IPC-6012B class 3A allows 0.5% Bow and twist. May be you should call out
class 3A on your drawing.
Mahendra Gandhi
Northrop Grumman Space Technology
-----Original Message-----
From: IPC-600-6012 [mailto:[log in to unmask]] On Behalf Of Joseph C
Schmidt
Sent: Wednesday, May 10, 2006 3:21 PM
To: [log in to unmask]
Subject: Re: [IPC-600-6012] IPC-6012B Bow and Twist Requirements
I voice the same opinion as the other to not tighten the spec. If a
design requires a tighter spec, the designer should call it out on the
drawing.
I have always wondered about bow and twist requirements. I used to
work
for a PWB fab shop. When a board failed inspection, we baked the board
with a plate on it to flatten it out. The board stayed flat enough to
pass inspection. There was no guarantee that the stresses would not
cause the board to bow with time, especially if the card required
double-sided SMT assembly or if it sat in a warehouse for a few months.
Joe Schmidt
Raytheon Product Development Engineering P. O. Box 11337 Bldg. 807 MS J6
Tucson, Arizona 85734 Tel (520) 794-4229 Pgr (520) 489-9286 Email
[log in to unmask]
John Perry
<[log in to unmask]
G>
To
Sent by: [log in to unmask]
IPC-600-6012
cc
<IPC-600-6012@IPC
.ORG>
Subject
[IPC-600-6012] IPC-6012B Bow and
Twist Requirements
05/10/2006 12:48
PM
Please respond to
"(Combined Forum
of D-33a and
7-31a
Subcommittees)"
<IPC-600-6012@IPC
.ORG>
Colleagues,
IPC continues to receive queries from industry as to whether or not
there will be a tightening of the bow and twist requirements for printed
boards that are given in both IPC-2221A and IPC-6012B.
I would like the members of this forum to provide their feedback on
these requests.
There are currently two arenas where we are getting requests for
alteration to the requirements: BGA devices and screen printing
technology.
Regarding BGA technology, Don Dupriest of Lockheed Martin Missiles and
Fire Control provided a .ppt file at the IPCWorks 2005 meeting for these
groups where the recommendation for bow and twist acceptance for
isolated BGA areas is 0.50%.
This presentation file is available at
http://www.ipc.org/committeedetail.asp?Committee=D-33A
Within "Drafts", under the titled "PCB Bow and Twist - BGA Devices"
Regarding screen printing technology, we recently received the following
request:
We had a board that was slightly warped, make contact with and damage
the camera in one of our screen printers. Of course, when I measured the
bow and twist, it was well within acceptable limits for bow and twist.
My question is this: Are the bow and twist specifications tight enough?
For over 15 years, I have been "jumped on" by the line operators and
maintenance guys to fixed warped boards only to find that the boards are
usually not any where near the limits. Since I have worked for multiple
companies, I see that this is an industry wide dilemma. Board
manufacturers typically can hold bow and twist requirements, but
different SMA machines (especially screen printers) aren't able to run a
board that has bow or twist near but within the specification limits.
Should I be asking why machines built for the surface mount industry
can't run boards that meet industry specifications, or if the
specifications that once fit, are in need of revision?
Larry D. Roberts
Quality Engineer
Andrew Corporation
Thanks for taking the time to review and respond to these industry
requests.
Regards,
John Perry
Technical Project Manager
IPC
3000 Lakeside Drive # 309S
Bannockburn, IL 60015
[log in to unmask]
1-847-597-2818 (P)
1-847-615-7105 (F)
1-847-615-7100 (Main)
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