Buy any BGA built on glass composites and turn it over under a microscope. You will see mask defined pads. Xilinx is the biggest purveyor of "do as I say, not as I do". In their guidelines, they insist, using the logic Vladimir puts forth, that non-mask defined pads are superior. We usually work with small balls. The "weak link in the chain" with these is the adhesion of the pad to the laminate, not cracks in the solder. Kyocera did a pretty decent study on this a few years ago, and concluded that the most robust designs used mask defined pads. The big problems with mask defined pads are mask contamination on the pads and poor solder mask resolution on behalf of most pcb fabricators. If the fab can do a good job on the mask, you will get stronger, more uniform as-reflowed balls. Maybe the big parts work totally different from my small ones, but I have not seen any problems with mask defined pads other than those stated above. Wayne Thayer ________________________________ From: TechNet on behalf of Igoshev, Vladimir Sent: Tue 3/18/2008 9:18 AM To: [log in to unmask] Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability There is a very short answer: Fracture Mechanics :-) When we have an SMD pad, there is an unavoidable stress concentration in the vicinity of the triple junction "solder/solder pad/solder mask". Non-SMD pads are free from that kind of stress concentration, as they are "wrapped up" with solder. Regards, Vladimir -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Kirk, Graham (GE EntSol, Intelligent Platforms) Sent: Tuesday, March 18, 2008 8:06 AM To: [log in to unmask] Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability Hi Joyce, I realize that CTE mismatch is responsible for the fatigue, I am trying to understand why the solder mask has so much effect on the solder joint. Graham Kirk Senior Technologist Radstone Embedded Computing -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Joyce Koo Sent: 18 March 2008 11:07 To: [log in to unmask] Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability Fatique, due to CTE mismatch or poor design. My 1st guess. Jk -------------------------- Sent using BlackBerry ----- Original Message ----- From: TechNet <[log in to unmask]> To: [log in to unmask] <[log in to unmask]> Sent: Tue Mar 18 06:18:08 2008 Subject: [TN] Solder Mask Defined Pad BGA Reliability Hi All, Can anybody tell me what the actual mechanism is that propagates cracks in the device to ball joint on plastic BGA's (Leaded balls) with SMD pads under thermal cycling? Graham Kirk Senior Technologist GE Fanuc Intelligent Platforms Ltd --------------------------------------------------- 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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