Hello,
When you state that the stresses during cycling are concentrated around the
area where the soldermask comes in contact with the solder joint are you
stating that part of the cyclic creep-fatigue issue is due to inadequate
masking of the pad area? Could differences in the quality of the solder
masks also make a difference?
Thank you in advance for anwering my questions.
David Tremmel
Value Recovery
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Stadem, Richard D.
Sent: Martes, 18 de Marzo de 2008 09:30 a.m.
To: [log in to unmask]
Subject: Re: [TN] Solder Mask Defined Pad BGA Reliability
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
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