I believe Ivan Straznicky from Curtiss Wright in Ottawa published a
study that demonstrated misregistration of the pad to the PTH produced
no discernable reduction in cycles to failure. Ivan presented his finds
at IPC Expo some years ago I believe.  I think he found that well
registered and poorly registered pads produce similar data in thermal
cycle testing.

It is my experience that, although the idea of a pad and trace acting
like a leaver, copper appears to be pretty malleable and it does not
produce significantly reduced cycles to failure.

Material induces pad rotation that causes corners to crack but the
registration of the hole to the pad does not appear to shift results
significantly. 

I offer this as food for thought.

Grid size and the visco-elastic properties (Young's modulus) of the
material are likely the influences that dominate corner cracks. If you
think of the PTH as a rivet the dielectric material stress relieves by
squeezing out between PTHs. The greater the distance between PTH the
easier the material can stress relieve. The more liquid the material is
at test temperature the more it can stress relieve.


Sincerely,

Paul Reid 

Program Coordinator 
PWB Interconnect Solutions Inc. 
235 Stafford Rd., West, Unit 103 
Nepean, Ontario 
Canada, K2H 9C1 
613 596 4244 ext. 229 
Skype paul_reid_pwb 
[log in to unmask] <mailto:[log in to unmask]> 

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: October 5, 2010 3:02 PM
To: [log in to unmask]
Subject: [TN] PTH reliability - pad to surface trace

I've been reading about PTH reliability, mainly through Paul Reid's
articles and old TN posts from Werner....

If you have a PTH with a surface trace attached to the top side pad,
would there be more stress (and thus propensity for cracking /
separation between the pad and joining trace) at the pad / trace
interface area with a smaller or larger pad size?

The "pad rotation" pivots about the knee.  A larger pad dia puts the
pad-to-trace connection further away from the pivot point and thus the
interface area would be subjected to greater deflection.  Alternately, a
smaller pad area (I believe 6012 requires .002" min) would have less
connecting surface area ???

Or is the pad to trace interconnection not a primary failure mode when
compared to barrel or corner cracks?

Thanks!

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