Ask the Experts
December 8, 2008
Recommended DI Water Conductivity For Ultrasonic Cleaning
What is the proper DI water conductivity in micro Siemens for ultrasonic cleaning?
Expert Panel Responses
Deionized water is often used in precision cleaning processes. It's a tried-and-true process, but one with a number of hidden pitfalls. With this response, I'd like to try to "skim the treetops" with my answer, which hopefully will provide enough information for you to fine-tune your research in the direction of the details that pertain to your specific application.
There are three general grades of water: tap water, distilled water and deionized water. In terms of precision cleaning, neither tap water nor distilled water are sufficiently pure to handle the job as both are contaminated, to greater or lesser degrees, with minerals and organics. So you must jump to DI-water.
The quality of DI water usually is measured by the water's resistance to electric current (in OHM-cm). Quoting from http://www.finishing.com/2600-2799/2733.shtml: "Deionized water quality depends on a variety of factors, including raw water composition, ion exchange types and quantities, and the number of resin tanks in the system. Two-bed deionizers use separate tanks, one containing cation resin, the other containing anion resin. A two-bed weak vase deionizer typically produces water with electrical resistance of about 50 kOHM-cm. A two-bed strong base deionizer typically produces water with electrical resistance of about 200 kOHM-cm."
Now, how pure is that?
Obviously, the cost, energy consumption, through-put and handling issues all jump exponentially as the purity of the water increases. The more pure the water, the more hungry it is for ions, and the more contamination it will attract unless the packaging and handling is tightly controlled.
- 50 kOHM is pretty standard and can be produced easily and inexpensively by many in-house deionizing systems.
- In my experience, 1 megaohm of resistivity is the minimum required for true precision cleaning.
- In the really high-end world, such as the semiconductor industry, 20 megaohms is the norm. 20 megaohm water is so hungry for ions it will cut through steel.
Now, you didn't note in your email the level of purity you were seeking, nor the application into which you were working. But if we assume it is some aspect of electronics assembly, beware: unless you have an elaborate and energy-intensive DI generating system, your water will generally not be clean enough for highly aggressive cleaning of normal PCBs.
I have seen people try to use DI-water, for example, on the benchtop. This is futile. If the water is pure enough to be an aggressive cleaner, it will be instantly contaminated as soon as the bottle or container is opened, and at that point you might as well be cleaning with distilled water.
People also try to use DI-water in ultrasonic tanks. I just visited a company with thirty 5-gallon ultrasonic cleaners, and they were changing the DI-water every 60 minutes in every machine. This was because they found that after an hour or so the cleaning had stopped no matter how long they ran the machines.
Again, the issue here is handling: an open-topped ultrasonic cleaner holding DI-water is always going to be recontaminated within minutes, and ordinary water won't clean (without additives).
The only viable option is a tightly-sealed, closed-loop system which purifies the water, performs the cleaning task, and then recycles the water. These tend to be expensive, energy-hungry, and relatively slow in through-put.
The one exception to this generalization is ionograph testing. This is a less fast and user-friendly bench top system used for the measurement of PCB cleanliness. The machine features a narrow dip tank with very limited surface area exposed to air, a tiny on-board DI-ionizing system, and some computer systems for data capture and processing.
This process requires the board to be immersed in a bath of deionized water, which the machine is constantly regenerating. After the water has had a few moments to circulate around the components, the water automatically is tested for ionic contamination. Since the DI water will dissolve ionics from the boards, any decrease in resistivity of the water would indicate the relative dirtiness of the board.
The Omegameter 600SMD probably is the industry standard for ionic testing. The system provides an accurate, repeatable and rapid method for determining cleanliness and complies with industrial specifications including MIL-STD-2000A, IPC-TM-650 and ANSI/J-STD-001B. A quick Google search will help clarify these options.
Given the complexities of using DI water, and the worries about the environmental impacts of energy-intensive systems, many PCB makers have switched to cleaning by vapor degreasing. Vapor degreasers are simple, fast and reliable cleaning systems that out-perform aqueous cleaners in most applications.
Vapor degreasers and their special solvents can easily get into and around parts which are extremely small or have extremely tight clearances. Vapor degreasers have very short cleaning cycles, so even a modest batch machine can usually outperform an in-line water cleaning system costing two or three times the price.
Because the solvents evaporate so quickly, the vapors are easily removed from complex shapes, deep holes, blind vias and other designs where slow-drying solvents could be trapped. This keeps costs down and quality up. Water often leaves unacceptable water spots which vapor degreasers never leave, making them perfect for cleaning optics and other super-critical applications.
Vapor degreasers and their solvents can easily clean almost every type of contamination, up to and including heavy greases, oils and even waxes. And perhaps most importantly, vapor degreasers are very inexpensive to purchase and use. A modern degreaser can limit solvent consumption to dollars per day.
In short, vapor degreasing -- especially with the MicroCare solvents -- delivers the highest quality of cleaning in the shortest possible time, with the least risk of damage to the boards, components or the environment.
Mr. Jones is an electronics cleaning and stencil printing specialist. Averaging over one hundred days a year on the road, Mike visits SMT production sites and circuit board repair facilities in every corner of the globe, helping engineers and technicians work through the complex trade-offs today's demanding electronics require.
This is a very wide open question. The correct answer is that the DI water resistivity is whatever you put in your ultrasonic cleaning system. A better question would be "what type of DI water quality should be used in ultrasonic cleaning".
The answer to this question would be determined by your specific cleaning application. If you are cleaning stencils using an ultrasonic cleaning system, the answer would be 1 M-Ohm. Anything more is a waste of money.
If you are cleaning circuit assemblies, or any other part where ionic contamination is undesirable, then 5 M-Ohms or greater is recommended.
One should also keep in mind that most cleaning processes utilize both a wash and a rinse. For the wash, DI water quality is not all that important. If one has hard water, DI water would be better as the chemical may react with the minerals in the water, leaving white residue.
For the rinse, higher grade DI water ( greater than 5 M-Ohm) is vital for ionically clean results.
Mr. Konrad has been in the electronic assembly equipment industry since 1985. He is founder and CEO of Aqueous Technologies Corporation, a manufacturer of automatic de-fluxing equipment, chemicals, and cleanliness testing systems.
1 to 10 microsiemens-cm as 10 microsiemens-cm being maximum not more than that.
Application Technology Manager
Mr. Tosun has published numerous technical articles. As an active member of the SMTA and IPC organizations, Mr. Tosun has presented a variety of papers and studies on topics such as "Lead-Free Cleaning" and "Climatic Reliability".