Brian,
Thanks for the information about the background to the 100 Mohm criterion. I
suspected it was something like that. It is not uncommon that the limits of
the test equipment set the acceptance criterion. A big problem is that this
information is not given in the standard so it is impossible for a user to
judge the relevance of the requirement. Although one can understand that
standards organisations do not want to admit that the scientific base for
the criterion is so poor. Most people, I believe, are convinced that if a
criterion of 100 Mohm is given in the standard, there must have been done an
investigation showing that this is what is required to have a reliable
product, and since it has been used for so long time without no one
questioning it, it surely must be a well-grounded criterion. But then we are
back in a discussion held previously at TechNet of what information should
be given in standards and I have a feeling of that I am a good way on
qualifying among the heretic (probably did already in my first email).
I liked the iceberg metaphor and the proposal of how to get it moving and
agree to 100 %.
Cyprus, yes that would be nice. I have to talk to my wife about that.
Per-Erik Tegehall
IVF
> ----------
>
> Per-Erik
>
> Thanks for your support. I knew, when I wrote what I did, that I was
> opening a big can
> of worms and that many would be shocked and a few thinkers, like yourself,
> would agree,
> at least partially, with what I admit were my controversial statements.
>
> However - and, unfortunately, in my state of semi-retired bliss, I cannot
> substantiate
> what I say with references as I have been forced to reduce my library from
> an ad-hoc
> room lined with shelves to one small bookshelf - I think I may be able to
> supply the
> answer to the 100 megohm dilemma. It was simply the practical limit that
> electrometer
> valve (tube, west of the Atlantic) instruments could measure (do I
> remember right? Was
> it the Philips EF80?) before the FET was invented? At that time, most
> people used the
> Megger, being far cheaper than electrometer systems. This was a
> magneto-type device
> where you turned a handle until a clutch slipped, when it was supposed to
> be generating
> a very rough waveform 100 V or 500 V, depending on the model. A double
> moving coil
> without hairsprings would measure the volts and amps and indicate the
> insulation
> resistance. The meter scale was non-linearly calibrated and the 100 megohm
> mark was the
> first one, about 1/2 mm from the infinity mark and the same again from the
> 50 megohm
> mark. The crunch was that turning the handle transmitted sufficient
> movement to the
> instrument, even if you pressed it hard onto a table, that the meter
> needle would
> oscillate by about +/- 2 mm, so the 100 megohm pass/fail criterion was
> little more than
> an inspired guess. It is also because the Megger was available in 100 and
> 500 V
> versions that these voltages were used in all the early specs - and to
> hell with all
> notions of voltage gradients. This is what I mean about inherited
> legacies. Those were
> the days, my friends, and, unfortunately, I am old enough to remember
> having used these
> methods! :-( For you younger guys, you will be able to post on the
> Technet, in due
> course, that we used to interconnect with a putrid system called the
> printed circuit
> and we actually used lead solder to fix the the components, believe it or
> not: your
> turn cometh! :-)
>
> Now, coming back to the clean slate: how can we get this moving?
> Unfortunately, as an
> old has-been, I am no longer financed by anything other than the pension I
> live on plus
> the odd consultancy job. My moving and shaking is therefore more or less
> limited to
> this beautiful island. Unless... However, my first imperious suggestion is
> that we coin
> new terms to replace all the old ambiguous ones and define them absolutely
> with
> scientific rigour. Then, before we can start thinking about new standards,
> we need a
> scientific study of what all these phenomena are all about and the
> contributions of
> such things as the ions in the substrate materials, the effects of the
> lack of
> homogeneity (reinforcements), the effects of curing the substrate resins,
> the effects
> of humid air (far from negligible), etc. Parallel with this, we should try
> to correlate
> reliability with both the theoretical and practical parameters. Only then
> should we
> start to formulate standards. Believe me, guys, what I know about SIR -
> and I venture
> to suggest the same applies to my illustrious colleagues on this thread -
> is just the
> veriest tip of the iceberg. But I am stating a fact and not just blowing
> my trumpet
> when I state that I know a helluva sight more about the subject than 80%
> or more of the
> guys who use the technique, slavishly adhering to standards that date from
> cable
> insulation resistance, but not understanding a thing. OK, this is a
> zillion dollar
> programme I'm talking about but it will all have to come out in the wash,
> sooner or
> later (especially when the EU start issuing Directives on the subject!!).
>
> If I can help, I will.
>
> Per-Erik, if you happen to be thinking of taking your holidays in Cyprus
> this year,
> please let me know and we could get together (this is not a spam for
> island holidays!!)
>
> Brian
>
> Tegehall Per-Erik wrote:
>
> > Brian, Doug, Graham,
> >
> > I have just read last weeks discussion with great interest since this is
> my
> > favorite subject.
