Dear Ed;
Measuring the electrical resistance of solutions is an interesting task.
We are dealing with a three dimensional piece of solution between the two
electrodes. So, not only the area of the electrode surfaces are important,
but so is the distance between the electrodes. The electrodes used to
measure conductivity are referred to as "cells"
The traditional resistance probe consists of two square platinum electrodes
parallel to each other and held some fixed distance apart. Resistance
probes used in cleanliness testers are two cylinders, one inside the other,
with holes in the outer cylinder to allow the test solution to flow through
the probe. The solution has many of the same electrical properties as
wire. Two properties that effect resistance measurements are cross
sectional area and distance.
The resistance of a solution is INVERSELY proportional to the cross
sectional area of the electrodes. This is similar to resistance in wires.
A large gauge wire with a large cross sectional area has a low resistance.
A small gauge wire with a small cross sectional area will have a higher
resistance. If the cross sectional area increases, the resistance
decreases.
The resistance of a solution is DIRECTLY proportional to the distance
between the electrodes. This is also similar to electrical resistance in
wires. A long wire will have a higher resistance than a short wire. If
the distance between the electrodes increases, the resistance increases.
If we combine these two relationships in a mathematical formula, we get the
following equation:
R=K *d/A
Where R = ohms, or what the ohm meter measures
K= specific resistance, the value that gets reported
d = distance between the electrodes
A= cross sectional area of the electrodes
The "specific resistance", or K, is the value that is reported. If we
solve the equation for K, we get this equation:
K = R *A/d
The ratio A/d is known as the "cell constant" and is used to make the
reported value of K independent of the cells used to make the measurements.
The dimensions of A/d are cm**2 / cm, or centimeters. R is measured in
ohms or megohms. Therefore, K is reported in megohm centimeters.
To convert between resistance measurements and "micrograms / square inch"
it would probably be best to make your measurements in units of
conductivity, or inverse ohms (units = mhos), as inverse ohms are more or
less directly proportional to the concentration of ions in solution. The
units for specific resistance are "mhos / cm". Make several different
concentrations of NaCl and measure the conductivity for each NaCl
concentration and make a calibration curve. Then measure the washings from
your circuit boards. Your NaCl standards should be made with the same
solvent that the circuit board is extracted with, typically 75% isopropyl
alcohol - 25% water.
IPC-TM-650, test # 2.3.26.1, Ionizable Detection of Surface Contaminants,
paragraph 5.4.1 has a formula that should be able to convert the megohm-cm
readings into micrograms NaCl/sq inch. I have never used this formula to
calculate ug/in**2 levels before and do not know how accurate the formula
and empirical constants are. If you try to use this formula, let me know
what your results are.
If you have any other questions regarding resistance / conductivity
measurements, please do not hesitate to contact me.
Eric Higbie
phone 317.655.3673 X216
fax 317.655.3699
email [log in to unmask]
----------
>
>Can anyone tell me how to convert Megohms / sq. inch into (g / sq. cm ?
>Also, section 5.0 of IPC-A-600E states the resistance limitations for
>solvent resistivity to not be less than 2 x 10 to the 6th power / cm.
>Should not the cm be squared to provide a surface area? All help is
>appreciated.
>
>Thanks,
>
>Ed Cosper
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