Hi Richard,

I know you quoted the spec but I don't understand why you would measure
multiple boards in panel form. It seems to me that if you had an array that
had large tab-route edges and spacers in between the boards that additional
area could give you a bow or twist that exceeds the allowable limits but the
individual boards could be OK.

I'm I missing something here?

KennyB 

-----Original Message-----
From: Stadem, Richard D. [mailto:[log in to unmask]] 
Sent: Tuesday, July 14, 2009 8:50 AM
To: [log in to unmask]
Subject: Re: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability

I understand now what you mean.

IPC-6012B states the following on bow and twist:

3.4.3 Bow and Twist Unless otherwise specified in the
procurement documentation, when designed in accordance
with 5.2.4 of IPC-2221, the printed board shall have a
maximum bow and twist of 0.75% for boards that use surface
mount components and 1.5% for all other boards. Panels
which contain multiple printed boards which are
assembled on the panel and later separated shall be
assessed in panel form.
Bow, twist, or any combination thereof, shall be determined
by physical measurement and percentage calculation
in accordance with IPC-TM-650, Method 2.4.22.



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