Ask the Experts
October 8, 2018
Deionized Water Sample Testing
We have an in-line sensor connected to aresistivity meter at the exit pipe of our deionized water system. The resistivityreadings of the deionized water are above 15 megohms. The meter is calibrated per themanufacturer recommendations and the sensor has been verified with acceptableresults.
We have taken samples of the deionized water to an outside lab for testing. The resistivity results of thesamples tested by the outside lab have been approx. 1 megohm.
Is thereany explanation for this big discrepancy? Can the deionized water samples change by exposure to the atmosphere? How should we transport samples of deionized water to lab for testing?
Expert Panel Responses
That is exactly what is happening. As soon as the DI water comes out ofthe system and is exposed to the atmosphere, it dramatically loses resistivityas it quickly becomes ionized again. It is also true that it will loseresistivity the moment that it comes in contact with any container due tocontaminants inherent in the bottle. We have a DI water system that we use formaking 18.2mOhm deionized water for use with our ionic chromatography testinglaboratory 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) leavesthe system, it very quickly drops down to about 1mOhm so what you're seeing is typical.
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.
Deionizedwater 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. Onceexposed to the air, DI water will quickly absorb (eat up) CO2 from the airsatisfying its acidity and reducing its resistivity very quick. This makes gettingan "outside lab" report somewhat difficult, unless you can find a way to draw,transport and test the sample without exposing it to air.
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 willabsorb Carbon Dioxide in the air and this will deteriorate the megohm readings ofthe material. Since the DI water is so clean it will attack anything to changechemically. You need to be able to transport the water in a specializedcontainer to make sure it is sealed from the environment.
For more information visit: http://www.tmasc.com/di%20water%20specs.htm
I would recommenddoing a second test with a different piece of test equipment directly at thesite to verify the goodness of the DI system. One of the recommendations frommany of the sites is to maintain the correct DI readings it is recommended thatthe DI water is continuously processed through the DI columns. So to take it toanother location it will deteriorate over time which is probably why you areexperiencing the change in DI values.
Vice President, Technical Director
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 atmosphereas well as very small amounts of contaminants from containers can dramaticallylower the resistivity of deionized water. If your meter is calibrated, rely onit. The delay in sample processing will introduce very large discrepancies thatmake lab testing less than useful. What lab testing can tell you is whetherthere 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 contaminantspresent, testing may be worthwhile.
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 almostimpossible to take an ultra-pure water sample to the lab and compare that tothe reading from the closed system in the facility. It takes so littlecontamination -- from either in the method of taking the sample, or in thesample container, or in the exposure to the atmosphere, or in the analysismethod in the lab -- to change the resistivity from 15 MOhms to 1MOhm. Ifyou think about how much dissolved solids are in ultra-pure water:
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 thesample during the transfer from the in-plant system to the analyticalinstrument in the lab. That's not very much contamination!
DI water will change within minutes of exposure to theatmosphere. Ultra-pure water is one of the most aggressive solvents andwill scavenge anywhere for ions. It will also absorb CO2 from theatmosphere which will drop the resistivity. There is almost no way oftaking a clean sample to an outside lab for analysis. I was told a fewyears ago that a properly regenerated set of ion-exchange columns should giveyou between 16 and 18 MOhms, but there was no way to use a sample to calibratethe resistivity meters. I don't know if that information still holdstrue.
Therewas a related (although not exactly on this question) previous discussion onCircuitnet in 2009 on this topic: http://www.circuitnet.com/experts/59891.shtml
|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 |
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 youare getting a reading of 15 mOhms. Is that at the discharge from the pointwhere the water is returning to the recirc system or at the entry into themachine from the DI system?15 mOhms is very goodquality DI water.Di water exposed to theatmosphere will degrade as it absorbs ions from the surrounding air.Please clarify where you are taking/getting yourreadings from.
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 thewater 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 theequipment that the outside lab is using?
