The levels were not set arbitrarily. They were set on basis of ionic
residues causing vesiccation & subsequent failure of military assemblies in high
humidity environments (Vietnam). Specifically, ionic residues from RMA fluxes on
conformally coated assemblies- most often radios. (See Navy report
MRR-3-72).
Mechanization of the original manual ionic testing method led to the
different nbrs used for the various automated test instruments, as published in Navy
report MRR-3-78. I wrote a column for SMT magazine on this issue; if you'd
like a copy of the column, email me separately at [log in to unmask]
(mailto:[log in to unmask]) as it would be stripped off if I tried to email it to
TechNet as an attachment.
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526
Tel: 970.207.9586 Cell: 970.980.6373
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