I hate to disagree with you, Wayne. Soldermask-defined pads typically have the soldermask up on the pads. Therefore they reduce the amount of pad area for the solder to form a good IMF because the design seldom allows for a larger pad surface area to compensate for the maskant reduction. They also preclude solder wetting down around the sides of the pads. Having solder totally cover the pads including the sides provides a much stronger (mechanically) solder joint. I do know that on boards with soldermask on top of the pads, there is often a very thin layer of the maskant over the whole pad. I have yet to see that happen with non-soldermask-defined pads. The stresses during cycling are concentrated around the area where the soldermask comes in contact with the solder joint. I have seen the same boards with and without soldermask defined pads temperature cycled and the soldermask-defined boards had daisy-chain failures as much as 50% fewer cycles than non-soldermask defined pads. Same was true of BGAs with soldermask-defined pads versus non. -----Original Message----- From: TechNet [mailto:[log in to unmask]] On Behalf Of Igoshev, Vladimir Sent: Tuesday, March 18, 2008 8:18 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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