I've struggled with this, too!  The % is the amount of change over the
temperature range of the test.  Fortunately, Nelco includes the range of
the test in its literature (not all suppliers do this, by the way).  For
3.5% over a range of  50 to 288C here's the conversion I've used:
1,000,000*(0.035)/233 = 147ppm/C

One thing that I've found that makes using Z-axis CTE data difficult to
use is the inconsistency of reported data.  There appears to be no
standard for reporting the values.  For instance Merix builds boards
with FR 4 type materials from 3 different suppliers.  One reports a
Z-axis CTE of about 70ppm the other two are at 130 ppm.  But is the
lower material really better?  It shouldn't be since the 70 ppm value is
for a material with a Tg of about 130C and the "higher" CTE materials
have Tg over 170C.  So why the difference?  The lower reported value is
for a temperature range that does not include the Tg transition.
Testing all three materials over the same temperature range shows that
the low Tg material has a CTE of 155ppm, not 70.  It's easy to see how
the data can lead to incorrectly selecting the appropriate materials for
an application.  When comparing values, it's imperative that you know
the units (ppm or %) as well as the temperature range of the test.

It would be nice to see some standardization of how these results are
reported.  My assumption is that two values are important to the end
users of the materials.

1)      How much will the material expand during assembly?
2)      How much will the material expand during operation in the field?

I know there are a wide variety of temperature ranges for both items
that make settling on a single range for each difficult.  At least all
the data would represent the same thing and allow somewhat intelligent
comparisons.

By the way IPC-TM-650 Method 2.4.24 (Glass transition temperature and
Z-axis thermal expansion by TMA) does take a stab at this, recommending
that any test be done to at least 30C above the material Tg.  This
appears to introduce some standardization until you look at the
different materials that I use in production.  The Tg for PTFE-based
materials is about 25C while Rogers 4000 series materials have Tg's
above 250C.



        ----------
        From:  PAUL BROWN [SMTP:[log in to unmask]]
        Sent:  Tuesday, September 07, 1999 11:08 AM
        To:  [log in to unmask]
        Subject:  [TN] Woh Nelco.............

        Does anybody know the CTE Z for Nelco N4000-2, 6 or 13. I
checked there
        website and unlike the values for x and y the z values are
specified in
        %. if 3.5% is in fact correct would that not mean  CTE of  3500
PPM??
        Help would be great...............Paul

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