Graham
You asked for a reply, so here goes:
1. Where I'll be going, the heretic will be burnt sooner or later!
2. I'm glad you said British and not English: otherwise, I'd be after you.
3. Keep moving? I was 35 years in Switzerland. If that does not show staying power,
please tell me what does. At least, in Cyprus, I get the number of days of sun
tripled, so that I can look after my orange trees.
4. Target to hit - I was always that: the price for being outspoken. I must shamefully
admit that, over the past 20 years, I have become an even easier target, having put on
some 30 kg of pure fat and not an ounce of muscle.
5. IC v. SIR v. ionic contamination testing: each of them serve their purpose and they
are certainly not interchangeable. With IC, you can measure the quantities of
**specific** ions, provided you have the appropriate columns/detectors. It does not
measure ALL ions. For example, if you set it up to measure, say, Cu, Sn and Pb as
cations and Cl, Br and SO4 as anions and somebody neatly salts your assembly (e.g. from
an onshore storm or from the salt aerosol thrown up by tyres on a salted road), then
you will measure nothing. You need numerous columns to get the overall picture. It is
also very difficult to measure some organic anions. On the other hand, ionic
contamination testing will give an indication of the aggregate of all the ionic
species. It has the supreme merit of being the only practical process control method.
SIR, as I've indicated in earlier postings, measures an aggregate of different
resistances in parallel, some of which are contamination related, some of which are
not. There is great confusion as to the definition of SIR. It is the resistance, as the
name implies, of the surface insulation of a substrate. It is NOT detecting the
presence of dendrites. That is ECMR (electrochemical migration resistance). Neither
will give you a picture of the reliability of a given assembly due to contaminants, at
least without many qualifications. To illustrate this let me present a hypothetical
situation:
A flux (or paste) is qualified using one of the standard IPC test patterns and the
overall fluxing/soldering/cleaning process is seen to be OK. On the basis of this,
production starts. One of the components is a large fine-pitch QFP with a very small
standoff. First of all, residues enter under the component. They are partially removed
during the cleaning. The resultant diluted liquor is pushed to one side by the air
knifing, where it slowly evaporates off but not before it has dissolved some ionic
species from the component itself. As the volume diminishes, the liquor will, by
surface tension, concentrate itself in the spaces between the QFP leads, exactly where
the resultant dried contamination will cause the most harm. No amount of SIR testing
will forecast the reliability when this happens - and happens it does. Furthermore, the
substrate used may be the same as the IPC test coupon, in that they are both FR-4, but
that does not mean that the epoxy chemistries are the same and the reaction between the
flux and the substrate may be VERY different, even by more than an order of magnitude
from make to make of FR-4, let alone other substrate types. Why this difference? Simply
because the physico-chemical bonds between the flux residues and the substrate are
different: this depends much on the epoxy chemistry and the MW of the surface
molecules. I have observed half an order of magnitude difference between different
batches of the same substrate and, I suspect, possibly a difference on whether the
laminate was pressed on the outside or the inside of a packet.
In other words, there is no way that you can predetermine the reliability of an
assembly. We have imperfect tools, each one of which may give us a small piece of the
jigsaw puzzle, but a lot of the pieces are missing from the puzzle box.
The big weakness of SIR is the time it takes to conduct a test. In most factories, the
assemblies would already be delivered to the customer before a test is terminated. It
is therefore useless as a process control tool. However, in 1988, I did propose an
overnight non-destructive SIR test for cleaned assemblies, but this never caught on.
Retsina is actually a soldering flux. However, the clean slate is proposed to rid
ourselves of all the bad legacies SIR and ECMR have been associated with, and still
are. Basically, SIR is a function of the substrate and not of the contaminants and,
until this realised, confusion will reign - for ever and ever. I know I'm not alone in
this school of thought: there are several well-known names within our industry who
agree with me.
Now, to set the ball really rolling: a) the de Bono type corrosion tests are similar to
ECMR in that they do not reproduce the conditions obtained in practice and b) what is
the influence of substrate chemistry and contaminant chemistry on the electrical noise
generated?
Going from Latin to French, bonne chance (it's lucky I cannot e-mail you in Greek!)
Brian
Graham Naisbitt wrote:
> Good Morning everyone,
>
> Doug, Graham is not more restrained than you - I've been out for a couple of
> days visiting customers! However, burning the heretic Brian, is perhaps a
> little strong - even for you. Always bear in mind that Brian is a British
> exile first (to my knowledge) to Switzerland now to Cyprus. SIRBrian has to
> keep moving because that makes him a more difficult target to hit!!
>
> Scott, sorry I had to get that bit out of the way first.
>
> You state:
>
> > I have been observing this thread with interest. We run standard SIR
> > coupons monthly through our production floor to monitor our cleaning
> > processes (SMT, wave, DI wash, water-soluble flux chemistry, standard
> > production processes and handling).
>
> Can you please inform me the method you use, the type of coupon and is the
> test pattern overmounted with components?
> Are you running this test using coupons from each process stage or simply on
> "completed" coupons?
> >
> > My question: Is all of this necessary? Would it be possible to gauge
> > acceptable cleanliness levels based on another type of measurement, such
> as
> > Ion chromatography? Or does IC miss things that SIR picks up?
>
> Is it necessary? Well what is the level of deviation you have seen from your
> SIR results?
