Terry- much of what you say is correct. But we have to look at some of the
1. The ionic cleanliness test was originally developed to determine if
conformally coated military equipment would survive the harsh temp/humidity
conditions of S.E. Asia. In the words of the day, is the floor clean enough so we
can put down the wall-to-wall carpet?
2. While most of the PWAs were assembled with RMA moderate to high solids
rosin fluxes, many of the electronics makers wanted to use stronger fluxes to
cope with the marginal component solderability of the era. Thus we soon had the
RA-Mil activity level fluxes to help with that issue.
3. The designs were generally through-hole, surface mount wasn't in common
use until much later, esp. for mil use. Thus flux residue entrapment wasn't the
issue then that it became after the adoption of low clearance restricted
cleaning agent flow SMT parts.
4. The Task Group work you have cited pointed out other concerns that
weren't big issues with PTH/high solids fluxing agent designs; such as the 'dead
band' in cleaning instrumentation response.
5. All in all, these deeper investigations helped us all to understand some
of the nuances of ionic cleanliness testing, which had taken on a standing
that was interpreted as an absolute guarantee of reliability, when it was really
developed as a process monitoring tool. Now in the test's favor, the level
was set to reflect ionic contamination levels based on clean and failed PWAs
from the field (U.S.Navy MRR-3-72), and confirmed by the lab work of W. B.
Wargotz at Bell Labs (see my previously offered SMT column for the reference)
so the pass/fail was not arbitrarily chosen.
6. To address your point about spot or localized high levels on ionics,
Wargotz has published on his work of making mini-cells on the PWA surface by
gluing a piece of Pyrex tubing to the surface using RTV, then putting in a
controlled volume of test solution, a micro conductivity or resistivity cell to
obtain spot readings in known entrapment areas. Hopefully, such work would
result in design changes so the overall average ionic contamination level would be
indicative of good field performance. (This paper may have given at one of
the IPC World Conferences, I'll have to check my files to see if I still have
7. Terry- thanks for bringing your points up, so they can be discussed in a
way that helps members of the TN community better understand ionics and
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