1. We often have wished for a simple conductivity/resistivity test for
plated metals, but there is such a range of cleanliness levels needed from bridge
girders to precision gyros, that it is not as easy as it was for electronics.
(Many other tests are used for metal cleaning, such as torque signature
charts for precision ball bearing, tape adhesion tests for paint on
refrigerators, etc. etc.)
2. That said, the first application of a conductivity change test was put in
place at Western Electric by Bell Labs. The goal was to determine if
connector parts were clean enough to gold plate. In the test developed by Tom Egan
(ca. 1973), atmospheric equilibrated distilled water was used, with
conductivity measured before and after rinsing a certain amount of parts in a specified
amount of water. (I still have a copy of the original paper in anyone is
interested in reading it.)
3. Naturally, plain water didn't work well for cleaning off/dissolving flux
residues, so enter IPA/DI water mixtures. The goal was to liberate all the
ionics in the flux residue into the solution so they could be measured, with the
results expressed as sodium chloride equivalents; i.e. how much sodium
chloride would be needed to give the same change in conductivity/resistivity.
Sodium chloride equivalents was a good choice, since sodium chloride in DI water
was a readily available cheap calibration solution. Most of the ionic
measurement devices ship with sodium chloride calibration fluid.
4. Residual ionics became important during the Vietnam era, when radios that
had functioned well in Europe for a decade failed after a few weeks in the
S.E. Asian jungles. Careful examination of the PWAs showed blisters or
'vesiccation' of the coating. Lancing the blisters showed they contained liquid
water, which combined under the coating with enough residual ionics from the RMA
flux residues of the time to cause current leakage paths and subsequent
failure. NAFI (Naval Avionics Facility, Indianapolis) set an acceptance level based
on ionic levels seen on PWAs that functioned reliably under such adverse
conditions. This work was done by Martz, Hobson and DeNoon, who were
instrumental in preparing MIL P-XXXXX, which became MIL P-28809 in 1975. The original
graph used to set the acceptance level can be found in MRR-3-72.
5. The original NAFI wash bottle test was automated in various types of
measuring equipment, so NAFI/NAC Labs called on the measuring equipment makers to
submit equipnment for testing. This was done, then we all reconvened to
assess and standardize the results. Jack Brous, Ed Wolfgram and I attended this
meeting & came up with the idea of basing all the results as multiples of the
average of the two conductivity meters already approved for such use. Thus
the development of "equivalence factors", published in MRR-3-78, which are
still used today.
5. Bottom Line- we sure 'lucked out' in the electronics industry when NAFI
gave us a simple monitoring test for ionics that worked very well for all the
rosin-based soldering fluxes of that era. Metal cleaning just didn't have it
as easy as we did, due to the much wider range of applications and materials
of construction.
Bill Kenyon
Global Centre Consulting
3336 Birmingham Drive
Fort Collins, CO 80526
Tel: 970.207.9586 Cell: 970.980.6373
email: [log in to unmask]
---------------------------------------------------
Technet Mail List provided as a service by IPC using LISTSERV 1.8e
To unsubscribe, send a message to [log in to unmask] with following text in
the BODY (NOT the subject field): SIGNOFF Technet
To temporarily halt or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL)
To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest
Search the archives of previous posts at: http://listserv.ipc.org/archives
Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-615-7100 ext.2815
-----------------------------------------------------
|