>
> IC will inform you EXACTLY what is on the surface - but, unless you are Doug
> it cannot inform you if the product is reliable.
> SIR will inform you if the product is reliable, it cannot inform you what is
> on the surface that may be causing a reliability problem.
>
> >
> > It's plausible that localized contamination, say underneath a low profile
> > QFP, might not raise the overall contamination levels much, but would have
> a
> > detrimental localized affect.
>
> Agreed. As I always point out, the industry desire to use a simple pass /
> fail ROSE level of <1.5microgrammes/square centimeter NaCl equivalence (in
> line with another current TechNet thread that Joyce posed) also implies that
> it is acceptable to leave UP TO that level on every square centimeter of
> your assembly. If you manufacture fine-line fine-pitch and / or use CSP
> devices, then this level will almost certainly prove to be too high. As an
> example, I know that certain sections of IBM employ a level < 0.2.
>
> In respect to Brian's suggestion to "start with a clean slate", I cannot see
> why, unless you have discovered a new application for Retsina? The NPL
> carried out an extesive research project some years ago, into different
> methods of defining "clean, reliable" (my interpretation) electronic
> assemblies. They listed some 22 different methods most of which are
> destructive.
>
> The 2 best were considered IC and SIR. There was also a consideration for
> Impedence Spectroscopy, but that hasn't materialised yet.
>
> As I have just stated, there is an application for both methods to be used.
> SIR to inform you the level of reliability by its measuring the degree of
> current leakage from REPRESENTATIVE coupons processed in parallel with your
> production. This at least seem to be what Scott is doing at Benchmark.
>
> Then if a problem is discovered, IC would be useful in determining the EXACT
> nature of the dendritic failure.
>
> And if my Latin isn't too rusty - Brian suggests he is "I am (either) a
> develish (or) diabilical advocate (lawyer or attorney) who is in the dog
> house!?"
>
> As I said in my old latin school book from dumpty-dum years ago: Latin is a
> language as dead as dead can be, it killed the ancient romans and now its
> killing me".
>
> I look forward to Scotts reply - and no doubt Brian and Dougs!!
>
> Regards,
> Graham Naisbitt
>
> [log in to unmask]
>
> WEB: http://www.concoat.co.uk
>
> CONCOAT Ltd
> Alasan House, Albany Park
> CAMBERLEY GU15 2PL UK
>
> Tel: +44 (0) 1276 691100 Fax: +44 (0) 1276 691227
> > -----Original Message-----
> > From: Brian Ellis [mailto:[log in to unmask]]
> > Sent: Thursday, February 24, 2000 10:36 AM
> > To: [log in to unmask]
> > Subject: Re: [TN] AW: [TN] components cleanliness
> >
> >
> > Doug
> >
> > I thought that one would get you to rise to the bait :-) Am surprised
> Graham
> > hasn't reacted, either.
> >
> > Seriously, I consider this an important problem as very few assemblers
> test
> > the
> > bare boards they receive, let alone the components. I consider that there
> > are
> > also many other ignored parameters, some of which I brought up during
> > various
> > IPC and ISO WG meetings, to be shot down - for the most part - in flames.
> >
> > Unfortunately, I do not have an answer to all these problems. I'm a great
> > one
> > for actually putting the test patterns on the real PCBs, as close to
> > critical
> > components as possible, but even that is less-than-ideal :-(
> >
> > If the truth be told, I honestly believe that we should forget all we have
> > in
> > the way of ECMR and SIR standards and start again with a totally clean
> > slate,
> > beginning with the basic definitions which have become so distorted as to
> be
> > misleading. There are FAR too many legacies from test methods and
> standards
> > that
> > date from 30, 40 or more years. Not only did we not have an understanding
> of
> > what it was all about, then, we didn't have sufficiently good instruments
> > and
> > even most of those available today leave a lot to be desired. I have never
> > seen
> > an in-depth scientific analysis of all the mechanisms that can influence
> the
> > results. From time-to-time, I've brought up some practical observations,
> > over
> > the past 20 years, some of which have created discussion and others,
> > yourself
> > likewise, have done the same. Furthermore, we need to address present
> > issues,
> > let alone future ones. Who, e.g., has done anything about SIR on HDISs
> with
> > track spacings down to 25 um?
> >
> > Advocatus diaboli sum (and probably in dome canis est, by your lights!).
> >
> > Best regards
> >
> > Brian
> >
> >
> >
> > [log in to unmask] wrote:
> >
> > > In a message dated 02/23/2000 3:09:10 AM US Eastern Standard Time,
> > > [log in to unmask] writes:
> > >
> > > >
> > > > This is the main reason why SIR testing is, in many cases, useless to
> > > > qualify a flux: you qualify it under lab conditions, totally ignoring
> > > > the fact that its behavious will alter in practice due to it mixing
> > with
> > > > external contaminants not present on your test vehicles.
> > > >
> > >
> > > SIR USELESS? Gasp!! Burn the Heretic!!!!!!!!!!
> > >
> > > SIR can be useful, even as a lab tool, you just have to use the right
> > > combination of materials, processes, environment and data
> interpretation.
> > I
> > > would agree that we have a long way to go before our existing methods
> are
> > a
> > > good and true measure of field performance.
> > >
> > > Doug Pauls
> >
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