If I understand it correctly the board is populated sent out to the next higher assembly and mounted in a bent position. If that is the case, perhaps it could be heated in an oven and bent before cool off to the right shape. There will be no stress on it.
Regards,
Ramon
-----Original Message-----
From: TechNet [mailto:[log in to unmask]]On Behalf Of Simms, Michael
Sent: Wednesday, January 12, 2005 10:36 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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