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August 2002

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Mon, 12 Aug 2002 11:29:18 +0800
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Graham, Doug and other wise persons on this topic,

In the absence of an absolute cleanliness figure for boards, for all the
goods reasons given, is there then a reliable procedure / formula /
guideline or even recommendation that could tell me how to determine what
the cleanliness level for our boards should be? i.e. instead of giving us
the fish (or not), can you teach us how to fish so that we can catch our
own?

If we can at least calculate what the cleanliness figure SHOULD be for,
say, our multilayer class 3 I/O board, made of FR4, finished with HASL,
ENIG, ImAg, Immersion Tin, etc, with an LPI solder mask, assembled with
water soluble fluxes, coated with an acrylic, used in a tropical climate of
high humidity and high ground temperature, or over the North Pole for up to
15 years, with a known vibration profile for the 'plane it's being used in,
etc., etc., - if we can calculate a cleanliness figure for that, then we
know what we have to test for.

The only questions then are "what are the best cleaning machines and
methods to achieve that figure?", and "what are the best equipment and
procedures for measuring that the figure has been achieved?".

Is the SEC test all-embracing - i.e. is it actually a fair measure of how
clean a board is of all 'hazzardous' contaminants, or are there other
contaminant types that the SEC test won't recognise but still cause
problems with boards in the field? Should we be conducting a series of
cleanliness tests in order to cover all bases? In other words, should
different cleanliness tests be used depending on the chemistries that went
into the boards' manufacture.

Is bare board cleanliness different from assembly cleanliness, should it be
and, if so, why?

Peter
(Assistant pot stirrer)



[log in to unmask]   09/08/2002 11:08 PM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to dopauls

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: Re: [TN] Board cleanliness








Graham,
May I respectfully suggest you switch to the decaffeinated tea!

You  bring up some good points, but Susan did not write the MIL-P-55110,
nor did Bev write the Bellcore document.  They, like most industry
standards have been with us a long time, and have evolved over time.  They
retain the old values until someone gives them a better test and a better
value to put in its place.  The same can be said of IPC specs, and I
suspect, ISO specs as well.

Take a look at J-STD-001, Rev C.  What does it have as a default assembly
cleanliness level?  1.56 ug/cm2.  Same as 1980 mil specs.  The most forward
looking part of the cleanliness requirement is that it takes the stance
that cleanliness is whatever you define with your customer.  The 1.56 value
only comes into play if neither you nor your customer have a clue as to
what it SHOULD be.

Why does this modern spec have values we know to be antiquated?  Because
the people on the committee (and I am one of them) knew that a one size
fits all cleanliness requirement for assemblies was ludicrous.  Think about
it.  Would you have the same required cleanliness for a garage door opener
as you would for a pacemaker or a flight controller?  The same argument
applies to bare board cleanliness.

IPC-60XX series for bare boards are pretty good documents, but they use the
old mil spec levels until someone else gives them something better to put
in its place.  You will have different board cleanliness requirements for
class 1, 2, 3 boards, different for high speed, RF, high voltage, high
power, etc.  The best that I think you can hope for is a set of recommended
guidelines as starting points if you have no current information (which you
can find now in J-HDBK-001, section 8), and a standard protocol for
determining how clean YOUR boards need to be for YOUR application for YOUR
end use environment.

I have talked with many companies about their bare board cleanliness
requirements.  Some use the 1.0 ug/cm2 figure in Bellcore.  Some use values
of 0.5 ug/cm2 because "they want to be twice as clean as Bellcore" (you
cannot imagine the stunned silence on my end when I first heard that).
Other use values like 0.02 as Graham mentions.  Ask them why they have such
a value and you will usually get a puzzled expression and an "I dunno".
Some companies, like Delco Electronics, have put a great deal of time and
energy into determining how clean their boards have to be with good science
behind the decision.

If you are waiting for a single value that you can put in your board
purchase specs - AIN'T GONNA HAPPEN.

Doug Pauls
Rockwell Collins
(Who, in all fairness, should seriously consider decaffeinated Mt. Dew)







Graham Naisbitt <[log in to unmask]>@ipc.org> on 08/09/2002
09:19:50 AM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
       to Graham Naisbitt <[log in to unmask]>

Sent by:    TechNet <[log in to unmask]>


To:    [log in to unmask]
cc:

Subject:    Re: [TN] Board cleanliness


Hi Bev and Susan,

Permit me a  W O A H!

This is such a misleading spec. To say that something should be cleaned
to a level of less than 1.5 microgrammes / cm2 also means that it is
acceptable to leave UP TO 1.5 microgrammes of NaCl equivalence on every
square cm of your assembly! But if you are manufacturing to modern
fine-line, fine-pitch and components to suite....then that level will
almost certainly be too high.

I have many customers who have, as a result, imposed their own empirical
cleanliness value at something less than 0.02microgrammes/cm2 for Class
3 assemblies. As has been stated already, the ionic extract cleanliness
test is/was only intended to be used for process monitoring. The
Pass/Fail can and has got a lot of people into troublesome problems.

I also register concern at the use of "hot" ipa / water mixture - safety
issues apart, there has been a good deal of research demonstrating that
prolonged immersion in such a solution can induce the bromine flame
retarder to rise to the surface of the board and....this is a problem!
Would you care to comment?

Regards Graham Naisbitt

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> -----Original Message-----
> From: TechNet [mailto:[log in to unmask]] On Behalf Of Bev Christian
> Sent: Friday, August 09, 2002 03:44
> To: [log in to unmask]
> Subject: Re: [TN] Board cleanliness
>
>
> Patrick,
> Susan mentions a standard (US military MIL-P-55110E), but I
> would like to point out another standard - GR-78 CORE from
> Telcordia, which is used extensively in the
> telecommunications industry.  It requires that the finished
> bare board cleanliness be less than (or equal to?) 1
> ug/square centimeter.  For you Americans, that's 6.5
> ug/square inch.  The standard goes on to state that for a
> finished assembly that has gone through a wash cycle that the
> circuit pack have a SEC value less than (or equal to?) 1.5
> ug/square centimeter.  There is no standard for no-clean
> boards, but then there are other criteria that the no-clean
> flux/solder paste must meet first
> - copper mirror, halide ion spot test, SIR and
> electromigration. Some companies still require a fluoride ion
> spot test and SIR/EM testing of possible mixtures of wave
> soldering fluxes/solder pastes/cored wires. And some
> companies have set process indicator levels for SEC testing
> of circuit packs manufactured with no-clean fluxes,
> especially for wave soldering. Note, I said "process
> indicator", so don't bring fire and brimstone down on me,
> fellow Technetters!  :)
>
> regards,
> Bev Christian
> Research in Motion
>
> -----Original Message-----
> From: [log in to unmask] [mailto:[log in to unmask]]
> Sent: August 8, 2002 4:41 PM
> To: [log in to unmask]
> Subject: [TN] Board cleanliness
>
>
> Hi Technetters,
>
> I would like to know how to specify board cleanliness on the
> Fab drawing. We use class 3  boards. Does it make any
> difference on cleanliness standard? Is it OK to ask for
> cleanliness test results/reports shipped with every lot?
> Which department in a company should be responsible for this
> board cleanliness issue, R&D, Quality or Production?
>
> Thanks in advance to everyone sharing this information.
>
> Best regards,
>
> Patrick
> ----------------------------------------------------------
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