Late input here. Look at your boards as being incremental elements of the
entire panel. The bow and twist on one board is proably very small, which,
if you were processing individual boards, would not matter. The actual
positional displacement of component locations would be tiny. However, on a
large panel where the displacement has been compounded many times, the
displacement (b/t) becomes significant. Hence the reason why panelled
boards shall be considered as panels and not as individual boards.
Peter
"Marsico, James"
<James.Marsico@D To: [log in to unmask]
P.AIL.COM> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST
Sent by: TechNet Aero/ST Group)
<[log in to unmask] Subject: Re: [TN] Board Bow and Twist
>
03/14/02 09:29
PM
Please respond
to "TechNet
E-Mail Forum.";
Please respond
to "Marsico,
James"
I look at it from a different perspective, reliability. If you populate
(and reflow) warped boards, what happens to the solder joints when the
boards are flattened when installed in the next higher assembly? Now that
I
think about it, probably nothing, since the solder will eventually relax.
Right? But why else would there be a more stringent flatness requirement
for SMT boards than TH since most P&P systems hold SMT boards down by the
edges?
Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879
-----Original Message-----
From: Franklin D Asbell [SMTP:[log in to unmask]]
Sent: Thursday, March 14, 2002 8:25 AM
To: TechNet E-Mail Forum.; Marsico, James
Subject: Re: [TN] Board Bow and Twist
James,
An afterthought, the bow and twist requirement is there to provide
ease of
board population as well as end use fit. If the board is warped
prior to
assembly, this indeed would affect population thus potentially
making the
boards unsuitable for end use...just something to think about, or
pass along
to your supplier.
Franklin
----- Original Message -----
From: "Marsico, James" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Thursday, March 14, 2002 6:48 AM
Subject: [TN] Board Bow and Twist
> I think this is a no-brainer, but here goes... We were delivered
boards
in
> panel form from one of our suppliers. At Incoming Inspection,
the
panel
was
> rejected for excessive bow (>75% for SMT) and returned the
supplier. The
> supplier called me and said that even though the panel exceeded
the bow
> requirement, the individual boards were acceptable. When I
quoted
the bow
> and twist requirement from IPC-6012, "Panels which contain
multiple
printed
> boards which are assembled on the panel and later separated shall
be
> assessed in panel form." , he said yes, you measure the
individual
boards
in
> the panel. I need clarification.
> Thanks,
> Jim Marsico
> Senior Engineer
> Production Engineering
> EDO Electronics Systems Group
> [log in to unmask] <mailto:[log in to unmask]>
> 631-595-5879
>
>
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