Hello, Using the analogy I used with my kids to explain this: If I had a smore made up with 1,000 mini marshmallows then any CTE mismatch between the graham crackers would be absorbed as cyclic strains by the marshmallows and deform no matter what. I understand that the corner marshmallows would be under the greatest strain because of their DNP (distance to neutral point) but I don't understand how the strain would increase with compression. Then again, to make a long story short, I was thinking that lead-free marshmallows had higher bulk mallow / interface failures and the heat sink compression would greatly encourage that. I think I need to get some lead-free marshmallows and do some experiments. Please pardon the humor, I appreciate your time in answering my question !! Thank you, David Tremmel http://ValuRecovery.com -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Werner engelmaier Sent: Monday, December 08, 2008 5:36 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 -----------------------------------------------------