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 meteris 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 closelyas you can. Make sure you're sampling from the same area asthe monitor. Any significant length of plumbing could affect theresults. If there are any metal pipes or unions the water is travelingthrough prior to the sample point or if the sample valve is metal, the waterwill a) be acidic due to metal ion pickup and lack of buffering and b) willshow a decrease in resistivity. PVC or polypropylene are common materialsfor DI water plumbing. Eliminate any sources of metal in the system.
Contaminants from the transport container can certainly causereading differences. In other words, don't send the sample to the lab ina Coke bottle (yes, we've seen this!). A good reference for watersampling techniques can be found in Standard Methods for The Examination ofWater and Wastewater. Sterile, sealed water sampling containersare available through most lab supply companies.
Ask your lab about the test method, test equipment, and rangelimits. Differences in methodology and test equipment can result indiscrepancies, although a 14 meg swing is huge. Contamination can alsooccur at the lab, so check on their protocol. Also question your watersupplier and get advice from them. You could ask a competing supplier totest the water from the same point you do. This can usually be done at nocharge.
As far as beingexposed to the open atmosphere - yes, it can absorb contaminants from the airand through improper handling (ie sulfur compounds, ammoniated compounds, saltsfrom your hands etc) that will affect the outcome. Any ionic species thatcan be picked up by the water will cause large swings in the resistivity due tolack of (initial) buffering capacity of the water.
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 forjust pH measurement can change over time from absorption of air constituentslike CO2. That is why EPA requires analysis of samples for pH within 15 minutesof sample collection.
Also, the type of sample container used is crucial to obtaining accurate labresults. I don't know what type of sample container you used for your outsidelab testing, but I highly recommend that you use glass, and not plastic. It isknown that certain constituents in plastic containers, like the plasticizerDEHP. DI water is very "hungry" and will dissolve anything it contacts if itcan.
Finally, re: sample containers, you need to ensure that there are nopreservatives in the clean (unused) sample container. Depending on the analysisdesired, some labs will supply sample bottles with a preservative in thecontainer. Also, you should check to see what type of sample bottle lidmaterial is used. I recommend one of Teflon since it is inert. Hope thesetips help.
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.
Principal Materials and Process Engineer
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. DIwater is only DI water in its purest form at the source.
Once transported through pipe, sprayed in air, heated or evenexposed to various metals the quality level of where it started to whereit is drops rapidly. Deionized water is exactly that - water that hasessentially been stripped of all of its ions. Water likes to be balanced in itsnatural state, however, and this means that it adds ions to itself to achievethat goal. Therefore, DI water grabs ions from everything it touches that canbe dissolved or absorbed. It is about a close as you can get to a UniversalSolvent.
Honestly I see you spending money for tests that can be easilydone in house with a simple meter. I have seen some customers put a tap off thefeed line just before it enters the machine where they can grab a sample andtest it with a resistivity meter. Now with that said;
IPC-AC-62A,Chapt. 10.6, "Goodquality deionized water is the preferred rinse medium . . . Waterin the 1 to 5 megohm-cm region will be satisfactory for most operations. Thetemperature of the rinse water should be as high as possible, but comparativewith parts and process."
For practical purposes, the maximum level of deionization ofwater results in resistivity of approximately 18.2 megohm-cm. There is arelatively small difference in parts per million (TDS) between 1 megohm and 18megohm water. There is an exponential relationship that has a significant costimpact as well. Operating a system at 18 megohm (or in your case 15) will besubstantially more costly than operating one at 1 megohm, yet the benefits incleanliness will be minimal. Typically, DI water above 1 megohm is a sufficientsupply for the final rinse of a cleaner.
In summary, the resistivity of the feed water into final rinsestage 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.
Technical Expert Sales Support
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 technicallypossible for water to absorb CO2 from the atmosphere which will formacid, that in turn will decrease the resistance of the water, thisprocess is however relatively slow and requires exposure to air. A more likelyexplanation is the ionic cleanliness of the container used to hold andtransport the samples. Water will leach ionic material from many/most materialsgiven intimate contact and time.
Senior Applications Chemist
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.