Ask the Experts
June 27, 2022 - Updated
July 18, 2018 - Originally Posted

Adding Chlorine to a DI (Deionization) Water System

We have had a recent issue of bacteria accumulation in our DI (Deionization) reservoir. In order to combat this we have put in a floater (similar to one found in swimming pools) containing chlorine tablets. Is this dangerous to use in our system? What limit should the chlorine levels be in the DI reservoir?


Expert Panel Responses

The addition of Cl to DI water fundamentally turns it back into city water. The fact you have a DI system would tend to indicate that you need DI! When events like yours occur you need to dump the system, bleach it, rinse thoroughly and start again with a new charge of DI.

Are you using a UV bug killer with your system? If not, you should investigate getting one, if you are, then either the bulb is broken or on its way out.

Gerard O'Brien
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.

Depending on the specific use for the DI water, it may not be necessary. I assume it's for process use, in which case the free chlorine in the water will drastically change the resistivity/conductivity readings of the water. This is due to the fact that the processed water has no buffer to speak of.

Depending on the amount consumed by the bacteria, there may be (probably will be) adequate amount left to initiate corrosion on any substrates cleaned or rinsed with it.

A simpler solution is to install an in-line UV light on the discharge side of the reservoir close to the point of use. This is a common practice to eliminate bacteria in a DI system. Your water consultant should be able to install this at the proper location with minimum down-time to your system.

Pierce Pillon
Laboratory Mgr.
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 would like to caution you from using the DI water that is fine for swimming but very dangerous if you are using the DI water for cleaning of electronic assemblies, boards or components, or feed water for environmental chambers.

To remove the bacteria from the DI system I would recommend using UV light in the DI water loop to destroy the bacteria. I would be very concerned about the contamination of the DI water system that now will leach chlorine into the new clean water. I would ask what is the DI water used for and what quality were you expecting?

Terry Munson
President/Senior Technical Consultant
Mr. Munson, President and Founder of Foresite, has extensive electronics industry experience applying Ion Chromatography analytical techniques to a wide spectrum of manufacturing applications.

Bacteria growth in DI water storage tanks is a common problem. It is sometimes necessary to clean out the bacteria through a sanitization process. A simple way to do that is to use bleach and allow it to soak for a time, followed by thorough rinsing with DI water to remove the bleach residues. Multiple rinses may be required to remove all of the bleach.

Using a pool chlorinator will have a similar effect on the bacteria as the chemicals in the chlorine tablets dissolve into the water. This causes the water to no longer be De-ionized because ions from the chlorine tablets have been intentionally added to the DI water. The purity of the DI water may be to low for the intended use. For example, the free chlorine in the DI water can potentially cause corrosion or dendritic growth if the DI water is used on circuit board assemblies.

It would be best to use the pool chlorinator only as long as necessary to clean the tank. Then perform thorough rinsing with DI water to remove the chlorine tablet chemicals. Once the DI water purity is back to normal, then it can be used normally.

Tony Lentz
Field Applications
FCT Assembly
Tony has worked in the electronics industry since 1994. He worked as a process engineer at a circuit board manufacturer for 5 years. Since 1999, Tony has worked for FCT Companies as a laboratory manager, facility manager, and most recently a field application engineer. He has extensive experience doing research and development, quality control, and technical service with products used to manufacture and assemble printed circuit boards. He holds B.S. and M.B.S. degrees in Chemistry.

My colleague Joe Russeau at Precision Analytical Labs(PAL) and myself recommend the following SOP to address bacteria issue:
  1. Place a large UV light on the incoming water side of your reservoir. The UV light has to be large enough to handle standard incoming water pressure from the city.
  2. Place a second smaller UV light in your circulation line. This is for any stragglers that were missed by the first UV.
  3. Place a 0.2 micron filter/s in two locations on the recirculation line and at the top of the tank where the incoming air line is( in case your tank has an air port to prevent any development of a vacuum). Routine maintenance is important on the filters and UV bulbs. The frequency of replacement depends on the amount of water being generated. For a lab, that frequency may only be a once a year or every two years depending on water usage.
    For large scale production, it may be every 3 - 6 months.The system makes Class I water. The suggested chloride level should be less 1 ppb (ug/L). Below is a link to help you depending on what your working with.

