Hi Brian,

 Many years ago when tyranossourus rex made PCB's I was involved in the
insulation resistance test in relative humidity@ a potential of 50V in a
pair of parralel tracks. The requirement was a resistance of I think 10^8
ohms at 16 and 21 days. What we discovered was that samples can pass at the
required test time whilst failing at say 15 and 20.
My contention at this time was due to ingress of NaOH during stripping of
dry film and the mechanism of good/bad rinses on the bulk/surface ionic
contamination/diffusion.
I have quite a bit more off technet if anybody is interested.

Regards,

Ken Mc Gowan
----- Original Message -----
From: "Brian Ellis" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Saturday, April 28, 2001 6:54 AM
Subject: Re: [TN] SIR Definition


> Chapter 2
>
> Is there a correlation between ionic contamination testing and SIR. The
> easy answer is "maybe". Let us take four hypothetical situations:
> 1) a board is contaminated with, say, salt (e.g. from sea spray) and
> nothing else. There would be a close correlation between the two.
> 2) a board is contaminated with, say, sugar and nothing else. Ionic
> contamination would say the board was perfectly clean but SIR would tell
> you it's hopeless.
> 3) a board is contaminated with well-hardened, aged, halide-activated
> rosin flux residues. Ionic contamination testing will dissolve just
> sufficient to register severe contamination, while SIR testing may
> (depending on the conditions) reveal no problems.
> 4) a board is contaminated with freshly soldered "no-clean" organic acid
> activated flux residues. Ionic contamination testing may reveal
> impossibly high (by cleaned standards) contamination, while SIR testing
> may reveal it to be safe (because the chosen temperature was so high
> that the residues sublimated).
> So....! Pragmatically, one must say there is no sure correlation and
> probably none at all with the usual porridge of contaminants encountered
> in practice.
>
> However, there is another enormous difference: ionic contamination
> testing is a simple, low-cost (equipment and operating), rapid (15
> minutes), workshop-floor, process control system. SIR is a high-cost,
> laboratory, slow (days, weeks or months), qualification procedure. Ion
> chromatography is a moderately high-cost, laboratory, fairly rapid
> (hours), test very useful for identifying ionic contaminants.
>
> They are therefore all complementary to each other and, if you wish to
> know the whole story, at all stages, you must do them all, regularly. I
> say "the whole story" but, with these three alone, you still will not
> have it. For example, spray some mould release (silicone or
> fluoropolymer type) or many oils on a circuit and none of these methods
> will budge a hair, but the contamination will be there and cause
> potential problems (e.g. no adhesion of conformal coatings). In fact, it
> is conceivable that some of these contaminants could even improve ionic
> contamination or SIR figures! A "pressure cooker test" (vesication test)
> could reveal the presence of these contaminants.
>
> _______
>
>
> Other things that must be considered with SIR testing are:
> - absorbed contaminants (epoxy substrates will absorb contaminants of
> all types, especially over the Tg or with small molecules). These may
> desorb on a time scale ranging from hours to years.
> - adsorbed contaminants (hydrogen bond linking with the organic
> substrate or chemisorption). This is often fairly permanent if the
> adsorbed molecule is stable, but it may have a hydrophilic end on the
> outer surface, creating an increase of hygroscopicity with electrical
> consequences. Less stable molecules may break down by any one of a
> hundred mechanisms (heat or thermolysis, IR, light, UV, up to cosmic
> radiation or photolysis, reaction with hydroxyl radicals or hydrolysis,
> bacteriolysis, plasmolysis, electrolysis, chemical reactions etc.) to
> release harmful contaminants that could lower the SIR
> - the substrate chemistry. For example, most epoxy resins are formed
> (simplistically) by an initial prepolymerisation, usually of an epoxide
> substance (typically epichlorohydrin which has an epoxy ring C-O-C and a
> chlorine atom stuck on the end) and a complex phenolic molecule such as
> bisphenol A (or tetrabromobisphenol A for flame-retarding). A hydroxyl
> group is needed to complete the reaction and this is supplied by a third
> component, sodium hydroxide, where the sodium atom has the supreme merit
> of capturing the chlorine one, which would otherwise be a darned
> nuisance. Yes, you've read it right, there is sodium chloride in the
> epoxy prepolymer, although most, but not all, of it is removed in the
> electrical grades. This prepolymer (tube A in your hardware store) is
> then crosslinked using such horribles as amines or dicarboxylic acids
> (contained in tube B). Under ideal conditions, you have exactly the
> right number of molecules in tubes A and B that every reactive epoxy
> group in the prepolymer clicks into a molecule of the crosslinker, with
> no excess of either, and that the mixture is perfectly homogeneous to
> allow this. However, this is utopian and there is always a local excess
> of one component or the other, especially at the surface. If it is the
> prepolymer, it means you have some highly reactive epoxy groups just
> waiting to capture anything that comes along. If it is the crosslinker,
> then these are usually nicely ionic or can break down into ionic
> species. In either case, you can expect some salt molecules near the
> surface, as well. All this has an influence on SIR measurements, whether
> you are testing the quality of the substrate or trying to find the
> effect of possible contaminants.
> - moist air. Yes, moist air - and we are talking of 80-95% RH - is a
> conductor of the same order as some of the other factors we are looking
> at. A student at the Swiss Federal Institute of Technology who was
> working on the electrical characteristics of epoxy resins for his
> dissertation stretched two 1 mm diameter wires copper wires between PTFE
> insulators, 1 mm apart, across a humidity chamber and obtained readings
> of the order of 1E9 ohms (if my memory is good, the "test pattern" was
> about 50 cm long). Control wires cut off at the insulators were abour
> two orders of magnitude higher. This is therefore not negligible.
> - history of the measurement. If you apply a voltage gradient to the
> substance of a damp epoxy resin, the salt molecules (and any other
> ionics) will dissociate and start to migrate. This migration requires
> energy which is supplied by the applied voltage in the form of an
> additional current (i.e. an apparent lowering of resistance). I don't
> know the distances involved, probably in the order of angstroms, but I
> do know it takes hours for equilibrium to be reached (i.e. the
> resistance reaches maximum). If the gradient is removed, it will take
> days for a new electrical equilibrium to be reached, because the
> re-associative forces are much lower, but this can be accelerated by
> reversing the field which causes an initial relatively high peak of
> current as the ions are re-pushed towards one another. This is why the
> power factor of most epoxy resins is never brilliant, even at low
> frequencies. It is also why it is very important that the SIR
> measurements should always be made with exactly the same time interval
> between applying the test voltage and actually making the measurement,
> especially important if a lower, no or reverse bias voltage is used.
>
> In conclusion of Chapter 2: make sure you know exactly what you are
> measuring.
>
> Brian
>
> Bev Christian wrote:
> >
> > OK, let's see what I can stir up today.  :)
> >
> > I am looking for a clear, succinct definition of surface insulation
> > resistance.  If I look in IPC-9201 "Surface Insulation Resistance
Handbook",
> > I don't find what I am looking for. (Sorry, Joe, Doug et al).  What I
find
> > on page 2, section 2.1, Definitions is the following: "It represents the
> > electrical resistance between two electrical conductors separated by
some
> > dielectric material(s).  This property is loosely based on the concept
of
> > sheet resistance, (see ASTM-D-263), but also contains elements of bulk
> > conductivity, leakage through electrolytic contaminants, multiple
dielectric
> > and metallization materials and air."  Whew!
> >
> > Any takers?  Hmmm?
> >
> > regards,
> > Bev Christian
> > Research in Motion
> >
>
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