|
That is exactly what is happening. As soon as the DI water comes out of
the system and is exposed to the atmosphere, it dramatically loses resistivity
as it quickly becomes ionized again. It is also true that it will lose
resistivity the moment that it comes in contact with any container due to
contaminants inherent in the bottle. We have a DI water system that we use for
making 18.2mOhm deionized water for use with our ionic chromatography testing
laboratory where we test the cleanliness of printed circuit board assemblies.
We have a resistivity/conductivity meter and it reports the exact same thing.
As soon as the water (which is measured by the instrument at 18.2mOhms) leaves
the system, it very quickly drops down to about 1mOhm so what you're seeing is typical.
|
David Bonito
Sales & Marketing Manager
Technical Manufacturing Corp.
David has been active in all areas of the contract electronics manufacturing industry for over fifteen years. He is currently in charge of all Sales and Marketing related activities for Technical Manufacturing Corporation.
|
|
Deionized
water is a very "hungry" acid. However, it's hunger is very easily satisfied.
DI water fresh from a DI bed will easily be >15 meg Ohm resistivity. Once
exposed to the air, DI water will quickly absorb (eat up) CO2 from the air
satisfying its acidity and reducing its resistivity very quick. This makes getting
an "outside lab" report somewhat difficult, unless you can find a way to draw,
transport and test the sample without exposing it to air.
|
Paul Austen
Senior Project Engineer
Electronic Controls Design Inc
Paul been with Electronic Controls Design Inc. (ECD) in Milwaukie, Oregon for over 34 years as a Senior Project Engineer. He has seen and worked with the electronic manufacturing industry from many points of view, including: technician, designer, manufacture, and customer. His focus has been the design and application of thermal process measurement tools used to improve manufacturing processes like: mass reflow and wave soldering, bread baking, paint and powder curing, metal heat treatment and more.
|
The De-ionized water will
absorb Carbon Dioxide in the air and this will deteriorate the megohm readings of
the material. Since the DI water is so clean it will attack anything to change
chemically. You need to be able to transport the water in a specialized
container to make sure it is sealed from the environment.
For more information visit: http://www.tmasc.com/di%20water%20specs.htm
I would recommend
doing a second test with a different piece of test equipment directly at the
site to verify the goodness of the DI system. One of the recommendations from
many of the sites is to maintain the correct DI readings it is recommended that
the DI water is continuously processed through the DI columns. So to take it to
another location it will deteriorate over time which is probably why you are
experiencing the change in DI values.
|
Leo Lambert
Vice President, Technical Director
EPTAC Corporation
At EPTAC Corporation, Mr. Lambert oversees content of course offerings, IPC Certification programs and provides customers with expert consultation in electronics manufacturing, including RoHS/WEEE and lead free issues. Leo is also the IPC General Chairman for the Assembly/Joining Process Committee.
|
|
Absorption of ionic species such as CO2 from the atmosphere
as well as very small amounts of contaminants from containers can dramatically
lower the resistivity of deionized water. If your meter is calibrated, rely on
it. The delay in sample processing will introduce very large discrepancies that
make lab testing less than useful. What lab testing can tell you is whether
there are non-ionic dissolved solids, such as organic compounds, in the water.
Normally, this is not an issue, but if you suspect that there might be contaminants
present, testing may be worthwhile.
|
Fritz Byle
Process Engineer
Astronautics
Fritz's career in electronics manufacturing has included diverse engineering roles including PWB fabrication, thick film print & fire, SMT and wave/selective solder process engineering, and electronics materials development and marketing. Fritz's educational background is in mechanical engineering with an emphasis on materials science. Design of Experiments (DoE) techniques have been an area of independent study. Fritz has published over a dozen papers at various industry conferences.
|
As to the question about DI Water Sampling: It is almost
impossible to take an ultra-pure water sample to the lab and compare that to
the reading from the closed system in the facility. It takes so little
contamination -- from either in the method of taking the sample, or in the
sample container, or in the exposure to the atmosphere, or in the analysis
method in the lab -- to change the resistivity from 15 MOhms to 1MOhm. If
you think about how much dissolved solids are in ultra-pure water:
| Resistivity (MOhms) |
Total Dissolved Solids (mg/L CaCO3) |
| 18 |
0.028 |
| 16 |
0.031 |
| 14 |
0.036 |
| 12 |
0.042 |
| 10 |
0.050 |
| 5 |
0.100 |
| 2 |
0.250 |
| 1 |
0.500 |
So to drop from 15 MOhm water to 1 MOhm water, we only need
(0.500 - 0.034 = 0.466 mg/L). If the sample was a typical 500 mL bottle,
then we only needed 0.233 mg of TDS either in the bottle, or added to the
sample during the transfer from the in-plant system to the analytical
instrument in the lab. That's not very much contamination!
