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July 1999

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Subject:
From:
Bev Christian <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 27 Jul 1999 20:40:20 -0500
Content-Type:
text/plain
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text/plain (85 lines)
Darrel,
I can't claim to having seen the situation you describe below, but let me
offer a wild scenario just to get people thinking...

On one or more pads not enough solder paste was put down and/or some of the
balls of the BGA were undersized.  When reflow was finished there was just
the barest mechanical (as opposed to metallurgical ) contact between the now
melted and resolidified solder from the paste on the pad and the BGA ball.
If the current was high enough going through a small enough contact area
this might account for excess heating.  Of course it could be just because
you were looking for something it seemed hot to you, I don't know.  Had you
compared the feel of it to other identical BGAs used in the same circuit on
different boards for a true comparison? Anyway, with this whisper thin
contact any slight temperature change, board torsion, system vibration could
have been making/breaking the connection or, even more unlikely, controlling
current flow and thus giving squirrelly results.  Once you hit it with
freeze spray you shrunk the materials such that the contact area opened up,
electrical flow stopped and perhaps an incredibly  little piece of solder
moved such that on rewarming to room temperature and re-expansion of the
reflowed pad and BGA ball, connection could not be re-established to reheat
the former connecting members.

Anyone else willing to give their crazy (or maybe not so crazy ) scenario?

regards,
Bev Christian
Nortel Networks

> -----Original Message-----
> From: Darrel Therriault [SMTP:[log in to unmask]]
> Sent: Tuesday, July 27, 1999 8:01 PM
> To:   [log in to unmask]
> Subject:      [TN] Freeze Spray and BGAs
>
> TechNet,
>
> A curious situation happened when we used freeze spray on a BGA; it quit
> working.
>
> The BGA seemed a bit hot to the touch and we were getting some flakey
> operation, so
> we decided to hit it with some freeze spray.  It quit working altogether.
>
> I have not had a chance to check out the package for a failure mechanism,
> but
> I will be replacing it to see if it was that part causing the failure.
>
> Does anyone know of any specific thermal issues with using freeze spray on
> BGAs??
> We use both TBGA and PBGA, so I thought I'd ask the question before I go
> and
> inadvertently destroy more of our parts.
>
> Regards......DT
>
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