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

From: Peter C. Wallace <pcw_at_mesanet.com>
Date: Fri Apr 26 09:00:15 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"?
> >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...

Forgot that one, yes if you have removed the BGA parts from their sealed
desiccant packed containers for more than a few days, you need to dry them by
baking in an oven at around 150F for a few days. The Epoxy will absorb
moisture from the atmosphere and when heated to soldering temperature, the
steam created may crack the epoxy if not dry (popcorn effect). This applies to
all SMT parts, not just BGAs. We've had troubles with 208 QFP parts that the
assembly house has left out for too long. Problem is worse on large parts.

> >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

Peter Wallace
Mesa Electronics
Received on Fri Apr 26 2002 - 09:00:15 BST

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