I would suggest that the 1.56 microgram value was a default suggestion 
for all three classes of hardware and is easily achieved.  There is 
substantial data suggeseting that as the PWA designs go to tighter 
spacings (circuit and part/board) and higher speeds, this requirement 
will probably get tighter.  Below is a summary of some literature on 
this subject that I collected for our management.

     a) "For bare PWB levels of contamination in the 1 microgram NaCl 
     (eq)/ cm2 ranges (6.45 microgram/in2 )...are reasonably 
     consistent with leakage requirements of today's devices."-  A 
     Correlation between Surface Insulation Resistance and Solvent 
     Extract Conductivity Cleanliness Test,  A. Chan and T. Shankoff, 
     Circuit World, Volume 14, #4, 1988, pp. 23-26
     
     b) Summary of Contamination Limits; Process Stage:  Bare PWBs 
     before solder mask, contamination limit (actual production) 1  
     microgm/cm2 (6.45  microgram/in2); after assembly limit is 1.5 
     micrograms/cm2 - Bellcore Standard TR-NWT-000078
     
     c) "Many assembly manufacturer's use a bulk resistivity limit of 
     1.0 to 3.3  microgram/square INCH of NaCl as their cleanliness 
     requirement test for incoming boards" - Component Cleanliness, T. 
     Munson and M. Ford, Circuits Assembly, March 1993
     
     d) "(Permissible Ionic Contamination) For surface mounted 
     assemblies this level is better set at 0.5  microgram/cm2..." - A 
     Scientific Guide to Surface Mount Technology, C. Lea, 
     Electrochemical Publications, 1988
     
     e) (Suggested Arbitrary Limit Values for Ionic Contamination as 
     Measured by a Commercial Instrument) 
          Aerospace/military applications - 1  microgram/cm2
          Ultra-critical SMC application - 0.5  microgram/cm2 
     - Cleaning and Contamination of Electronic Components and 
     Assemblies, B. N. Ellis, Electrochemical Publications, 1986
     
     f) "...a chloride contamination of greater than 4  microgram/in2 
     resulted in a loss of insulation resistance, development of 
     corrosion and blistering in the conformal coating." - Evaluation 
     of Salt Contamination on Insulation Resistance, C. Littlefield, 
     EM/P 120, (~1975)
     
     g) "A study done by the Naval Weapons Center (China Lake, CA) 
     showed that certain types of fluxes used in PWB processing would 
     fail SIR testing even after meeting the requirements of 
     MIL-P-28809" - No Clean Fluxes:  New Technology Needs New Test, 
     E. Small, Circuits Manufacturing, December 1989



______________________________ Reply Separator _________________________________
Subject: PCBA Ionic Contamination Levels
Author:  [log in to unmask] at esdigate
Date:    10/3/95 3:41 PM


ANSI/J-STD-001 and other specifications require ionic contamination 
levels of less than 1.56 micrograms/cm2 (10.06 micrograms/in2).  This is 
a reasonable level for overall ionic contamination on Class 3 PCBA's, and 
is a good maximum level for monitoring the cleaning process.  But, what 
about maximum levels for Class 1 (Consumer) and Class 2 (Industrial) 
grade PCBA's ?  Afterall, there are different levels for rosin residues.

Does anyone have ACCEPTABLE (maximum) levels for ionic contamination on 
Class 1 and Class 2 PCBA's ?  Thanks . . .

Ed Valentine ([log in to unmask])
ProTronics, Inc.
Knightdale, NC
Phone: (919) 217-0007, FAX: (919) 217-0050 
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