True, David, true, and I'm going to tackle that issue when I get my hands
on the board. I'll have to examine it with X-ray equipment, since one of
the factors I deliberately omitted from my story is that the BGA's are
underfilled and the board is coated. Of course, I could not repair a single
BGA ball manually with underfill in place even if it were possible to, and
had I mentioned the underfill, I would have introduced a confusing
distraction from the question I was asking.
Why did it fail? I don't know for sure that it has yet - and why I don't
know (apart from the supplier telling me), is a whole other story too
gruesome for this tender young forum. I might have to use bad words.
Peter
David Douthit <[log in to unmask]> 05/02/2003 01:17 PM
To: "TechNet E-Mail Forum." <[log in to unmask]>, DUNCAN Peter/Asst Prin Engr/ST
Aero/ST Group@ST Domain
cc:
Subject: Re: [TN] Single BGA ball rework
Peter,
I think the more important question is why did it fail in the first
place!!!
David A. Douthit
Manager
LoCan LLC
[log in to unmask] wrote:
> Dear All,
>
> Thanks to everyone who responded to my last plea for information on
> adhesive for teflon. Tetra-etch is the obvious etchant - I must be
getting
> old not to have thought of it, though the bigger problem is actually to
> separate the still-bonded teflon from its flange in order to clean it up,
> re-etch it and bond it back again. Timing is everything though. and I was
> shown a memorandum yesterday, announcing that the OEM of the component is
> changing the teflon component material to GRP. So maybe we'll buy GRP
> replacement bits and bond them on instead of trying to recover the teflon
> mouldings.
>
> OK. Today's topic will either bring out amused smiles or cries of
outrage.
> One of our suppliers designs boards for us (functional, schematic diagram
> stuff only - though they do basic testing of built boards, once someone
> else has specified the layout design rules the PCB, done all the
> manufacturing stuff, etc.). One board is now failing, and they have
> diagnosed that one ball on the outside row of a BGA is no longer making
> contact with the board. Although only a prototype board, it is a Class 3
> type that could ultimately fly.
>
> For various reasons, the supplier is very reluctant to reflow the entire
> BGA or to replace it in order to solve the contact problem (the BGA is
very
> expensive). Instead, they came up with the idea of manually soldering the
> defective contact. [ I can now sense incredulous reactions from here!].
> They discussed their proposal with a local assembly house, who gave them
> the impression that the idea is possible to carry out (with suitable but
> unspecified equipment). Since then they have been insisting that they be
> allowed to repair the faulty BGA with this method. I used my power of
veto,
> strongly..
>
> I thought it might be a subject to put before learned council here,
though,
> in case anyone else has been thinking of trying this sort of repair. Has
> anyone out there come across any [successful] procedure for - or actually
> tried - manually 'touching up' a single BGA ball? If you have, is the
> procedure approved for anything, what is it approved for (board class,
> specific incidents/circumstances, etc), and where can I get a copy of it?
>
> The BGA in question is a large Xilinx XCV600E FPGA (Field Programmable
Grid
> Array, not a pin grid array), 40mm (1.6 inches) square with 5 rows of
balls
> and an open centre portion. The entire top body surface is covered with a
> metal plate. Soldered standoff height is about 20 mils.
>
> Have fun.
>
> Peter
>
> [This e-mail is confidential and may also be privileged. If you are not
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