Peter,
I think Doug and probably Brian will answer most of your questions (with at
least a few "it depends"), but I will chime in on a few.
In-situ techniques are SEM/EDX and Fourier Transform Infrared Spectrometry
(FTIR). However, (oh don't we love THAT word) the former will only give
you elemental mapping, so you don't know whether the material is in ionic
form or not. Conventional FTIR can ideally identify covalently bonded
compounds. Why so many modifiers in my sentence? Because if you have a
large number of materials, like more than three, FTIR by itself cannot
deconvolute the superimposed spectra to tell you what the individual
compounds are. Can a chemist make some guesses? Probably. Also simple
salts like sodium chloride or potassium bromide do not have pure covalent
bonds and the lattice vibrations are so low in frequency that the optical
material and materials in the atmosphere of your spectrometer will absorb
all the radiation in the band width of these vibrations and you will never
see them. Also, both of these techniques are line-of-sight so if your
residue of interest is under a component that you can't tear off the board,
forget it.
Some companies have also perfected techniques for building little dams on
printed circuit packs to puddle solvents for local extractions for ion
chromatography, but this is time consuming and very expensive as a result.
Looking forward to other replies.
regards,
Bev Christian
Research in Motion
-----Original Message-----
From: [log in to unmask] [mailto:[log in to unmask]]
Sent: August 13, 2002 6:38 AM
To: [log in to unmask]
Subject: Re: [TN] Board cleanliness
Doug Wise-Guy-Person
Very many thanks for the time you've taken to share such a full and
valuable response to my questions. If I may, I have some supplementaries:
1. I have a theory from what you've said that may stand some debate. If the
two main elements of this problem are (a) identifying residues on a board
and (b) carrying out temperature/humidity tests to failure then analysing
the failure modes, then I think it is best to try and identify the residues
in situ. By identifying them in situ, one can them build a 'map' of where
they're located on the board and this can be used for correlation to the
failure results. What is your opinion on this?
What are the main drawbacks to measuring in-situ and measuring extracted
residues? The main drawback to measuring extracted residues are twofold, as
far as I can see - (i) you don't know where on the board they came from and
(ii) you don't know local concentrations. Can one tell concentrations by
measuring in-situ, though?
Do you (or anyone else) know if there is a software package that can map
contaminant locations and concentrations/levels on a board? What measuring
equipment can the computer be linked to to provide this map?
2. Residue problems seem to fall largely into two camps - corrosion and
dendrite growth. Is this fair to say? Do you need to be much of a chemist
to determine if/when residues are likely to be detrimental to adjacent
materials that are part of the board, or is there a good layman's
guide/chart to indicate reactivity?
3. You talked generally about the expected levels of various ionic
contaminants that might be found on boards of a certain material, with a
certain finish, etc - are you talking about acceptable levels after
cleaning or levels that might be detected prior to cleaning? It's what
post-cleaning levels need to be for reliability that we're interesting in
determining. They seem to be pre-cleaning levels, so we're still stuck with
finding reliable post-cleaning levels.
4. Can you comment on whether or not IPC-TM-650 covers all known test
methods, or are there other good ones outside of that spec? How might I
reliably find out about them, given that this process is only a small part
of my working life and I work largely alone (not many/any people here seem
particularly interested in the minutiae of processes, so I don't know how
they determine reliability and causes of failure without this
understanding)
5. What I was trying to get at with the PCB vs. PCA thang is should the
acceptable cleanliness level for a PCB be different (cleaner) from the ACL
for an assembly and if so, why. How clean do I tell my fab house to make
the bare boards?
6. Following right along, if the bare board residues are so hard to remove
come assembly time, what contaminant testing/measuring methods are
effective? - surely not the extraction type, or I would use the measuring
equipment to clean the board.
Enough for now (more than, probably)
Peter
<[log in to unmask]> 12/08/2002 10:37 PM
To: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group@ST Domain
cc: [log in to unmask]
Subject: Re: [TN] Board cleanliness
Graham, Doug and other wise persons on this topic,
**Wise person? Never. Wise guy.... maybe......
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?
**Well, when going after walleye, the king of good-eatin' fish, I prefer a
white Mr. Twister on my flyline and..... oh, wait, you were speaking
metaphorically.....never mind.
**In order to understand what value your boards SHOULD be, you need to
understand two things: (1) the residues present and (2) the effects of
those residues on circuit board reliability in your end use environment.
**In order to understand (1), you need to understand the residue detection
methods, their pros and cons, and the proper time and place to use them.
