Phil:

The 6.0 micrograms equivalent of sodium chloride per square inch or 1.0
micrograms equivalent of sodium chloride per square centimeter came from Bell 
Labs.  I believe it was Bernie Wargots' group who first published this number as
an upper limit to ionic contamination, following studies on corrosion of PCBs 
using sodium chloride as the contaminate.  It began as an AT&T requirement and 
later became a Bellcore requirement.  Many PCB manufactures have adopted this as
their standard.  It is relatively easy to obtain and maintain this level of 
ionic cleanliness in a production environment.  The more difficult task 
is insuring the absence of detrimental organic or non-ionic contamination.  This
can be monitored via SIR testing, which measures the corrosive effect of any 
contaminate left on the board.

Harry


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______________________________ Reply Separator _________________________________
Subject: Fab:Re: Ionic Contamination  
Author:  [log in to unmask] at SMTPLINK
Date:    7/30/96 4:23 PM


Nancy 

The ionic contamination number of 10.06 micrograms/in. sq. or 1.56 mictograms
per cm.sq. is pretty universal around the industry.  IPC-RB-276, IPC-6012,
J-STD-001, MIL-P-55110 and MIL-PRF-31032 all list this number as the max. for
cleanliness measurement and has been the value for a long time.  I am not
sure where the 6 micrograms/in.sq. or 0.96 micrograms/cm.sq. came from or on
what it is based, but it is not one of the more common callouts. As chairman
of the 276 task group, I found that our discussions alway led to retaining
the higher number which appear to be working for us.  If anybody has any
input into the 6 micrograms number or why it is used or needed, I would like
to hear from you.

Thanks
Phil HInton 
Hinton Engiineering 
[log in to unmask]        

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