In message <[log in to unmask]>, drilbert
<[log in to unmask]> writes
>I thought one advantage of HTE Cu was higher reliability circuits.  This
>is what the inner layer guys tell me where I work. 
>Even though it is difficult to drill, aren't there other problems that
>will occur if HTE Cu is not used?  Can someone point out any technical
>papers that have covered this subject?  Is there independent data that
>shows HTE Cu gives the customer a better product?  

I was not aware that it was any more difficult to drill. Can anyone
explain why?

The reason for HTE foil on inner layers is to prevent cracking due to
the elongation stresses from thermal expansion. Standard foil is more
brittle and will crack at relatively low elongations. HTE foil can
withstand much greater elongation and is equivalent to the plated copper
in the holes. The stress increases the further away from the centre of
the hole you go so it is more important on thicker laminates.

I have seen cracking on inner layers running back into the foil after
thermal shock testing through 5 cycles, but the cracks are difficult to
see. Chances are they would not cause a problem but who wants to take
that chance. If you have fine traces running into holes with minimum
land it could cause failure if someone uses a really hot soldering iron
out in the field.

We used to specify HTE before it became standard. Now all multi-layer
laminates are HTE clad and there is no price difference.

Regards
Paul Gould

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