Werner Surely "it depends" is too short for a consultant How about something like The significance of the scenario under discussion is dependent on an undetermined number of un-quantified factors whose relevance is yet to be determined. Regards Mike -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner engelmaier Sent: Wednesday, December 10, 2008 7:26 PM To: [log in to unmask] Subject: Re: [TN] Xbox360 BGAs Hi Vladimir, The old consultants' mantra applies: 'It depends.' The damage process in ACT is somewhat different than in product, because the solder grain structure coarsens preferentially in bands in the locii of highest loading; for product, the whole SJ grain structure coarsens because of the much longer time scale. In any case, grain coarsening comes first, micro-voiding and grain boundary intersections next, micro-cracks forming from these micro-voids, growing and coalescing to failure. The first parts of the process are enhanced by higher Ts, but once cracks are formed, crack propagation is faster at lower Ts. Werner -----Original Message----- From: Igoshev, Vladimir <[log in to unmask]> To: [log in to unmask] Sent: Wed, 10 Dec 2008 11:50 am Subject: Re: [TN] Xbox360 BGAs Hi Bob, I want to add to the previous Werner's e-mail. To be exact, solder joints fail because of crack growing. Obviously, there are shouldn't be any cracks in as formed solder joints, so a crack in a joint has to be formed to result in final failure. Overall plastic deformation of solder joints (accumulating creep-fatigue damage, as Werner said) will result in formation of a crack. However, I'm not sure whether the overall plastic deformation or localized processes around the crack tip will play a role in its further propagation. Regards, Vladimir -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Robert Kondner Sent: Tuesday, December 09, 2008 6:57 PM To: [log in to unmask] Subject: Re: [TN] Xbox360 BGAs Hi, Ok, thanks. Dare I ask further but: If the material creeps at normal/high temps that would tend to reduce the stresses, right? Why then would a joint tend to fail on repeated temperature changes, especially going hot? That Xbox photo showed really squished solder balls. At elevated temperature the creep was probably high and the balls squished due to the force of the bottom board warping. (An Al heat sink probably holds the top chip pretty flat.) If balls got that squished to the thickness of paper I would almost expect shorts to form between squished balls. Do we know if the unit failed with opens or shorts. Would placing a rigid metal stiffener under the bottom side (Bottom side heat sink) help reduce warp? Would under fill on a large BGA help do the same thing? Thanks, Bob Kondner -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner engelmaier Sent: Tuesday, December 09, 2008 6:30 PM To: [log in to unmask] Subject: Re: [TN] Xbox360 BGAs Hi Bob, Forget what you know about metals when talking solder---most of it does not apply; we do not use any other metal this close to ts melting temperature. At Th>=0.5 [Th=T[K]/Tm[K]] all metals will creep significantly. Thus, any elastic strains get converted to plastic strains in short [depending on T] order, thus reducing any stresses. So the pre-stressed concrete analogy does not apply. At those T's stresses are not the issue, it is displacement strains. Werner -----Original Message----- From: Robert Kondner <[log in to unmask]> To: 'TechNet E-Mail Forum' <[log in to unmask]>; 'Werner engelmaier' <[log in to unmask]> Sent: Tue, 9 Dec 2008 8:57 am Subject: RE: [TN] Xbox360 BGAs Hi, I have a general question about: Solder joint strains cause by thermal expansion differences. It would seem to me that cooling of a assembly after reflow would introduce built in strains. It seems very much like a pre-stressed concrete beam. It would seem that exposing a device to cold temperature would increase these stresses. Question: Do crystals in a solder ball "Flow" at room temperature? Can a solder ball be annealed with a soak at some temperature? Thanks, Bob Kondner -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner engelmaier Sent: Monday, December 08, 2008 6:36 PM To: [log in to unmask] Subject: Re: [TN] Xbox360 BGAs Hi David, You are somewhat off. Nothing a SJ can do to stop being deformed---whatever the expansion mismatch is will wind up in the SJ. HOWEVER, for the same thermal expansion mismatch, different solder joint heights will result in different cyclic strain ranges [ that is why IBM makes solder columns]. The corner balls seethe highest thermal expansion mismatch because of the largest DNP, and the strain is further increased because of the compression. Werner -----Original Message----- From: David Tremmel <[log in to unmask]> To: [log in to unmask] Sent: Sun, 7 Dec 2008 10:17 am Subject: [TN] Xbox360 BGAs Hello Technetters, Since I have had some people wanting to see pictures, I have resubmitted my email with some corrections. I had someone who got a used Xbox360 that failed and when we disassembled it, we found they had put an aftermarket heat sink on two of the largest BGAs. The original Microsoft heatsinks make contact with the dies (the BGAs are flipchip designs) and have four arms that extend past the corners of the BGA and then have posts which go through the board to a latching piece on the bottom of the PCB directly underneath the BGA which adds tension to make good physical contact with the top of the die and the bottom of the heat sink but does not add sufficient pressure to deform the PCB. The aftermarket heatsinks have posts with spacers above the PCB and washers below the PCB but the spacers above the PCB are not the proper length and after time, the PCB warps upwards and the spacing at the corner of the BGAs was severely compressed to the point where I could not put 2 pieces of paper between the corner of the BGA and the PCB. The solder spheres are 25 mil and are probably being compressed to less than 10 mil. Anywho, this is what I think has the failure mechanism is and I would appreciate some feedback from the gurus: While the chip operates, it generates heat which expands the device to some degree in the X,Y axis Part of the job of the solder spheres is to sink the heat to the logic board so the thermal expansion of the BGA does not act like a sheer force at the solder ball interfaces. The logic board also expands due to the heat and any CTE differences between the BGA and the logic board deform the solder spheres to some degree. The BGA substrate size is 35mm x 35mm and the solder spheres on the corner of the BGA are subject to the greatest sheer forces which, in this case, is detrimental. Due to the compression of the solder spheres at the corners because of the poor heatsink design, they are unable to deform and any CTE difference between the BGA and the logic board are turned into sheer forces and cause ball/interface failures. I also think that the constant pressure of the lead free solder spheres would greatly increase the chances of tin whiskers. I have a picture of just how bad the solder sphere compression is if anyone is interested. Am I way off base? Thank you in advance for any correction in my theory. A confirmation would be better!! Thank you, David Tremmel <http://valurecovery.com/> http://ValuRecovery.com --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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 ----------------------------------------------------- --------------------------------------------------- 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 email, its content, and any files transmitted with it, are intended solely for the addressee(s) and may be legally privileged and/or confidential. 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