| Thank you all for your replies. I guess my age in this industry is showing. This military spec. does not appear to be in existance. So, let me make my request a little more to the point: | | Is anyone aware of a specification specifically referencing the Zero Ion ionic contamination system? I recall an equivalency factor that showed "acceptable" limits of contamination for Zero Ion and the Omega Meter SMD. If I recall correctly, the amount for the Zero Ion was 37 micrograms per sq. inch. | | Thanks, | Mike. | | | Is anyone aware of a web page for military spec's? I am looking for Mil-P-28809. | | | | Thanks, | | Mike Demos | | | | Mike
I think that it was in 1978 that MIL-P-28809B introduced the notion of so-called equivalency factors for a limited number of commercial instruments. This was done by a totally artificial test method (the test boards were not even soldered, so the flux was raw) to try and equate the results to the already extremely flawed reference method.
Shortly afterwards, the UK Ministry of Defence attempted to do something similar to their reference method, which was much better than the US one, with many more parameters defined (but still not perfect). They were not able to do so, because different test boards gave widely different results. Because of this, the MoD refused to depart from their reference method (which was practical, based on a Swiss instrument, but could be determined in a lab without the instrument, albeit laboriously). This was incorporated in DEF-STAN 00/10-3 (1986). For the anecdote, the scientists at HMS Aquila research labs called the equivalency factor the "fiddle factor".
I'm not sure of the exact date - probably about 1982 - the European Space Agency (ESTEC in Holland) commissioned the IVF in Sweden to evaluate instruments and equivalency factors. They did a whole matrix of tests round several instruments and 5 or 6 flux types and cleaning methods, with a single test board. Interestingly, they found that the equivalency factors between instruments could vary according to the flux type and cleaning method by as much as a whole order of magnitude. Furthermore, if Instrument I gave a considerably higher reading than Instruments II and III when measuring a board with components soldered with Flux A and cleaned in Solvent X, it could be I II when using F B and S Y and I III with F C and S X (I think there were only two solvents in the matrix). In the conclusion of the long report (Bergendahl, C.G., and Dunn B.D. 'Evaluation of Test Equipment for the Detection of Contamination on Electronic Circuits", European Space Agency Technical Memorandum ESA STM-234, Paris [1984]), it is clearly stated that equivalency factors are totally meaningless.
Furthermore, the instruments used for determining them in the 1970s are no longer made and have been replaced by much better ones. If the flawed 1970s tests were repeated with today's instruments, even bearing the same trade names, the results would be VERY different, indeed.
I'll also state that the fiddle factors were determined long before SM techniques became the norm and take no account of modern manufacturing processes.
My advice is therefore to forget that these fiddle factors ever existed and determine your own limit value according to your process and your instrument.
Brian
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