DI water will change within minutes of exposure to the
atmosphere. Ultra-pure water is one of the most aggressive solvents and
will scavenge anywhere for ions. It will also absorb CO2 from the
atmosphere which will drop the resistivity. There is almost no way of
taking a clean sample to an outside lab for analysis. I was told a few
years ago that a properly regenerated set of ion-exchange columns should give
you between 16 and 18 MOhms, but there was no way to use a sample to calibrate
the resistivity meters. I don't know if that information still holds
true.
There
was a related (although not exactly on this question) previous discussion on
Circuitnet in 2009 on this topic: http://www.circuitnet.com/experts/59891.shtml
|
John Sharp
Corporate Product Compliance Manager
TriQuint Semiconductor, Inc.
John has focused on Product Compliance and Environmental & Safety issues throughout his career. He has a B.S. in Chemical Engineering, an M.S. in Chemical Oceanography, an M.S. in Environmental Engineering, and is a Professional Engineer in the fields of Chemical and Environmental Engineering.
|
|
You indicate that you
are getting a reading of 15 mOhms. Is that at the discharge from the point
where the water is returning to the recirc system or at the entry into the
machine from the DI system?
15 mOhms is very good
quality DI water.
Di water exposed to the
atmosphere will degrade as it absorbs ions from the surrounding air.
Please clarify where you are taking/getting your
readings from.
|
Jerry Karp
President
JSK Associates
Based in. Northern California since 1971. Founded JSK Associates in 1979. Actively involved in soldering, cleaning, chemistries. 30 years experience in EOS/ESD control.
|
|
Exposure to atmosphere will definitely impact the resistance of the
water as will the container you are using -- but this is a major reduction in resistivity.
Have you asked to see the measurement uncertainty calculations for the
equipment that the outside lab is using?
|
Gerard O'Brien
President
S T and S Testing and Analysis
Gerald O'Brien is Chairman of ANSI J-STD 003, and Co Chairman of IPC 4-14 Surface Finish Plating Committee. He is a key member of ANSI J-STD 002 and 311 G Committees Expert in Surface finish, Solderability issues and Failure analysis in the PWA, PWB and component fields.
|
About the disparity of the test results - Remember the meter
is sampling from a closed system. You're securing a second sample and "breaking" the system, so it's important to compare apples to apples as closely
as you can. Make sure you're sampling from the same area as
the monitor. Any significant length of plumbing could affect the
results. If there are any metal pipes or unions the water is traveling
through prior to the sample point or if the sample valve is metal, the water
will a) be acidic due to metal ion pickup and lack of buffering and b) will
show a decrease in resistivity. PVC or polypropylene are common materials
for DI water plumbing. Eliminate any sources of metal in the system.
Contaminants from the transport container can certainly cause
reading differences. In other words, don't send the sample to the lab in
a Coke bottle (yes, we've seen this!). A good reference for water
sampling techniques can be found in Standard Methods for The Examination of
Water and Wastewater. Sterile, sealed water sampling containers
are available through most lab supply companies.
Ask your lab about the test method, test equipment, and range
limits. Differences in methodology and test equipment can result in
discrepancies, although a 14 meg swing is huge. Contamination can also
occur at the lab, so check on their protocol. Also question your water
supplier and get advice from them. You could ask a competing supplier to
test the water from the same point you do. This can usually be done at no
charge.
As far as being
exposed to the open atmosphere - yes, it can absorb contaminants from the air
and through improper handling (ie sulfur compounds, ammoniated compounds, salts
from your hands etc) that will affect the outcome. Any ionic species that
can be picked up by the water will cause large swings in the resistivity due to
lack of (initial) buffering capacity of the water.
|
Pierce Pillon
Laboratory Mgr.
Techspray
Pierce Pillon is the Laboratory Manager and lead formulations chemist at Techspray, a division of Illinois Tool Works (ITW) and a leading manufacturer of chemical products for the electronics industry.
|
I don't have direct DI sampling experience, but do know that water samples for
just pH measurement can change over time from absorption of air constituents
like CO2. That is why EPA requires analysis of samples for pH within 15 minutes
of sample collection.