Mark Northrup
VP of Advanced Technical Operations
IEC Electronics
Mark has over 25 years' experience in electronics fabrication, quality and reliability while working for IEC Electronics, GE, Motorola, ORS, etc. He has most recently established IEC Electronics Analysis and Testing Laboratories (IATL), LLC in Albuquerque, NM for electronics and material analysis testing in the military, medical, and industrial industries. His area of expertise includes PCB, PCBA, components, analytical and electrical analysis techniques.

I'm not too familiar with swimming pools and their chemical balance, however the addition of chlorine to the water being used to clean boards may be or should be of some concern.

With enough chlorine added to the system it could lead to an acidic condition which could dry on the boards and this would then create a condition that is susceptible to the absorption of moisture creating a conductive path across conductors which would enhance dendritic growth.

I would suggest getting touch with labs that specialize in the evaluation of cleanliness and have this conversation with them regarding chlorinated residues left behind due to this modified process.

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.

Chlorine works large water systems like swimming pools & potable water tanks. If you have a DI water tank and add Cl ion your water is no longer considered to be de-ionized.

Your case requires an expert service to determine the source of the bacteria. The bacteria could be lodged in the water line, or in the filters, or in any other component leading to the tank. The experts will be able to determine the way to remove the bacteria from your system.

Edithel Marietti
Senior Manufacturing Engineer
Northrop Grumman
Edithel is a chemical engineer with 20 year experience in manufacturing & process development for electronic contract manufacturers in US as well as some major OEM's. Involved in SMT, Reflow, Wave and other assembly operations entailing conformal coating and robotics.

We assume you're talking about a deionized reservoir for an aqueous cleaner. Adding chlorine will remove bacteria from the cleaner itself but not necessarily from the entire system including the piping and exchange tanks.

It is generally recommended to include an ultraviolet light source in the system since UV light inactivates microorganisms causing them to become sterile. Adding a UV light source in combination with adding chlorine is highly suggested to keep bacteria accumulation under control.

Carlos Bouras
General Manager
Nordson SELECT
Carlos Bouras is the General Manager of Nordson SELECT and has over 30 years of experience in the electronics manufacturing industry. Carlos's expertise is in process engineering, product development and manufacturing operations. For the past 15 years Carlos has focused specifically on automated assembly issues and is the holder of several US patents for non-contact dispensing and precision dispensing of adhesives for the packaging of microprocessor devices.

This is not recommended. You may as well just use tap water it would be safer. Most bactericides are chloride or bromine. Both halides are very ionic and will be conductive /corrosive with moisture.

Bacteria and fungus control is an issue in VOC free flux as well. I would suggest that you clean and purge your tank. Run a UV light in a recirculation circuit or place a UV light in the tank. This is dependent on the tank configuration. UV light has to be shielded as it can cause eye damage and other issues so consult a water UV specialist.

After this you will need to add a particulate filter on the exit of the tank.

Karl Seelig

Deck Street Consultants
In his 32 years of industry experience, Mr. Seelig has authored over 30 published articles on topics including lead-free assembly, no-clean technology, and process optimization. Karl holds numerous patents, including four for lead-free solder alloys, and was a key developer of no-clean technology.

If you're adding chlorine to the water after it is deionized, you've defeated the purpose of deionization. Chlorine is very ionic, and in fact is one of the primary soils you want to avoid in electronics. You don't want this in your DI supply.

If you have bacterial growth in the DI reservoir, the system may not be functioning as it should to begin with, since bacterial need nutrients, and deionized water has next to none. I'd take a two-pronged approach:
  • Review the function of the system, to ensure it is actually producing high-quality DI water, and that it does not need further work
  • Install a UV light source in the tank (there are UV sanitizing lights made for this specific purpose)

Fritz Byle
Process Engineer
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.

Reader Comment
Just a note on the use of the UV light. If the water becomes stagnant, say the system is shut down for several days over a holiday or a plant shut down, the UV is not doing anything except to the water that is currently directly exposed. Any water in a holding tank or piping can still develop bacterial growth. It is a good idea to have the water circulating past the UV light 24/7.
Alan Woodford, NeoTech

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