In the past, when we've experienced minor twisting or bowing, it usually is a
result of internal full planes not evenly spaced within the construction, or
insufficient cure of the multilayer in the press. Usually, if not due to an
internal plane, it can be addressed by reviewing the mfr's cure cycle, or very
low temp TG material being used. It's definitely not something that will happen
on a properly designed/constructed board.
I'd ask for the cure temp/pressure and the material TG used, if you don't
specify it. Try to keep your FR4 above 125 deg. C TG (transistion temperature,
which is the softening state).
In the past, we've put them in an oven at <200 F with weight on top of them to
correct. But have never seen such a severe twist as you describe.
A good reference book is the IPC PE-740, Trouble shooting for printed wiring
manufacture and assembly.
Ernie Crump, Raytheon Systems Canada Ltd.







"Michael R. Forrester" <"Michael_Forrester/NEWYORK/LECROY%LECROY"@LECROY.COM> on
10/11/2000 15:07:06

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
      "Michael_Forrester/NEWYORK/LECROY%LECROY"@LECROY.COM








 To:      [log in to unmask]

 cc:      (bcc: Ernie Crump/WLO/Raytheon/CA)



 Subject: [TN] PCB Warp & Twist and then Flat again?








A batch of PCBs we received twisted during reflow.  We sent a few bare PCBs
from the same batch through the reflow cycle and as expected also
twisted.  The strange thing is that the PCBs are "untwisting" as time goes
on.  The twisted PCBs returned to flat after 20 minutes.  Other boards
twisted
during the board bake-out before assy.  On one board we originally measured
a bow & twist of .450 in. per IPC spec.  One week later it is down to under
.200 in. just sitting on a shelf.  The memory usually is in the twisted
mode not the flat mode.  The boards are 6 layer FR4, cap lamination
(cores on the outer layers) with a well matched construction and are all
from the same date code.  Our PCB house has never seen this before, and has
not explanation for it.  Since we do not have a resolution on the problem I
am concerned we may see it again on another date code.  Has anyone ever
experienced this or have a possible explanation for it?  Are there any
suggestions on what further testing can be done to come to an answer?
Thank you.

Best Regards,

Michael Forrester
Sr. Manufacturing Engineer
LeCroy Corp.

---------------------------------------------------------------------------------

Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
Search previous postings at: www.ipc.org > On-Line Resources & Databases >
E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700
ext.5315
---------------------------------------------------------------------------------

---------------------------------------------------------------------------------
Technet Mail List provided as a free service by IPC using LISTSERV 1.8d
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives
Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional
information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315
---------------------------------------------------------------------------------