If the epoxy is trying to expand in x and y, wouldn't that set up force
against the holes, causing compression?

The drilbert

Leslie O. Connally wrote:
>
> Hi Werner,
>
> Your comments to Chris were Interesting and I agree for the most part, but I
> have a question with regard to the comment about radial barrel compression
> during elevated temperatures. It is my understanding that as the board expands
> in X & Y that the hole will also increase in diameter. Due to the
> disproportionate difference in CTE I would expect that the barrel would be in
> tension and not in compression (a hoop stress). This phenomena has been
> demonstrated in the Physics classroom on many ocassions with Ring and Ball
> apparatus. It's good to see you responding to some of these interesting
> problems showing up on Technet. So what do you think about this analysis?
>
> Les Connally,
> [log in to unmask]
>
> >  From: [log in to unmask], on 8/22/97 5:48 PM:
> >  Hi Chris,
> >  If you get the ITRI Report 'PWB Hole-To-Land Misregistration: Causes and
> >  Reliability, Project Final Report", you may be a little careful in applying
> >  the conclusions. I am not saying that the conclusions are wrong, but that
> >  based on the test performed, the conclusions may be a stretch. From a
> >  reliability point-of-view of the PTH barrel-to-land interconnect, the most
> >  severe loading occurs during excursions to soldering temperatures not
> >  thermal
> >  cycling, even from -65 to +125C. That is because the largest thermal
> >  expansion of the epoxy that causes the radial barrel compression and land
> >  rotations occurs only at temperatures above Tg.
> >
> >  Werner Engelmaier
> >  Engelmaier Associates, Inc.
> >  Electronic Packaging, Interconnection and Reliability Consulting
> >  23 Gunther Street
> >  Mendham, NJ  07945  USA
> >  Phone & Fax: 973-543-2747
> >  E-mail: [log in to unmask]
> >
> >