Well, I guess I have decided to wade in as well - with respect to using FTIR
(Fourier transform infrared spectroscopy). Basically one is allowing
infrared light of different frequencies to interact with the changing dipole
moment as covalent bonds vibrate at infrared frequencies.
Let's first look at what this means for ionic solutions. Salts with cations
like sodium, potassium, magnesium and calcium and anions like fluoride,
chloride and bromide are going to be completely disassociated in the water
and there will be no IR spectrum due to these salts. They are completely
ionic entities and there are no covalent bonds.
What about sodium nitrate and sodium sulfate? Again the salts disassociate,
but in this case the anions are multi-atom entities that stay together (NO3-
and SO4-2). Nitrate has four vibrations around 1040, 820, 1440 and 700 wave
numbers. I say "around", as the book I have only shows the values for the
solid salts and the values vary from salt to salt. In solution only the
latter three vibrations are active in the infrared. Many spectrometers use
NaCl cells and optics with the cutoff being 625 wave numbers, so for the
band at 700 it may be buried in the decreasing transparency as one
approaches 625 cm-1. This doesn't give one a lot to work with. If it is
mixed with organic molecules, which have a messy "fingerprint" region
(usually defined as below 1500 wave numbers), then hand me the aspirin.
Multi-atom ions will have more active bands in solids because of crystal
effects splitting bands and making those active which are normally not
infrared active in the free gas or liquid phase. However, this also can be
said for the other materials the salts may be mixed with. What a mess.
FTIR is NOT the technique to be looking for inorganic salts.
Now if one is looking for a common flux residue, coffee, Coke/Pepsi/Mountain
Dew, yes, we can tell the difference. :)
Regards,
Bev
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Joe Russeau
Sent: Tuesday, May 27, 2014 2:41 PM
To: [log in to unmask]
Subject: Re: [TN] PCB Cleanliness Via Ion Chromatography Poll (informal)
Graham, Graham, Graham,
I about choked on my afternoon tea when I read your post. I must disagree
with you that IC is an unsuitable tool. In fact, I would say it is highly
suitable for evaluating bare board cleanliness. And the items you point out
about bromide and false positives or false negatives are not necessarily a
function of the tool, but perhaps more from improper use or inexperience
using the tool or in understanding what it is that is being evaluated. The
gist of Rich's question (I think) is regarding what the industry experts
consider as an appropriate metric for evaluating IC test results. Rich
correct me if I am wrong on that.
To add my 2 cents, I have said for many years now that there is no
one-size-fits-all to this question. Documents like IPC-5704 provide a
general guideline for the evaluation of IC data on bare boards, but you have
to answer the question of how applicable that criteria is for your
product(s). If you have no clue of where to begin, then 5704 provides a
remedial starting point. As you begin to gather data, take the time to log
it and evaluate it. Hopefully, you are pulling data across multiple product
designs. I tell clients all the time that every board has its own threshold
for how much residue it can tolerate. So if you are only evaluating one
board type, you can set a misleading standard for your other products.
Case in point, I had a client a couple of years back that had set an IC
standard so low that none of their products could meet it. When asked how
they developed the standard, they told me that their competitor down the
road used the same standard and they knew if their competitor could meet it,
so could they. They neglected to take into account the material and process
differences they used compared to their competitor AND they never thought to
factor in their board supplier's process. They went off assumptions. We
still do that in industry even today and make assumptions about what we
think we know. In my world, data is king. So I would suggest that you
test, re-test and test some more and look across multiple product lines. I
would look at different board laminates and different surface finishes and
would tabulate the data that I collect. Over time you should begin to see
what the "typical" cleanliness picture is for your boards and then you will
have a standard that is relevant to you. Until then, I would recommend
consulting the experts and utilize IPC documents as they are suppose to be
derived from a consensus of industry experts that have knowledge and
experience in a given area or discipline.
My 2 pennies.
Joe Russeau
----- Original Message -----
From: "Graham Naisbitt" <[log in to unmask]>
To: <[log in to unmask]>
Sent: Tuesday, May 27, 2014 12:11 PM
Subject: Re: [TN] PCB Cleanliness Via Ion Chromatography Poll (informal)
Hi Richard
I was going to respond sooner but, being equally interested in the response,
thought I would wait to see what was said.
IMHO this is not a suitable test.
My reasoning being that the test is run by washing / "de-ionising" the
eluent until it squeaks. Then heating it to 80DegC, and immersing the
specimen / sample into the eluent for 1 hour.
Problem? The laminate closely resembles a sponge, it is hydrophilic and
contains more ions than you could shake a stick at.
Result? False negatives?
Why? Br could well be leached out through the mask onto the surface..but Br
is a requirement.
I have had several board fabricators asking for assistance on this matter as
they are struggling to get anything to pass... my answer has been: Use SIR
sir.
It does not discriminate between ionic and non-ionic contamination, it
simply examines changes to insulation resistance that might be pre-cursors
of unwanted electro-chemical activity.
Use IC when you need to know what is present that is causing the problem..or
use FTIR?
Use Process Ionic Contamination Testing (PICT) to control the process. Fast,
simple, relatively inexpensive and effective.
Always at your service,
BR Graham Naisbitt
On 23 May 2014, at 14:27, Richard Kraszewski <[log in to unmask]>
wrote:
> I actually sent this out on May 16th, but never got a single response.
> Hoping that was not due to lack of interest, but rather due to the TechNet
> being down. Hence, I think I'll try this one more time.
>
> I am hoping to run an informal " min straw poll " here.
>
> Questions stated are as follows:
>
>
> 1. "Is your organization testing incoming PCB cleanliness via
> ion- chromatography? Replies such as "YES" or "NO" will
> suffice, but more detailed explanations are also acceptable.
>
>
>
> 2. " If testing via IC, do you use IPC -5704 Table 4.1 limits or
> other? Replies such as "
> IPC" or "other" will suffice, but more detailed explanations are also
> acceptable.
>
> What's in it for you? I will summarize and post the results after a
> few days of replies.
>
> Rich Kraszewski
> Senior Process Engineer
> Plexus
>
>
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