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March 2008

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From:
"Thayer, Wayne" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum <[log in to unmask]>, Thayer, Wayne
Date:
Tue, 18 Mar 2008 10:19:52 -0400
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text/plain (185 lines)
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




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