We use a CMI PTH-100 unit. We found that on average the unit reads
0.3 mils lower than the microsection thickness as the panels come
off the line. However, if the same holes are measured a day later,
the unit measures very close to the true thickness. Right or wrong
we have attributed this effect to the room temp annealing process
that changes the crystal structure of electrodeposited copper. We
assumed that the resistance changes slightly when the crystal
structure changes and thereby changes the CMI readings. When we
"calibrate" we adjust the unit to add 0.3 mil to the reading.
Other observations with our unit: We created special "CMI coupons"
on our multilayer panels that have no internal pads. We felt that
the copper added by varying numbers of internal layers and varying
copper weights of those layers produced erratic readings (i.e. the
CMI unit would get much different readings on a 42 mil hole, 62 mil
thick, 10 layer M/L w/ 2 oz. copper internal pads, vs. a 42 mil,
62 mil thick, 4 layer M/L with 1 oz. internal pads provided they
were plated with the same amount of copper. I realize that it
would be better to measure the parts, but it is more important to
get a number can be trusted.
Also readings on panels less than 40-45 mils thick become highly
dependent on the parameters entered, whereas panels above this are
more insensitive to them. Ed, you didn't state how thick the
panels were that you were measuring. Also I wasn't sure if you
meant the errors you were getting have been happening all along or
it was a recent problem.
Randy Lyle
______________________________ Reply Separator _________________________________
Subject: [TN] FAB: CMI hand held copper thickness checker
Author: "TechNet E-Mail Forum." <[log in to unmask]>, at CCGATE
Date: 5/11/98 2:41 PM
Hi all,
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.
Ed Cosper
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