To me, the previous posting regarding greater number of broken bits with
HTE copper suggests that it is more difficult to drill.  Whether this is
due to higher Z forces through the HTE copper, or the fact that chip
formation in HTE copper is more difficult, I don't really know for
sure.  Possilby it is a combination of the two that leads to higher
frequency of drill breakage.
HTE copper is a material that has higher elongation therefore is a
tougher material.  The drill crush zone requires more thrust through the
copper, and the cutting edge requires a higher strain rate in order to
fracture the chips away from the copper.   A copper with lower
elongation gives up its chips more easily and is more forgiving to the
chisel edge in allowing passage through the copper foil.

Paul Gould wrote:
> 
> 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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