Hi, Brian,
I share in your lamentation over the demise of the much more accurate
contaminometer. I have previously spoken at some length with Graham
Naisbitt about the "current" contaminometer model, the CM11. It's main
advantage is that it's cheaper than most comparable instruments on the
market, but not demonstrably different or better than others (sorry,
Graham) inasmuch as it suffers from the same limitations as all the others.
i.e. it's designed to be compliant with IPC and MIL specs and nothing
better.
I think your much more sensitive instrument was allowed to die for a good
"Chicken and Egg" reason- folks didn't perhaps understand the nature of the
cleanliness testing process, and if they simply wanted a machine to tell
them if their boards passed IPC or MIL Std specs, the "normal" machines
would do that, so why buy a machine that was doing the job differently
and/or, (dare I say?) more expensively? Actually, I've no idea if it was
more expensive than other machines or not, but it might be a reason why it
isn't still around today.
Today, though, we do need machines that are more sensitive. Someone in an
earlier post mentioned requiring one with a starting resistivity of 450
MOhm-cm. I would welcome that, but it would require a whole different
exrtract solution to reach that level of resistivity and react linearly to
contamination. The old DI Water:Alcohol mix still has some life ahead of it
if the ROSE Tester software could be recoded to set a starting cleanliness
level at around 250 MOhm-cm rather than 150 MOhms that's the current upper
limit for most machines at present. 250 MOhms is about the limit for this
type of solution (75% Alcohol : 25% DI Water), but it would allow much
smaller levels of contamination to be measured accurately than present
set-ups are capable of.
Brian, large respect to your knowledge and experience but may I dare to
challenge you on a point? You also talked of 50:50 test solutions, and at
one time I would have agreed with that ratio as being better than 75:25. I
changed my mind because of two factors:
1) The lower proportion of alcohol seems to reduce the achievable
resistivity level of the test solution, thereby reducing the sensitivity of
the machine that's so crucial to measuring small amounts of contamination.
2) There's a trade off when determining the Alcohol to Water mix between
how much alcohol is required to dissolve non-water-soluble residues to
release trapped water-soluble residues, and how much water is required to
wash off the water-soluble residues that have been freed by the alcohol. My
reading on the matter indicates that 50:50 mixtures were succeeded by 75:25
mixtures as they were found to be more effective, especially when there
isn't much residual contamination to wash off anyway. (That's presuming the
cleaning process is working to required levels).
Thanks for joining in the skeet shoot.
Peter
Brian Ellis <[log in to unmask]> 26/05/2003 02:12 PM
Sent by: TechNet <[log in to unmask]>
Please respond to "TechNet E-Mail Forum."; Please respond to Brian Ellis
To: [log in to unmask]
cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
Subject: Re: [TN] ROSE for Flip Chip
Peter
To some extent I agree with you, but not entirely.
Using the pasted area is no better than using the board area. The paste
is deposited on metal. The danger lies on substrate and the paste
chemistry spreads over the substrate during preheat and reflow.
I agree that most current models of equipment do not meet the
requirements of cutting-edge industry. This is why I hatched the
Microcontaminometer in 1983, culminating in the MCM-2 in the late 1980s,
which was able to go down to 50 ng of absolute sodium chloride with a
50% solution (in other words over 4 orders of magnitude more sensitive
than your instrument). The conductivity measuring system was able to
measure 1.5 parts per billion (ppb) of salt in the solution! Multicore,
in their wisdom, decided to abandon this model after they took over the
range. They did make a vague effort in the late 1990s to resuscitate the
idea with the CM-20, I believe, but I don't think that got much beyond
the pre-production stage (I don't know for sure, as there were a lot of
communication difficulties). Maybe Graham Naisbitt of Concoat can tell
you what the current situation is, as I understand they are the present
successors to the Contaminometer legacy (I have absolutely no interest
in it, either technical or financial).
I agree that to calibrate and drive these ultra-sensitive instruments is
not great fun, unless you are a masochist, but they were able to produce
the desired results accurately, with care and, above all, knowledge. I
had several customers for them within the great names of the Swiss watch
industry (including Swatch!), the hybrid circuit industry and component
manufacturers and they were all able to use them without problems, even
if it required a day's training.
I see, in my book pp 316-320, I was suggesting that 0.5 ug/cm2 eq. NaCl
was necessary, even then (written 1983-5, published 1986, for hybrid
and SM circuits). So, what you are saying has a 20 year history, already!
Hope this helps.
Brian
[log in to unmask] wrote:
> Ah, Glen!
>
> I've been battling this one for far too long now myself, and one of the
> questions I posed was just the one you are posing here - is it imore
> realistic to calculate the solder paste area, since this is where all the
> flux contamination wil be concentrated, or go with the spec requirements
> which gives a cleanliness requirement calculated from 2x entire board
area?
