Kevin is correct, but even if you went with 0.5% for the bare PB that
may not be sufficient. Given a long axis the total deflection when
compressed for mounting will cause a stress strain relationship (tensile
and or compression) that will potentially cause premature solder joint
failure. There are several mitigating scenarios, but the use of a
compressible gasket may be a better solution.
When it comes to Fractured Solder Tales, I oughta know.
Dewey

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Kevin Glidden
Sent: Thursday, March 08, 2007 6:36 AM
To: [log in to unmask]
Subject: Re: [TN] Post Assembly Bow and Twist

0.75& max for an SMT assembly.

Reference IPC-J-STD-001D Section 9.3.

Test method is per IPC-TM-650 method 2.4.22.  You can download this
method
for free at:
 http://tinyurl.com/22trj9


-----Original Message-----
From: McGlaughlin, Jeffrey A [mailto:[log in to unmask]] 
Sent: Thursday, March 08, 2007 7:05 AM
To: [log in to unmask]
Subject: [TN] Post Assembly Bow and Twist

Technet and DesignersCouncil Gurus -

I have been asked by our mechanical engineering group to find out if
there
is a standard or specification for post assembly flatness of printed
circuit
cards.  This has never been an issue for functional failure but a new
design
requires that a row of LEDs mate to a display panel mask such that no
light
leaks from one LED cell to the next. Their concern is that post assembly
bow
and twist will over stress solder joints and board copper if they
mechanically stress the cards by pressing/pulling them flat to an
assembly
frame.  Can anyone direct me to a standard or advise me on the maximum
allowable bow and twist for this type of situation?


Jeffrey McGlaughlin, CID
Engineering Designer
Battelle
505 King Avenue
Columbus Ohio 43201-2693
(614)424-7582 Phone
(614)458-7582 Fax 

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