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October 2004

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Subject:
From:
"Paulus, Jim" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Paulus, Jim
Date:
Wed, 6 Oct 2004 09:31:29 -0500
Content-Type:
text/plain
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text/plain (137 lines)
After drafting a response with a snotty tone, I took a look at the TM
2.4.22.  And I find that years of my participation on the Test Methods
Subcommittee unfortunately did not include consideration of the
deficiencies of this TM.
So, hopefully my "restructured" comments will help:

You are looking for a quantification of deviation from absolute
flatness. The value will be relative to the size of the subject, whether
it's laminate, bare board, bare board array, or assembly.
If you're testing laminate panels of 18"x24", a .045" deflection of the
panel off a flat surface is not a big deal: less than .2% (.045 divided
by 24" (the dimension that is out-of-flat, usually the longest) times
100).
However, a 4x6" board with a .045" bow figures out to .75%, the max
generally allowed for SMB's.

The procedure is pretty much as per 2.4.22:
- Flat surface
- Feeler gauges to measure max deflection from supporting surface
- Identify whether the non-flat condition is bow or twist
- For twist, hold down 3 corners and measure deflection under the 4th
- Calculate % deflection:
        Bow:  Deflection/(Dimension affected) x100
        Twist:  Deflection/(Dimension from lifted corner to diagonal
corner) x100
- Per Tom G's response, assemblies or even bare boards that have
bottom-side protrusions may have to have the corners set on spacers of
equal thickness.  Then simply subtract the spacer thickness from the
measurement of deflection before calculating %.
Jim Paulus

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Tom Gervascio
Sent: Wednesday, October 06, 2004 8:53 AM
To: [log in to unmask]
Subject: Re: [TN] Twist & Bow

I would suggest taking the desired board and place three 0.125 inch high
spacers at the three corners of the board on a granite table. All three
spacers must be exactly the same height and the height must be known.
The board should now be elevated off the table by a known height (the
spacer height) . Not you can use a surface gage to run along the edges
of the board and determine the variation in height. The actual
calculations for warp and twist are contained in IPC-TM-650 Test Method
2.4.22.1c. It can be downloaded from
http://www.ipc.org/4.0_Knowledge/4.1_Standards/test/2.4.22.1c.pdf

>>> [log in to unmask] 10/06/04 09:35AM >>>
I too placed an inquire to the TechNetters about this Task.   I did not
receive an adequate response.  Good luck

Victor,

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Karnwal, Pankaj
Sent: Wednesday, October 06, 2004 7:14 AM
To: [log in to unmask] 
Subject: [TN] Twist & Bow

Dear Technetters hello,

Any body have the procedure for measuring the twist & bow as IPC 610 B
says
the .75% allowed for SMD populated module.
how we measure this .75 % , in respect to what ?

BR
Pankaj Karnwal
Jr. Engineer QA

Barco Electronics System  (P) Ltd.
mailto:[log in to unmask] 
www.barco.com 



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