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Hi Vladimir!
 
I believe Motorola was after the same issue I'm talking about.  Its not that you are shrinking the solder mask to encroach on the pad, you are growing the pad to encroach under the mask.  Obviously, its not quite that simple, but you can't grow the pad without running into the mask.
 
Wayne

________________________________

From: TechNet on behalf of Igoshev, Vladimir
Sent: Tue 3/18/2008 12:55 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability



I might be wrong but I think I heard a story that the idea of SMD pads
came out from Motorola as they had had a problem with pads being ripped
off during BGA repair. So, to hold the pads down they changed the design
from non-SMD to SMD.

Vladimir

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Thayer, Wayne
Sent: Tuesday, March 18, 2008 12:38 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability

Hi Werner!

SM holding a pad down???!

No!  The pad is held to the laminate material with CONTACT AREA.  Make
the pads as big as you can, then control solder flow with the mask.  And
make sure the mask has nice vertical walls and leaves no contamination
on the pad.

Wayne

________________________________

From: TechNet on behalf of Werner Engelmaier /*
Sent: Tue 3/18/2008 12:32 PM
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability



Hi Wayne,
SMD geometries are stress risers.

If you have a large thermal expansion mismatch, the weakest lik will let
go
first. That may be the SJ or the pad adhesion to the PCB (pad cratering)
or
BGA. Having a SM hold the pad to the BGA is something of a Band-Aid
approach.

Werner



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