In a message dated 98-05-11 17:41:39 EDT, you write:

<< Just a question. I have been having problems getting my CMI unit to read
copper thickness consistently. It has typically been reading low. For
instance, the meter will show .0005 to .0006 of copper on a standard .042 thru
hole but a section of the same hole shows .0012.  The meter reads fine to the
provided standard.  Has anyone else experienced this problem. I should note
that the meter was just calibrated a few weeks ago so calibration should not
be an issue.
  >>

We have been using a CMI PTX-200 for years and have never seen the deviation
you are talking about.   We do see some drift (on the order of 0.03 mils
typically but sometimes as much as 0.010 mils) and therefore we calibrate our
PTX-200 unit every shift.
Sources of deviation:
1.  There is a setting on the unit to compensate for different copper
thicknesses, but these only affect the reading by perhaps 0.05 mil.

2.  Cold temperature can decrease the reading by roughly 0.15 mils.

3.  The large inaccuracy you're talking about would make me suspect the cross
section technique.  Is this done in-house?  If so, a sample submitted to a
testing lab might show that the problem isn't with the CMI at all, but with
the cross section technique.  Which of the 2 numbers would you expect to get
based on your plating times?

Peter Blokhuis
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