I believe that there is a lot of confusion arising here. The deepest
study of the effects of w/s fluxes was certainly made by the UK Ministry
of Defence at Aquila, Bromley, Kent from c. 1980 to 1986, culminating in
their DEF-STAN 00-10/3, Although the MoD never published their findings
in journals or conferences, they were free with their information. I
know about this, because I worked quite closely with their Dr Alan
Stopes, who was in charge of the project (since retired, to a Welsh
sheep farm!).
The result of their research was that a) as long as assemblies met a
given ionic contamination test, the flux and soldering/cleaning process
could be freely chosen by the contractor b) the MoD preferred the
water-soluble flux/water wash, if possible, provided a) was met and not
all w/s fluxes would allow this. c) a 50% solution was preferable to a
75% solution for ionic contamination test as it gave double the
sensitivity and, consequently, a better precision. d) the integration
time should be 15-30 minutes, although they permitted shorter times
provided the curve had levelled to near asymptote. e) the so-called
"dynamic" method was less preferable to the "static" method; their
reference method for calibrating equipment was "static", albeit
dynamically (semantics!).
In terms of w/s flux residues, there were three main components: ionic
organics, ionic metal salts and non-ionic vehicles. The first two, by
their nature were usually the most dangerous in terms of reliability but
dissolved rapidly, by their very nature, except for a few lead salts
which tended to saturate with insufficient wash water. This was another
reason why they used 50% solution in their test protocol, as lead salts
would have double the solubility than in 75%, reducing the risk of
saturation.
The problem was with the vehicles used in w/s fluxes. These were usually
blends of diols and triols and their ethers. Depending on their
composition, these could form hydrogen bonds with epoxy substrates,
especially in pores. This adsorption ("Zado" effect) was hell, when it
happened because it could form a hydrophilic surface on the substrate,
lowering SIR and, of course, it was not detectable by ionic
contamination testing. The only cure was to choose the flux well. The
worst fluxes were those containing polyethylene glycols and their
cousins. Unpolymerised higher glycols were much better as was the main
triol, glycerol (used, for example, in Hughes' "lemon linctus" range of
fluxes).
At that time most w/s flux activators were eitheer amine hydrohalides or
amino acid hydrohalides. For example, the Lonco 3355 ST (not the pink
stuff which was 3355-11) consisted of glutamic acid hydrochloride as
activator, propylene glycol as vehicle and a mixture of IPA and water as
solvent. This gave excellent results, a very high SIR, very fast
solubilisation of residues in both water and the 50% ionic tester
solution and practically no adsorption. A typical ionic test would level
to asymptote in 2 - 3 minutes on nearly every substrate, usually at 0.1
- 0.5 ug/cm2 eq. NaCl if cleaned in a good machine (DEF Std limit 1.5
ug/cm2 eq. NaCl) and the SIR would be indistinguishable from that of
unsoldered laminate.
I hope this will put the church back into the middle of the village.
Brian
Carroll, George wrote:
> Ken,
> As the previous respondents have indicated, Ionograph results shouldn't be
> used as a go-no-go. Taking the process approach, what were previous results
> from this vendor? from other vendors providing similar boards? Are there
> components that would tend to hold flux under or near them? That could
> contribute to field reliability issues. There may be those that disagree
> with me, but 50 sounds a bit steep in comparison with no-clean boards that
> I've measured.
>
> George
>
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