That is probably what we experienced some years ago. FineFocus X-raying
showed what seemed to be voids, but analysis gave that some positions
under the BGAs had a 'white residue' around the balls...and..those balls
were severely corroded. We tried to analyse the white residues, but did
not succeed very well. Concoat told us, that they have registered as
many as 16 chemical compunds called white residues. GEC have found 20.
The most viable explanation was adipid acid from flux inclusions. Water
dropltes from the cleaning process stuck to the white residue, and lead
was solved from the balls. After adjustments of the paste printing and
use of Magic D7L, we could reuse the boards and have them accepted.
Analysis of the balls on new packages and failed ones showed that there
is a zone which is especially sensitive to corrosion, but that's a long
story with many involved.
I'll ask Steve to put up some dirty pictures, that show a fresh ball and
corroded ones.
Ingemar Hernefjord
Ericsson Microwave Systems
-----Ursprungligt meddelande-----
Från: TechNet [mailto:[log in to unmask]] För Dehoyos, Ramon
Skickat: den 18 oktober 2005 15:30
Till: [log in to unmask]
Ämne: Re: [TN] Voids in BGAs, again
Flux contains acids that when fully activated turn into inert
chemicals. It is the acids that when not fully activated that corrode,
so acids will corrode metals whether they are trapped or not. Salts
with moisture and voltage make dendrites. They do need free areas to
grow. In other words salts and resins or rosins will not cause any
trouble when they are trapped. For the most part with a good reflow
profile that activates all the acids of the flux will cause no harm if
trapped in a solder joint. My two non chemistry cents
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard
Sent: Tuesday, October 18, 2005 9:02 AM
To: [log in to unmask]
Subject: Re: [TN] Voids in BGAs, again
Technet,
Just wanted to say, yes, very few failures have occurred due to voiding,
and the situations I pointed out were extreme situations. But the
interposer attachment failures I described have also happened to others
in not very severe conditions. There is something about interposer
attached parts and voids that do not mix well.
Ioan, I may be mistaken, but I was told that flux entrapped inside of
voids is nothing to worry about because corrosion cannot occur due to a
lack of oxygen. The voids are sealed. Any chemical or metallurgical
engineers out there care to respond to this? I would like to know if
that is true.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tempea, Ioan
Sent: Tuesday, October 18, 2005 7:32 AM
To: [log in to unmask]
Subject: Re: [TN] Voids in BGAs, again
Concur with Richard,
good document, professional and academic. But if I am not mistakening,
it does not mention real failures from voiding.
BTW, your experience is very well appreciated, extreme conditions, bad
voids. I am wondering if regular testing, like HALT and the others
caused joints to fail due to voiding.
What about failures due to chemistries, WS flux entrapped into voids
that corrode their way out?
Best regards,
Ioan
> -----Original Message-----
> From: TechNet [SMTP:[log in to unmask]] On Behalf Of Stadem, Richard
> Sent: Tuesday, October 18, 2005 8:14 AM
> To: [log in to unmask]
> Subject: Re: [TN] Voids in BGAs, again
>
> Thank you Steve!
> This is an excellent and informational document. Thank you for posting
> it.
>
>
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