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January 2005

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Subject:
From:
"Dehoyos, Ramon" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Dehoyos, Ramon
Date:
Thu, 13 Jan 2005 13:27:15 -0500
Content-Type:
text/plain
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text/plain (378 lines)
    Hi Graham:
                   The method descried has worked for small flexing. How about using this method with the parts already built. Then send the board through a reflow oven after it has been bent to release stress on the joints. Parts should be perpendicular to the curvature to avoid stressing the part's body.
        Just a thought.
        Ramon
	

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Graham Collins
Sent: Wednesday, January 12, 2005 1:12 PM
To: [log in to unmask]
Subject: Re: [TN] FW: [TN] FW: FW: Flexible vs. rigid substrates


Hi Ramon
OK if you did that and then hand built the board, but I don't think
that is what Michael is talking about - if I'm understanding it
correctly he is talking about bending an assembled board into an arc.

The oven method would bend the board OK, but what about the parts
already soldered to it?  Their solder joints would not be so compliant
at 240F, so you would be leaving residual stresses in the solder joint.
As well you would be stressing the components.

OK for class 1 maybe, but not in my airplane please!!!

regards,

Graham Collins
Process Engineer,
L-3 Communications Electronic Systems Inc.
Halifax
(902) 873-2000 ext 6215

>>> [log in to unmask] 01/12/05 01:54PM >>>
        A regular board can be bowed after it is heated past the
necessary requirement then cooled and it will come back a little. Do not
need to redesign. Since the flexing is done while it is hot no damage is
done. Heat it at 220F for an hour flex it maintain it in the oven at
240F for two hours and slow cool it in the oven.
        Regards,
        Ramon

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Mcmaster, Michael
Sent: Wednesday, January 12, 2005 12:34 PM
To: [log in to unmask]
Subject: Re: [TN] FW: [TN] FW: FW: Flexible vs. rigid substrates


Just curious.  If you bowed the board in to its final configuration
with a
fixture, then assembled wouldn't the stress on the solder joints be
relieved?

Another solution would be to buy a warped board. As all the board fab
contributors out there know, it is relatively easy to design a stackup
that
will warp.  Usually our efforts are directed towards avoiding or
eliminating
this phenomenom, but for once here's an opportunity to allow the laws
of
physics and thermodynamics to work for you.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Simms, Michael
Sent: Wednesday, January 12, 2005 7:36 AM
To: [log in to unmask]
Subject: [TN] FW: [TN] FW: FW: Flexible vs. rigid substrates

Hello Guy,
Thank you for your response.  Our customer's concern is that
the solder joints involved are on a rigid substrate and the substrate
is
flexed to some amount of curvature.  The concern rests on the
presumption
that the solder joints are thus placed under some unknown but constant
stress.  Some of the engineers involved are concerned that no data
may exist for the reliability of solder joints placed under constant
stress.
The solder joints may also be subjected to high temperature excursions
during
their service life.
Some of the engineers reviewing the application have questioned
putting
solder joints
under constant stress during their service life.  The fear is that
under
constant stress,
even without repeated flexing, reliability of the solder joints is
compromised and
their service life unpredictable and unknown.
Wouldn't the use of flex material eliminate the stress?
I was asked to review IPC documents to find what the industry position
may
be regarding flexing of rigid assemblies.
So far, in my review of IPC documents and from some anecdotal responses
from
TechNet,
it appears that the industry may not have addressed the acceptability
of
this type
of application.
Regards,
Mike

Mike Simms
Chemist
Trace Laboratories - Central
1150 W. Euclid Ave.
Palatine, IL  60067

phone  847-934-5300
fax      847-934-4600
www.tracelabs.com

Notice:  This message is confidential and intended for the private use
of
the addressee only.



-----Original Message-----
From: Guy Ramsey [mailto:[log in to unmask]]
Sent: Wednesday, January 12, 2005 8:54 AM
To: TechNet E-Mail Forum; Simms, Michael
Subject: RE: [TN] FW: FW: Flexible vs. rigid substrates


I have know of an application where a thin FR-4 substrate is bent into
a
rather small diameter arc after components are mounted and soldered.
The
assembly is functional and reliable in its service environment. The
substrate does not see repeated flexing.

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Simms, Michael
Sent: Wednesday, January 12, 2005 8:52 AM
To: [log in to unmask]
Subject: [TN] FW: FW: Flexible vs. rigid substrates


Hello Joyce,
You seem to have a misunderstanding of the situation.
We have a customer who is building an assembly on a rigid substrate
and then flexing it.  The board is about the size of a 6-inch ruler.
Each end is deflected about half an inch.
Solder joints at the extremities are failing.  Resistors in those
areas are mounted parallel to curvature.  These joints see maximum
stress.
Although the contract manufacturer has changed the orientation of
these devices (to be perpendicular to the curvature), we don't
understand
why the assemblies are not built on flex material.
There is no IPC document which I can find that mandates the use of
flex material for this type of application.
Am I crazy? Is this an acceptable practice?
Regards,
Mike

Mike Simms
Chemist
Trace Laboratories - Central
1150 W. Euclid Ave.
Palatine, IL  60067

phone  847-934-5300
fax      847-934-4600
www.tracelabs.com


-----Original Message-----
From: Joyce Koo [mailto:[log in to unmask]]
Sent: Tuesday, January 11, 2005 8:10 AM
Subject: Re: FW: Flexible vs. rigid substrates


Michael,
who is the MFG of your flex?  they must have design guide, including
radius
of curvature, slot/hole location at bent, etc.etc.  some of them got
shielding requirements, impedance matching, etc.etc.  performance
driven and
material related (e.g. number of layers, thickness, etc).  if you just
want
to know 180 degree fold flex and design how it be done, many of
cellphone,
LCD got flex folded over, you can measure the thickness, components
mount,
encapsulation, etc.etc. (most of them are well designed), you can form
your
own design guide if you like...
                             jk (my 2 cents.)

-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Simms, Michael
Sent: Monday, January 10, 2005 5:50 PM
To: [log in to unmask]
Subject: [TN] FW: Flexible vs. rigid substrates


Hello T'Netters,
In the absence of any replies to a question that  I believe  many of
you
should have knowledge about, please allow me to re-post:

> Recently, the issue of the use of rigid printed wiring boards in an
> application
> which demands that printed wiring boards be pressed into an outer
shell
> with some
curvature has surfaced.
> I'm accustomed to the use of flexible substrates for any
applications
> which demand
that the board does not stay flat.
> However, I don't see any IPC document which mandates the use of
rigid
> boards
> for 180-degree assembly applications and flex boards (or rigid-flex)
for
> anything else.
>
Should rigid boards be used in applications which demancd that
the assemblies be flexed to some degree?
> Are there any guidelines for the selection of one type over another?
> Are there any reliability issues with the use of surface mount
devices on
> flex boards which don't also apply to rigid boards?
> Why would a manufacturer opt for a rigid board over a flex board for
> assemblies which
> are subsequently flexed?  Is this a choice based on cost?
Thanks,
Regards,
Mike

Mike Simms
Chemist
Trace Laboratories - Central
1150 W. Euclid Ave.
Palatine, IL  60067

phone  847-934-5300
fax      847-934-4600
www.tracelabs.com

Notice:  This message is confidential and intended for the private use
of
the addressee only.

>

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