Hans

I ran into this many years ago. I assume the basic equipment/measurement
hasn't changed much.

Are you density corrected? If not, I suspect all your gold measurements
are on the low side regardless of thickness......

Beta backscatter is essentially a measurement of the quantity of beta
rays backscattered  per unit area. That's why you have to calibrate for
each probe aperture (i.e. "measurement area"). The metal thickness per
unit area is proportional to the quantity of beta rays backscattered.
Thus, the density (i.e. weight per unit area) of the metal is affects
this measurement.

If your calibration stds are pure gold (i.e. 19.6 g/cm^3) and you are
plating hard gold (i.e. 17 g/cm^3) you will get a consistent error that
relates to the difference between the two different densities. You
correct for this either of two ways:

1) numerically correct by multiplying by the ratio of 19.6/17.0
.......easy method.

2) Making nickel/gold standards whose density is the same as your
plating and then getting them certified as "calibration
standards".......hard method.

good luck



-----Original Message-----
From:   Hans Rohr [SMTP:[log in to unmask]]
Sent:   Wednesday, November 05, 1997 8:37 PM
To:     [log in to unmask]
Subject:        [TN] Gold Thickness Measurement Error

We use beta backscattering technology to measure the thickness of gold
on plated fingers.  The instrument is calibrated at 0, 15, 50 and
"infinity" microinches.  In the past we have been plating at 50
microinches.  We recently starting plating at 30 microinches.  The
instrument is calibrated at the beginning of each shift.  Gold thickness
on product
is checked during plating after every 10 panels.  Calibration is checked
every two hours using the 15 and 50 microinch standard.

Recently we plated several hundred panels and the gold was subsequently
found to be consistently low (18-22 microinches or one half the measured
thickness) on every panel.  When the calibration was checked,  the 15
and 50 microinch stantards still read correctly.  The instrument was
recalibrated using a different calibration channel.
Then the fingers on the plated panels read 18-22 microinches.  When we
go
back to the original calibration channel, the problem is still there;
the plated fingers read 36 to 42 microinches.

Since the problem is consistantly repeatable, we have ruled out a
defective probe.  We have tried measurements holding the probe upside
down and beating it with a wood pencil.  The probe is as steady as a
rock.

Has anyone experience a similar problem?  If so, have you been able to
find a root cause for the problem.  Our gold salt supplier said he has
heard of this problem several times before.

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