These are merely tools and you have to understand when you can and can't
use them. Can't turn a phillips head screw with a 15 mm box end wrench,
that sort of thing. To understand (1) you need to think about the soils
that are present on a circuit board and how you measure them. Do you
measure them in-situ, as with surface reflectance FT-IR, or do you extract
them for analysis? Each has its merits and drawbacks.
**And, after you understand (1), you must determine what those residues
mean and if the residue levels found constitute a hazard to reliability.
So you find you have polypropylene glycol on your boards. So you find 15
micrograms of bromide per square inch on your assemblies. What does it
mean? Residues will generally negatively impact reliability in an
electrochemical failure mechanism. You need an ionic contaminant, a power
source, and moisture to fuel such a failure mechanism. Hence, you would
need to do some accelerated testing in a humid environment to determine if
the residues constitute a problem. What is a proper accelerating test?
Good question, wish I knew the answer. Do you have an environmental stress
test that you normally do to catch infant mortality kinds of failures?
Here at Rockwell Collins (maker of the worlds finest avaiation electronics,
thank you very much), we use thermal cycling on product before it goes out
the door. An accelerated test would be to do this kind of cycling under
power until failure occurs. Alternative, you could subject your hardware
to Bellcore conditions (35C, 85% RH) under power until failure. Look for
signs of dendritic growth, corrosion, or electrical leakage or false
function. Do a correlation study between the residue detection method and
the electrical accelerating test so that you can get an idea of what is a
good residue and what is a bad residue and where the breakpoints lie. What
is an acceptable amount of time in an accelerated test? Another question I
wish I had the answer to. Go as long as you can afford. 1000 hours has a
nice ring to it. As my good friend John Sohn once said "Every company has
to go and do the HARD work".
**To go back to your metaphor, I can't teach you how to fish because it
depends on freshwater or salt water, pan fish or trophy fish, spring summer
or late fall, US, Carribean, etc.....I could talk about how I fish, but
others have acceptable ways of fishing too. You know, now I gonna be
thinking about walleye fishing all day.......
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.
**If you are looking for a starting point, look in J-HDBK-001, section 8.
The values for ion chromatography come from CSL and represent a fair amount
of experience in determining what cleanliness levels should be for various
flux technologies. We found that for epoxy-glass boards, solder masked,
with tin-lead metalization (HASL or fused), chloride levels should be less
than 2.0 micrograms per square inch, bromide less than 12 micrograms per
square inch and sulfate less than 3 micrograms per square inch. For boards
with alternative plating (cold plated processes) like silver or palladium
or gold, chloride should be less than 1.0 micrograms per square inch,
bromide less than 10 ug/in2 and sulfate less than 2 ug/in2. These are for
ion chromatography per IPC-TM-650, method 2.3.28. You can look at the CSL
website (www.residues.com) if you want to go into more depth. I should
stress that these values are a good starting point if you have no clue.
You still have to do the correlation studies to determine what the residues
mean to your product.
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?".
**You would find as many answers to this question as you would people to
ask. Personally, I like the chemistries from Kyzen Corporation (we use
Aquanox SSA and Ionox FCR) and Envirosense (Envirogold 816), with in-line
aqueous cleaning. I like batch cleaning less and hand cleaning least of
all. It all boils down to the soils to be removed (i.e. what fluxes are
you using) and the susceptibilities (e.g. water intolerance) of the
hardware being cleaned.
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 it all embracing? Heavens no. Is it very common, yes. There are a
host of problems with SEC testers and you can go back through the Technet
archives and find where either I or Brian Ellis has expounded on the topic.
IPC also has some technical papers on the topic. I have seen many cases
where an ionic cleanliness tester found substrates clean as a whistle, only
to corrode and fail in the field. SEC testers are for process control, not
product acceptance, although they are frequently used for the latter.
Should different tests be used? Yes, more data points are better than one
data point or no data points, but the testing must be well founded. As
Susan Mansilla indicated recently, she views the IPA/water extraction as
bringing too much bromine to the surface and skewing the test. Maybe,
maybe not.
Is bare board cleanliness different from assembly cleanliness, should it be
and, if so, why?
**I would say yes. In my experience, residues that are on bare boards tend
to get locked in once the boards go through the first reflo cycle. They are
very difficult to remove after that, so even if you have cleaning in
assembly, you may not be able to remove the fabrication residues.
Determining bare board cleanliness is easier than assembly cleanliness
because there is usually no complicating factors like residues from
components or flux residues. Determining how clean bare boards need to be
is harder, since it depends a great deal on your materials sets, your
assembly process, and you end use environment. How much cleanliness margin
do you need for your process? How robust are your assemblies to residues?
There is no easy way to do this. This is the HARD work.
Doug Pauls
Rockwell Collins
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