The measuring of ionic contamination gives some problems:

1. if there is a required limit of ionic residues check the specific
acceptance limit
   of your equipment. MIL-STD-2000A Appendix C shows a table of different
Instrument       acceptance  limits and equivalence factors.

2.Some solder masks are a good source of ionic residues. I have made
measurements of        cleaned boards with results of 13 ug/cm2. If the
solder mask is the source you can notice it     if you need a long
measuring time till the values become constant. Another method is a  
second test after baking the assembly or bare board. The result should be
lower than the first.  

3.Test the bare board. It should have max. 1.6 ug/cm2 (=3.1ug/cm2 when
using the       Ionograph).

Karl Ring
 ZVE 
Germany 
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