> >
> > I agree that using SIR for qualifying a flux with a bare board with
> copper
> > comb patterns is of little value. Passing such a test is no guarantee
> for
> > that it will not cause SIR problem on a product when combined with other
> > contaminants (as shown by Adams et al., Circuit World, Vol 20, No 2, pp
> > 41-44, 1994) which I think you all agree with. Still, the test could
> have
> > some value since it could be used to sort out bad flux candidates early
> in
> > the developing process before too much money has been used up. To assure
> > that a flux will not cause any reliability problem on a product, it must
> be
> > evaluated using a test board representative for the materials and
> processes
> > that will be used for manufacturing the product, as required in
> J-STD-001B,
> > Appendix D. Thus, I am a devoted supporter of the methodology in
> Appendix D
> > using representative test vehicles. BUT...
> >
> > Something is seriously wrong with Appendix D. In the minutes from the
> > Soldering Subcommittee at PC/SMTA Electronics Assembly Expo '98, it was
> > concluded that "the industry appears to be ignoring the materials
> > compatibility part of 001B more and more. Few have used 001B Appendix D
> for
> > process qualification and those who have used it have not passed the
> test".
> > This means that either can we expect poor reliability of today's
> electronics
> > or else is the test method and the acceptance criterion not relevant. I
> > believe it mainly is the latter, although that does not excluded that
> many
> > products might not have an acceptable cleanliness.
> >
> > What is then wrong with the method and the criterion? I cannot but come
> to
> > the same conclusion as Brian. The problem is that there is practically
> no
> > scientific base for neither the test method nor the criterion. The whole
> > business is so full of distorted definitions, misunderstandings,
> > non-scientific practice and criteria that the best thing would be, to
> use
> > Brianīs words, "start again with a totally clean slate".
> >
> > There are many things that can be questioned. I will give some examples.
> > First, just measuring SIR is a problem since the measured currents are
> very
> > low. Perhaps more important is the fact that the measuring of SIR will
> > affect the measured value. In 1996, Chan showed that the contamination
> of a
> > board with 10 microgram per sq. inch table salt caused a decrease of the
> SIR
> > from 10 Gohm to 5 Mohm. When a bias of 100 V DC then was applied to the
> > contaminated board, SIR increased to 40 Mohm after 60 seconds, to 1 Gohm
> > after a few hours and to 8 Gohm after 100 hours. This is the reason for
> the
> > practice of having a stabilization time of usually one minute. Despite
> that,
> > this will create some uncertainty in the measured value. Another
> interesting
> > point is that after 100 h of applied bias, the SIR was about the same
> for
> > the contaminated board as for a clean board. This means that you cannot
> > discriminate a rather highly contaminated board from a clean one, not
> what
> > you would expect of a good method. The increase in SIR is brought about
> by a
> > depletion of ions due to the DC bias causing migration of the ions. By
> > reversing the bias, the ions will migrate in the opposite direction and
> > cause a decrease of SIR. Hence, many methods prescribe that measuring
> > potential should have reversed polarity and the measurements should be
> taken
> > after a stabilization time of one minute. In the investigation performed
> by
> > Chan, he showed that after reversing the bias, it might take one hour
> before
> > the SIR is affected and 10 hours before minimum in SIR is reached. So
> using
> > reversed measuring bias is a good method to assure that you have no idea
> of
> > what you have measured.
> >
> > Then we have the very high polarization and measuring voltages
> prescribed by
> > most standards. Is that really relevant for products using voltages of 5
> V
> > or less? Brian has written many good papers about this.
> >
> > What surprise me most is that, through the years, I have seen
> practically no
> > discussion of the relevance of the acceptance criteria in standards. The
> > criterion in Appendix D is that the SIR measured after 96 and 168 hours
> > should be minimum 100 Mohm (for non-coated assemblies). The criterion is
> the
> > same independent of the type of application and severity of the service
> > environment. It is quite obvious that that is not how it should be.
> Also,
> > bearing in mind the result found by Chan that a board contaminated with
> 10
> > ug/sq. inch had about the same SIR as a clean board after 100 h, it
> seems
> > strange to only have requirements for SIR measured after 96 and 168 h
> and
> > completely disregard the first 96 hours.
> >
> > The 100 Mohm requirement for SIR is not new. I have tried to track the
> > origin of this requirement and hoped for finding the reasons for why
> this
> > has been chosen as acceptance criterion, but with little success.
> > Interestingly, in an IPC technical paper from 1985 (IPC-TP-542) it is
> > mentioned that the practice at that time was to require a SIR of 100
> Mohm
> > but then only as a requirement for initial resistance. Why was that
> changed?
> > I cannot get rid of a feeling that this was done because it was to hard
> to
> > pass that requirement and not because it was found to be more relevant.
> Just
> > like the SIR requirements for printed boards once was lowered because
> > otherwise it was to difficult to pass the requirement for HASL boards
> when
> > they become popular to use. So how do we know that the present criterion
> is
> > relevant? Since I failed to track the origin of the 100 Mohm
> requirement,
> > but also the reason for changing from having a requirement on initial
> > values, I would appreciate if someone could shed some light to this.
> >
> > Much more could be said about the non-scientific approach of the present
> > standards for SIR testing but I think this contribution already has
> become
> > to long as it is. To sum up, I give Brian my vote for starting with a
> new
> > totally clean slate.
> >
> > Per-Erik Tegehall
> > IVF
>
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