I would have to ask where the stress is coming from.

Is the pad being stressed due to heat? (current through the conductor)
(might be an issue since the transition point can sometimes over etch)
Is the pad being stressed due to bending of the PCB? (I have seen failures
due to this)
Is the pad being stressed due to bending of the component lead inserted into
the PTH? (I have seen failures due to this)
What about a Flex PCB...the pad / conductor width ratio is important. (I
have seen failures due to this)

In general, if you have flexing force either on the PCB and / or component
lead, then match up pad / conductor ratio as best you can.
If there are no considerable flexing forces and amperage (heat dissipation)
is low, then ship it!
Also...when in doubt, teardrop the pads if possible.
Not sure about this "rotation" dealie though
Ken


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Glidden, Kevin
Sent: Tuesday, October 05, 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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