<< Concerning the 2.0 microgram/sq. in. limit:
Mark Shireman of Alliant Techsystems just presented a paper at China
Lake about a military low residue soldering production process. Based
on their experience, he recommends a bare board limit of < 2.0
micrograms/sq.in. for most designs. The data was generated using an
Omegameter 600SMD, 10 minute cycle, unheated. I think the limit
originated from a study conducted for the Army by Contamination Studies
Laboratory. >>
Good Day All,
The study referenced here was the Low Residue Soldering Task Force Report,
published in June of 1995, in which CSL participated. The 2.0 microgram per
square inch limit is specifically for chloride ions as determined by ion
chromatography. It is NOT 2.0 micrograms of sodium chloride equivalent, as
measured by an Omegameter. Do not confuse the two measures. An ionic
cleanliness tester relates the conductivity of the solution back to an
equivalent amount of a strongly ionizable salt (sodium chloride) that would
be needed to arrive at the same solution conductivity as the unknown. Ion
chromatography shows you how much of the specific ion is present. The two
are very different.
Incoming bare board cleanliness is often measured by ionic cleanliness
testers, with spec values of 4-6 micrograms of sodium chloride equivalence
per square inch. We have seen a number of cases where bare boards measure
clean by conventional wisdom, yet still fail from corrosion and metal
migration. For this reason, we prefer ion chromatography (IPC-TM-650, method
2.3.28) as a better cleanliness measure, to catch residues that an ionic
cleanliness tester will not detect.
For those who are interested in bare board cleanliness issues, I will be
chairing a new Bare Board Cleanliness Assessment Task Group (5-32c) at the
upcoming IPC meeting. We will be discussing bare board cleanliness, how it
should be measured, how to determine how clean is clean enough, etc. I hope
to see you there. If you cannot be there, please let me know if you are
interested and I will keep you electronically involved.
Since we will be setting the goals and the directions of the group, I would
be interested in hearing your concerns, what you would like to see, what you
currently use for specifying bare board cleanliness, etc. I will accept
input up to 4 pm EST on Friday.
Doug Pauls
Contamination Studies Labs
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