity helps
or does the solder surface tension take care of it? I assume that you don't
press the BGA part down? I've heard that moisture from humidity can cause
problems, sometimes refered to as the "popcorn effect". What can you tell
us about this? I've heard it has to do with component storage...

...
 5. Checking: When the PCB is cool you need to remove
the flux and inspect the
soldering. If the BGA is soldered properly the solder balls will have a
squashed appearance on all 4 sides of the BGA and you should be able see
through all the interball rows in both directions by along the underside of
the BGA. If the balls are not melted enough, you can re-apply flux and re-heat
...
 The worst thing that can happen is that your frame is
mis-aligned so far that
the balls ended up stikinf to 2 pads and have have merged underneath the BGA.
In that case you need to remove the BGA and try again (with a new part)

I have not been 100% successful but am close to 85% good solders first time...

Thanks for the advice. It doesn't sound too bad...

--tom