I would have to agree with the previous posts that are skeptical
of success with straightening at 150deg F. We typically use FR4 boards
here for characterizing semiconductors at 150deg C. It's not recommended
but it's a reasonably inexpensive way for us to accomplish our tasks
using prototype PCB technology. The boards will change shape at that
temperature if mechanical stress occurs and we do occasionally
experience board failures, but not very often.
 
Dan Enslen

The only reason time exists is so
everything doesn't happen all at once.

-----Original Message-----
From: Harry Selfridge [mailto:[EMAIL PROTECTED] 
Sent: Tuesday, July 20, 2004 9:56 AM
To: Protel EDA Forum
Subject: Re: [PEDA] Board warpage?

The important sentence below is "These boards perform an extremely 
important job..."  My vote would be to have the boards remade, and to 
ensure that the unpopulated boards pass inspection before population.

With respect to the existing boards - I'm skeptical that they can be
stress 
relieved at 150deg F.  It appears that whomever recommended the
procedure 
is hoping for some creep in the resin over time to fix the problem.  You

don't mention whether or not the boards are FR4; however, if they are,
the 
temperature suggested is about half of the lowest FR4 Tg normally found
in 
service (120deg C).  It is unlikely that any significant creep will
occur 
so far below the glass transition temperature.

Even if the boards themselves can be relieved (which I doubt), the 
components - and the pads to which they are soldered, will be under
stress 
if the board shape is changed after the components have been 
mounted.  Premature failure is almost certain as a result of component 
and/or joint stress fractures.

The cause of the board distortion should be examined carefully.  Some 
common causes are asymmetric stackup, improper lamination cooldown, 
improper reflow or wave soldering heating/cooling programs, and
mechanical 
stress when mounting large or heavy components.  Any one of the above,
or a 
combination, can cause the board to warp because of internal stresses.
A 
properly designed, properly laminated, and properly handled board
doesn't warp.


At 08:40 AM 7/20/04, you wrote:
>This email is not a Protel-specific question but instead concerns a
>generic problem with some boards I'm getting, so I apologize for being
>off-topic, but this forum has more PCB experience than any other I
>know of, so I would like to get your advice.
>
>A partner in a manufacturing effort is sending us full populated
>boards (ICs and passive components) that are badly warped.  We sent
>them back and rather than get the boards remade and repopulate them,
>they want to heat the boards up (fully populated) for three days at
>150 degrees Fahrenheit in order to straighten them out.  These boards
>perform an extremely important job and I'm worried about the stress
>on both the electrical components and the boards themselves.  Should
>I allow them to do this or should I demand the boards be remade?
>
>Thank you, Michael Robison
snip 







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