Resistivity readers of the de-ionized water are above 15 megohms. Resistivity results of the samples tested by the outside lab have been approximately 1 megohm. Is there any explanation to this big discrepancy? Phil Zarrow and Jim Hall, The Assembly Brothers, discuss this question. Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
With over 50 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis.
He has vast experience in SMT equipment, materials and processes.
Jim Hall
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics.
He is a pioneer in the science of reflow.
Transcript
Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers, pick and place. We are here to discuss your process situation, problems, methodologies. Today we have a water question Jim. This is from PC. We have an inline sensor connected to a resistivity meter at the exit pipe of our de-ionized water system. Resistivity readers of the de-ionized water are above 15 megohms.
The meter has been calibrated per the manufacturer recommendations and a sensor has been verified with acceptable results. We have taken samples of the de-ionized water to an outside lab for testing. Resistivity results of the samples tested by the outside lab have been approximately 1 megohm. Is there any explanation to this big discrepancy? Can the de-ionized water samples change by exposure to the atmosphere? How should we transport samples of the de-ionized water to the lab for testing?
Jim
First off, I want to compliment PC. You have your meter calibrated and you have validated when your measuring that 15 megohms that is what you are really getting. Now 15 megohms is really, really pure water. There is hardly any contamination. Naturally that water is a really good solvent for absorbing stuff. That is why we use it for the final rinse, to get everything off of the board. Can it absorb chemicals out of the atmosphere? Absolutely.
How fast, how much, what exposure? I don’t know. It will depend on the purity of your air, the level of chemicals in the environment that the water is being exposed to. Transporting samples to a lab, I am thinking use sealed containers, lab quality containers so that you know that there is absolutely no contamination whatsoever on any of the surfaces that the water is going to come into contact with.
Loading them into the sample bottles, or whatever you are using, there may be vacuum tubes that you can plug directly into your DI water as it is coming out of the machine and suck it in through the otherwise sealed tube. 15 megohms is really clean water and you can’t let it get near anything because of its ability to absorb moisture. I don’t think it takes a lot of contamination in a small sample to take it down from 15 megohms to 1.
Phil
One thing that I might add is ask the lab where you are sending your sample for analysis for their recommendations and what you should collect the samples in and transport them. They might even be able to supply you with containers that meet their specs and a methodology to extract the water. Of course if they hem and haw, you are probably going to the wrong lab. They should have a regiment that they can recommend as well.
You have been listening to Board Talk with Jim Hall and Phil Zarrow, The Assembly Brothers. We hope that we have enlightened you a little bit today. Whatever you’re cleaning your boards with, however that water has been analyzed please don’t solder like my brother.
Jim
And don’t solder like my brother.
Comments
Jay Postelwaite and Alexander Wirth both are correct as to the (main) cause of reduced resistance in deionized water. Consider: Most, if not all of the better de-ionized water "tank farms" used in electronics cleaning systems are capable of producing water that has a resistance of 15 megohms or better right at the output of the system. This is water of a very, very high purity. The less "contaminants" in the water, the higher its electrical resistance. Once that water comes out of the system and is exposed to ambient air, it quickly absorbs carbon dioxide from the air and subsequently forms carbonic acids.
This, along with exposure to even nanoscopic particulates both in the air and whatever else that water touches, will quickly reduce the pure water's ability to resist electrical current and will allow current to pass through the water which is how the resistance meter does its job; it is merely a miniature ohmmeter. The very best DI plumbing system can take 18 megohm water from the outlet of the DI system, but even traveling the very short distance from the outlet of the tank farm to the resistance meter inside the cleaning system results in a typical drop to about 500 Kohms to at most 1 megohm going into the machine.
For typical electronics production, WASHING with DI water that measures 500kohms or better is sufficient to provide a very, very effective cleaning process, especially when using a saponifier to reduce the surface tension such that its capillary action can penetrate into and under even a gap of .001". The drop in resistance for water leaving the tank farm is almost immediate, depending on the ambient air particles and many other factors. Everything in the natural world wants to be at equilibrium; hot water wants to be ambient, as does cold water. Electricity hates being either negative or positive, it wants to be at 0 volts. The higher the voltage, the hungrier it is to be at zero. The purer that water is, the more voracious its appetite for "contaminants" and so on and so forth.
All materials, everything, has a coefficient of thermal expansion and everything wants to be at equilibrium as dictated by the ambient temperature. The same is true of gravitational forces and inertia. Engineering is simply the science of exploiting these various forces that are striving for equilibrium. Having said all that, one understands that it is a very difficult task to keep pure de-ionized (hungry and starving) water pure, and we certainly cannot blame a laboratory for its "apparent failure" to keep 18 megohm water pure for resistance testing when in fact that water was probably at less than 500 kohms before it ever entered the outside lab.
In the case described in this article, if the incoming water inside the cleaner is anywhere between 500K and 1 megohm, that is probably one of the "better" cleaning systems and is perfectly normal; it is what is to be expected.
Richard Stadem, Analog Technologies Corp.
The process of transferring the water from PC's system to the lab is most likely to cause the significant drop in resistivity. If the containers are plastic rather than glass, if the containers are 'neutralized' before loading. just the motion of pouring from one source to another over an edge will generate a charge...
Ike Sedberry, ISEDS
Water, particularly if it has been recently prepared/purified to be ion free, has a natural tendency to absorb carbon dioxide from the atmosphere. This generates carbonic acid which naturally provides more acidity and associated free ions in the water. This diffusion and reaction continues until it reaches an equilibrium with the atmospheric concentration of carbon dioxide - higher CO2 levels yield higher acidity. Testing pH shift to see if this matches with accepted data from associated with this effect is one way to check if this atmospheric in source.
Alexander Wirth
Jim, There is a need to consider the absorption of carbon dioxide out of the air and forming carbonates in the water. These carbonate ions decrease the resistivity of water. Sample collection should be in a manner such that the water is not exposed to air (minimally), and there is no headspace in your sample bottles. The laboratory will need to help you with a sampling procedure and sample bottles.
Jay Postlewaite, Texwipe
I have experience with ultra pure water and believe the most likely cause of the low Ohmic reading at the test lab is quite likely a fault or basic discrepancy with the lab equipment. I suggest you take a sample to the lab and have them use perhaps half of the sample water from whatever container you utilized. Observe the test at the lab, confirm whatever reading the obtain, then return to your facility with the remaining water and re-measure the water with your own sensor. If, for some reason the container is contaminating the water then by the time you re-test the sample at your place the reading would be a lower Ohmic value.
A final step would to be taking your sensor and do a side-by-side comparison at the test lab. Seems most likely an equipment discrepancy. Now, when I was using ultra pure water the highly recommended lab approached me and asked for a good quantity of my water to use as a reference in the lab. It seems the reference samples they purchased were not as pure as my water. The measurements made in my case were both Ohmic and Mass Spectrum analysis. Summary: The lab could be at fault.
In any event a mass spectrum analysis will show what the contaminate is within your sample. Important to know if it is contaminated so you can find the source based upon the specific contaminate. Reference information: my filtration started with reverse osmosis then passed through eight resin-bed filters connected in series, followed by a Helium sparge to reduce the dissolved Oxygen content.
