I understand now what you mean. IPC-6012B states the following on bow and twist: 3.4.3 Bow and Twist Unless otherwise specified in the procurement documentation, when designed in accordance with 5.2.4 of IPC-2221, the printed board shall have a maximum bow and twist of 0.75% for boards that use surface mount components and 1.5% for all other boards. Panels which contain multiple printed boards which are assembled on the panel and later separated shall be assessed in panel form. Bow, twist, or any combination thereof, shall be determined by physical measurement and percentage calculation in accordance with IPC-TM-650, Method 2.4.22. The standard does not discriminate amongst CCAs that have been subjected to any other post reflow process, including HASS, thermal cycling, or rework. I believe the intention is to prevent CCAs that exhibit severe bow and twist (more than .75% per inch for SMT, and 1.5% per inch for PTH) from being used as production goods, especially in a high reliability (Class 3) applications, because of the expected reduction in reliability due to the flexural stress the solder joints would see if the is CCA stressed when clamped in the frame. The fact that some of your CCAs warped, and not all, is probably indicative of a laminate condition during fabrication, not their position in the chamber. The direction of the fiberglass weave can have a very great impact on the amount of bow and twist seen after reflow, or any other thermal excursion. For example, if the PWB fabricator maximized the number of PWBs (or panels) from a single flat by aligning an array 3 across by 5 rows, and had enough material left over in the flat to get two more across the bottom if they were turned 90 degrees, that could possibly explain why you get two or three that warp every so often. I'm not saying to rule the chamber location out, I'm just saying it is more likely to be something related to fabrication. Design and process issues also come into play. They can combine to bring some PWBs "over the fence" from a warpage standpoint. As far as whether you can use the CCAs that do not meet the specification, you need to decide whether they are usable for your particular application. I note you work for Dell, so I am assuming this is a computer motherboard, judging from the size. When your CCA is screwed to the bosses in the bottom of the cabinet, how much deflection takes place? There will be a one-time relaxation in the solder joints, but the amount and severity of that relaxation depends on the amount of warpage and location and the type of components. The initial relaxation or creep could reduce the expected product life by a small amount, but I don't have any documentation that states just how much that would be. I don't think anyone does, because a CCA with a pile of QFNs and LGAs and larger MLCCCs soldered with lead-free on an ENIG PWB is going to be much more of a risk if badly warped than one where most of the components have compliant leads (SOICs and QFPs) and are soldered with SN63/Pb37 to a HASL PWB. I would suggest you contain all CCAs that are warped and document them as non-conforming and investigate the flat layout, prepreg, design, and process factors to find the root cause. Concurrently, you can take the worst-case warped CCAs and screw them into a metal frame that precisely emulates the plastic cabinet. Then thermally cycle to failure those and compare the results with two or more non-warped CCAs in the same frames cycled inside the same chamber. That should give you some data as whether there is any loss of reliability as a result of the flexural stress applied and the basis for disposition. Once a decision is made, either scrap or maintain traceability on the suspect lots in case you need to get them back, either as a recall or by attrition through your repair outlets. The fixtures I described could be designed as reflow fixtures that would hold the PWB planar during reflow, and possibly during any post-reflow cycling as well. -----Original Message----- From: [log in to unmask] [mailto:[log in to unmask]] Sent: Tuesday, July 14, 2009 9:47 AM To: Stadem, Richard D. Subject: RE: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability Post soldered assembly were placed in a Thermal cycle environment in which very noticeable bow/twist was observed after the test. The assemblies did not exhibt signs of bowness prior to the Thermal Cycle event. Only a portion of the assemblies exhibited the bow. Perhaps the area within the chamber where they were located. Does the 1.5% bow/twist criteria apply to these samples? Victor, -----Original Message----- From: Stadem, Richard D. [mailto:[log in to unmask]] Sent: Tuesday, July 14, 2009 9:37 AM To: TechNet E-Mail Forum; Hernandez, Victor G Subject: RE: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability I am not sure what you mean by Post T/C? -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez Sent: Tuesday, July 14, 2009 9:31 AM To: [log in to unmask] Subject: [TN] Maximum Allowable Bow & Twist for Post T/C Reliability Richard, I did not receive any comments on my Post T/C Reliability inquiry so I will post again. What about Bow & Twist on Post reliability testing, T/C? Does the 1.5% still applied for SMT/PTH assemblies? The assembly is a rectangle, 10.5 x 8 inch, form factor. Victor, -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D. Sent: Monday, July 13, 2009 10:07 AM To: [log in to unmask] Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB Bev, I totally agree with you, but the point they are getting at in IPC 2221 is that no section of the PWB should have more than .75% per inch as a rate of warpage, extending in both the X and Y directions, after reflow. This is because if you have more warpage than that you are putting a stress on the resultant solder joints that leads to a reduction in reliability, especially with a lead-free joint and also especially if you need to straighten the CCA when mounting it into a chassis or frame. I think this specification, like so many others that were based on leaded solder, should be revisited for unleaded solders. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian Sent: Friday, July 10, 2009 3:24 PM To: [log in to unmask] Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB I contend this is old school and if you aren't measuring warpage at reflow temperatures how do you know you aren't suffering from head on pillow? I suggest the term head in pillow be dropped because you don't actually put your head IN a pillow. Bev RIM PS Waiting for my Dewism. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Genny Gibbard Sent: Friday, July 10, 2009 4:20 PM To: [log in to unmask] Subject: Re: [TN] Maximum Allowable Bow & Twist for PWB-PCB IPC-2221 5.2.4 ... Unless otherwise specified by master drawing, max bow and twist shall be 0.75% for boards that use surface mount components and 1.5% for all other board technologies. ... Values are measured as per IPC-TM-650 Method 2.4.22 Genny Gibbard, P. Eng Manufacturing Engineer, Vecima PTS dept. x.4229 -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Victor Hernandez Sent: July 10, 2009 2:15 PM To: [log in to unmask] Subject: [TN] Maximum Allowable Bow & Twist for PWB-PCB What is the maximum allowable Bow & Twist value? Not able to locate in IPC-2221, 2222, & 6012A Victor, --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 15.0 To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815 ----------------------------------------------------- --------------------------------------------------------------------- This transmission (including any attachments) may contain confidential information, privileged material (including material protected by the solicitor-client or other applicable privileges), or constitute non-public information. 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