Also, the type of sample container used is crucial to obtaining accurate lab
results. I don't know what type of sample container you used for your outside
lab testing, but I highly recommend that you use glass, and not plastic. It is
known that certain constituents in plastic containers, like the plasticizer
DEHP. DI water is very "hungry" and will dissolve anything it contacts if it
can.
Finally, re: sample containers, you need to ensure that there are no
preservatives in the clean (unused) sample container. Depending on the analysis
desired, some labs will supply sample bottles with a preservative in the
container. Also, you should check to see what type of sample bottle lid
material is used. I recommend one of Teflon since it is inert. Hope these
tips help.
|
Lee Wilmot
Director, EHS
TTM Technologies
Lee Wilmot has 20+ years doing EHS work in the PCB/PCBA industries, including environmental compliance, OSHA compliance, workers compensation, material content declarations, RoHS & REACH compliance. Active on IPC EHS committee and c-chaired committees on IPC-1331, J-STD-609A on labeling & marking, IPC-1758 on packaging and others.
|
|
Keep in mind that deionized water, cleaned to high
levels, is a very hungry solvent and will pull in ionic material from
wherever it can. The most common source is carbon dioxide from the air.
The degradation from 15 megohm-cm to 1 megohm-cm is most likely from
this. You might also have a container that contributed low levels of
ionics.
|
Doug Pauls
Principal Materials and Process Engineer
Rockwell Collins
Doug Pauls has a bachelors in Chemistry & Physics, Carthage College, BSEE, Univ of Wisc Madison. He has 9 years working experience for US Navy - Materials Lab, Naval Avionics Center Indianapolis. 8 years Technical Director, Contamination Studies Laboratories. 11 years Rockwell Collins Advanced Operations Engineering.
|
P.C. the DI water will always read highest at the source. DI
water is only DI water in its purest form at the source.
Once transported through pipe, sprayed in air, heated or even
exposed to various metals the quality level of where it started to where
it is drops rapidly. Deionized water is exactly that - water that has
essentially been stripped of all of its ions. Water likes to be balanced in its
natural state, however, and this means that it adds ions to itself to achieve
that goal. Therefore, DI water grabs ions from everything it touches that can
be dissolved or absorbed. It is about a close as you can get to a Universal
Solvent.
Honestly I see you spending money for tests that can be easily
done in house with a simple meter. I have seen some customers put a tap off the
feed line just before it enters the machine where they can grab a sample and
test it with a resistivity meter. Now with that said;
IPC-AC-62A,
Chapt. 10.6, "Good
quality deionized water is the preferred rinse medium . . . Water
in the 1 to 5 megohm-cm region will be satisfactory for most operations. The
temperature of the rinse water should be as high as possible, but comparative
with parts and process."
For practical purposes, the maximum level of deionization of
water results in resistivity of approximately 18.2 megohm-cm. There is a
relatively small difference in parts per million (TDS) between 1 megohm and 18
megohm water. There is an exponential relationship that has a significant cost
impact as well. Operating a system at 18 megohm (or in your case 15) will be
substantially more costly than operating one at 1 megohm, yet the benefits in
cleanliness will be minimal. Typically, DI water above 1 megohm is a sufficient
supply for the final rinse of a cleaner.
In summary, the resistivity of the feed water into final rinse
stage of the washer is fine at the 1 to 2 megOhm range measured at the source.
Or a TDS (total Dissolved Solids) of 0.4 mg/l as recommended by IPC standards.
Once it reaches the actual rinse tank however this number is way lower.
(typically 500 to 1000 k-Ohm will suffice)
If you would like to discuss this further do not hesitate to contact me.
|
Charlie Pitarys
Technical Expert Sales Support
Kyzen Corporation
Charlie Pitarys has over thirty years of industry experience and has been with KYZEN for twenty-one years. Charlie is a former Marine and a retired Sargent First Class in the Army Reserves. His previous employers include Hollis and Electrovert. Charlie continues to use his expertise on cleaning processes and machine mechanics to help KYZEN customers and partners improve their cleaning operations.
|
|
It is technically
possible for water to absorb CO2 from the atmosphere which will form
acid, that in turn will decrease the resistance of the water, this
process is however relatively slow and requires exposure to air. A more likely
explanation is the ionic cleanliness of the container used to hold and
transport the samples. Water will leach ionic material from many/most materials
given intimate contact and time.
|
Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
|