>
> I have my own opinion which I'll explain in a moment, but let's look at
> history first. The cleanliness figures in the IPC and MIL specs are now
> over 30 years old and date from the days when components were all PTH and
> wave soldered. With wave soldering, the entire board area is fluxed, and
> with the technology of those days, trace weights, widths and spacings
were
> much greater than they are today. Hence the cleanliness level required
was
> not as stringent as it needs to be today. The cleanliness level, taken
over
> the entire board area made sense then, but has not been updated since to
> account for the much finer traces, closer spacing and SMT technology of
> today. SMT technology, of course, largely uses Convection Reflow
soldering
> techniques in which solder paste (flux) is deposited only in localised
> areas.
>
> Doug Pauls from Rockwell Colins will probably give you a very
knowledgeable
> answer to your question, but my opinion is this:
>
> a) if you want your boards to meet IPC or MIL spec, it can be easily done
> using the current ROSE test equipment, which also has changed very
littlle
> in principle in 30 years. Take whatever reading you get from the machine
at
> face value, divide it by the total board area and you have the
appropriate
> figure against which to judge if you've passed or failed. However ...
> b) Industry concensus seems to have it that a much tighter cleanliness
> figure is now required than in days of yore - 0.2ug NaCl eq cm^-2 instead
> of 1.56ug is the figure that's been bandied about, based, I believe, on
SIR
> and Ion Chromatography results for modern boards. It's a figure I prefer
to
> believe than the IPC spec figure.
> c) Using the solder pasted area as the divisor for the cleanliness result
> makes the test much harder to pass, but will give you (to my mind) a more
> realistic idea of how clean the fluxed areas are. I think, too, that the
> real truth about overall board cleanliness will lie somewhere in between
> the two areas, though where I cannot say for sure but probably nearer to
> the pasted area than the overall board area.
> d) as boards get smaller and clealiness level requirements become
tighter,
> we run into problems with the ROSE testers, which are still happily
> claiming compliance with the IPC and MIL Specs and tend to assume a board
> area that may be much larger than you normally deal with. The problems
are
> those of working at the extreme limits of the machines' capabilities.
Sure,
> you can continue to do what you do now, which is to test a number of
boards
> at the same time to, as you say, give the machine something to measure,
but
> I believe it's time that the ROSE tester manufacturers looked to updating
> their technology.
> I've been giving Mike Konrad at Aqueous Technologies a hard time,
demanding
> "official" figures for how well their Zero Ion reads at low levels of
> contamination, because from tests I have carried out, machines tend to
> start reading inaccurately when total contamination weights drop much
below
> 1000ug NaCl eq. I have boards that measure just over 9" x 6" and have a
> target cleanliness figure of 0.2ug NaCl eq cm^-2. This means that the
> maximum permissible contamination weight must not exceed 143ug per board.
> The ROSE test tank I have is large enough to allow two boards to be
tested
> at one time, which means that the machine has to be capable of reading
> accurately 286ug NaCl eq (board area is irrelevant as it's just a divisor
> that you programme into the machine yourself - it doesn't affect the
> machines ability (or otherwise) to detect and measure the contamination
> level. 286ug is well below the level at which the machines seem to read
> accurately, or consistently. The variance at these levels is also great,
> and if my tests are accurate, makes the measurement at these
concentrations
> meaningless.
>
> As more people move to "no-clean" technology, the only folks left using
> ROSE testers will be the Class 3 board builders who continue to use
> water-soluble fluxes. Maybe that market is not big enough to justify
> re-evaluating the capabilities of the testers, in which case ways round
the
> capability problem have to be found.
>
> Glen, whether you choose to use overall board area or pasted area for
your
> cleanliness calculation is up to you. If you want a better idea about the
> board cleanliness, use pasted area, but if you want the boards to pass
test
> for shipment, use overall area adn no-one can legitimately criticise you.
>
> Hope some of the other gurus will give me a debate on ROSE tester
> capability.
>
> Peter.
>
>
>
> Glenn Pelkey <[log in to unmask]> 23/05/2003 09:59 PM
> Sent by: TechNet <[log in to unmask]>
>
> Please respond to "TechNet E-Mail Forum."; Please respond to
glenn.r.pelkey
>
> To: [log in to unmask]
> cc: (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
> Subject: [TN] ROSE for Flip Chip
>
>
>
>
>
>
>
>
> Hi Technetters,
>
>
> My buddy and I have a disagreement on using Resistivity of
Solvent
> Extract (ROSE) for measuring cleanliness on a Flip Chip board. So, we're
> throwing it to league of experts on this forum for feedback
>
>
> Here's the story: The board and assembly are very simple. One
> flip chip die at about 0.002 square inches and one board about 0.490
square
> inches. Flux is applied only to the flip chip die, reflowed, and DI
inline
> washed.
>
>
> Here's the disagreement: I say use 2X the surface area of the
die
> for the calculation. My good friend says use 2X the board surface. Of
> course, you can see one number is much higher than the other in ug/sq
inch
> NaCl equiv. My logic for using only the die area is to understand the
> process cleanliness at that location only, but I recognize the entire
board
> surface could be cross contaminated in some manner. We also loaded the
> machine with 32 of these units to get something measurable. When we only
> put 4 in, nothing showed up, but the smallest entry for surface area is
0.1
> square inches.
>
>
> Thanks for your input!
>
>
> Glenn
>
>
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