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
******************************************
|