Micrograms per square centimeter is a measurement of relative contamination.
The expression refers to the change in conductivity of a test solution of 
alcohol and water when you soak a sample of product in it divided by the 
square area of the sample.  

The test instrument is calibrated by adding an aliquot of sodium cloride 
and adjusting the output to read a standard value.  The units of the 
measurements are really "microgram gram equivalents of sodium cloride per 
square area processed".

This system was developed because there is really no way to measure the 
"resistivity" of a surface.  

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On Tue, 26 Sep 1995, Jim Marsico wrote:

> Can anyone tell me the correlation between micrograms/square centimeter and 
> resistivity for PWB cleanliness?
> 
> 
>