Steve

I days of yore, when knights wore armour, some parts had to be soldered together and
the knights didn't want rust to show after fighting in the rain... :-)

First of all, MIL-STD-2000 was a legacy of MIL-P-28809 (and others) and worked on the
philosophy that you MUST use an R or RMA flux approved to list MIL-F-14256 (I think,
from memory) and clean with a CFC-113 azeotrope or similar solvent. Let it be said here
and now that all the listed fluxes were liquid with a high solids content (>25%).
Pastes were not used in these days. This philosophy was totally flawed because many
combinations of approved flux/cleaner gave very bad results, in terms of cleanliness.
Then ionic cleanliness testing came along and the original NAFI test method was even
more flawed as there were 28 unspecified parameters and it was so easy to fiddle the
way of conducting the test that the results could be "adjusted" over a whole order of
magnitude. Then along came the instruments, initially the major contenders were the
Alpha Ionograph, the Kenco (later Alpha) OmegaMeter and the Protonique Contaminometer.
These were an improvement, the latter being the first to use IT techniques (1979) to
eliminate many of the unspecified parameters. These showed that the RMA/CFC combination
was, indeed, poor and that, in reality RA was even better than RMA, in many cases. At
this time, more interest was being shown in water-solubles which proved to be 1000
times more active and easier to use than RMA and 10,000 times easier to clean. However,
the proponents of the conservative approach cited the Zado effect (absorption of
polyglycols by the substrate, causing a lowering of SIR), without realising that some
W/S fluxes were formulated without such polyglycols.

Anyway, along came the Montreal Protocol which dropped CFC-113 and 1,1,1-trichlorethane
cleaners into disgrace and the military realised that a quick rethink was necessary.
They had already realised the old method was pretty worthless, anyway, as results,
although generally acceptable, were not infallible. Remember that many military
assemblies have a working lifetime of only a few seconds but a storage lifetime of 15
years and the object of the exercise was to prevent e.g. a tank-launched missile from
getting half way to the target, doing a U-turn, and coming back where it started from.
Other military hardware, with continuous use over many years, was not so critical a)
because maintenance and repair were current and b) because really critical assemblies
were du-/tri-plicated in redundant systems.

The military rethink was a complete volte face. Instead of specifying thou shalt use
such and such materials, it said thou mayest use what thou damn well pleasest, provided
thou art able to demonstrate the assemblies have the required reliability. This
philosophy makes sense because it means that the manufacturer can use W/S fluxes, if he
can master the cleaning, and thereby save a packet of money. If he cannot master this
process, he can use other methods, mostly with poorer results but with, perhaps, less
risk of the process going off the rails, even if it costs more, overall. However, the
onus of doing the homework shifted onto the manufacturer.

Therefore, the simple answer to your question is that it makes not a scrap of
difference technically. However, there is a proviso: some hardware is still governed by
old manufacturing standards because some obscure specs have not been updated. These may
still call for the RMA/CFC combination. As you can no longer obtain such
ozone-depleting cleaning solvents in non-Art 5(1) countries (developed), according to
the Montreal Protocol, you must negotiate a derogation with the procurement agency.
Such a derogation may allow you to use a W/S flux (non-Zado-types), if you argue your
case well and prove you can offer the required reliability.

That having been said, another variable has come into the equation, comparatively
recently: surface mounting. Don't believe a word of RMA being selected because of poor
clearance under SMDs. RMA fluxes were selected decades before there were any SMDs. No
matter which flux/paste type and cleaning method are chosen, this is a problem. I have
photos of RMA paste residues under components after the most careful and prolonged
halogenated solvent, with ultrasonics, and vapour phase cleaning in more powerful
solvents than CFC-113 azeotropes. In fact, I have never been able to guarantee removal
of RMA pastes with any cleaning method under many SM component types. I have shown that
adequate water-cleaning of some W/S pastes under the same components is just possible
under the most favourable conditions but with no margin of error. If this is done
correctly, an ECMR test under the component itself will reveal better results with the
carefully selected W/S paste. But you must have the right equipment and process: there
are few machines capable of doing the job adequately.

Hope this helps

Brian

"Stephen R. Gregory" wrote:

> Hi ya'll!!
>
> I gotta generic question about flux types. We do quite a few boards here that
> still call out that the assembly is to be built to MIL-STD-2000 specs, and as
> has always been here, RMA flux is always used. My question is, what is the
> REAL reason to use an RMA?
>
> I've been under the impression that because a RMA is not as active as an OA,
> any residues that might be left behind from the RMA wouldn't be as harmfull
> as OA residues would be. But if you always ensure that you clean your boards
> thoroughly, what difference does it make whether you use an RMA or an OA?
>
> -Steve Gregory-
>
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