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. 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