R.I.P. for D.I.Y.

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Thu Apr 25 22:56:01 2002

On Thu, 25 Apr 2002, Tom Uban wrote:

> >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"?

        No, there is always somewhere to tack solder the frame around the BGA.
I guess if the BGA was at the very edge of the card that might be a problem,
but thats not a likely layout...

> >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...

        I always have the board laying flat, because gravity helps, and it
also keeps the flux from running off... Dont press the BGA down! I did that a
couple of times (its fun because the BGA is "spring loaded" when the solder
balls are molten because of surface tension) -- if you are not really careful,
you will flatten some of the molten solder balls to the point that they merge
with adjacent balls - bad news. The surface tension is sufficient to align and
pull the BGA down.

> >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...

Its not so bad but does require practice, If you need a few practice BGA/cards
just ask...

> --tom

Peter Wallace
Received on Thu Apr 25 2002 - 22:56:01 BST

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