Jack,
I think you are correct.  The question was a hypothetical - I was trying to better understand the behavior.  I think Werner confirmed that pad to trace separation is far less likely a failure mode when compared to barrel or corner cracks.
Thanks!
Kevin

-----Original Message-----
From: Jack Olson [mailto:[log in to unmask]] 
Sent: Wednesday, October 06, 2010 9:37 PM
To: [log in to unmask]; Glidden, Kevin
Subject: Re: PTH reliability - pad to surface trace

I think you should be careful of the statement that
"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."

Using that logic, a direct-connect to a plane would be worst-case, because 
the whole plane will deflect! (grin) ...but it won't. The copper is more ductile 
than that, so I wouldn't think of pad rotation like a rigid see-saw (where the 
longer the see-saw the higher the damage). In fact, I would think the larger 
pad and connecting trace would tend to keep the copper "tacked down" to the 
material. The more important factor is probably going to be the angle of 
deflection at the knee, right? 

Jack


.
On Tue, 5 Oct 2010 15:02:13 -0400, Glidden, Kevin 
<[log in to unmask]> wrote:

>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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