Ask the Experts
December 20, 2018 - Updated
November 9, 2009 - Originally Posted

Hot or Cold DI Water Rinse

How important is the use of hot water in our deionized batch and inline cleaning systems? As I understand it, a hot water rinse will lower the surface tension to better clean under low profile parts. Could we get equally good results using a room temperature rinse within our DI systems?

R. S.

Expert Panel Responses

In nearly every circumstance, hot water (120-150F) will be better than room temperature water (70-80F) in terms of removing residues from soldering processes. While the surface tension concern that you mention is one aspect in this equation, the fact that solubility of most organic materials is better in hotter water is another strong case for going with hotter water. The greater solubility of organic materials in hotter water generally means that you will have better luck removing residues in hot water than room temperature water.

Brian Smith
General Manager - Electronic Assembly Americas
DEK International
Mr. Smith has been supporting customers in the electronics assembly industry since 1994. His expertise is focused on solder paste printing and reducing soldering defects. He holds a BS in Chemical Engineering and an MBA in Marketing. He has authored several papers in trade magazines and at industry conferences. He is an SMTA Certified Process Engineer.

The answer is "It depends"... Hot water will dissolve residues more quickly than cold water. In a time-critical process step, hot water will be more efficient from a through-put standpoint. Also, materials will typically be more soluble in hot water than in cold. So, cleaner and faster with hot water, in general..And, yes, hot water will have a lower surface tension.

Jim Williams
Polyonics, Inc.
Jim Willimas is a PhD Chemist in Polymers and Materials Science. He specialize in printing, cleaning, inks, and coatings used in electronics manufacturng operations. Williams has more than 30 years experience.

I would strongly recommend heated DI water as some wetting agents used in fluxes will Gel if hit with cold water instead of heated. Also heating the water will aid drying times considerably. Recommend 45-50C water. I'm not too sure about heating water to make it more searching for low standoff components but adding a small amount of Saponifier/cleaning agent will help with this considerably and neutralise any activators present.

Greg York
Technical Sales Manager
BLT Circuit Services Ltd
Greg York has over thirty two years of service in Electronics industry. York has installed over 600 Lead Free Lines in Europe with Solder and flux systems as well as Technical Support on SMT lines and trouble shooting.

Surface tension of water can be reduced and its solvency can be increased by increasing its temperature. Hot rinse water has better capability of creeping underneath components and rinsing off potentially re-deposited contaminants faster from the wash cycle.

Umut Tosun
Application Technology Manager
Zestron America
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".

You can get decent results but not equally good results. The rinse will be more active and do a better job in a shorter period of time. The old rule of thumb, double the activity for every 10 degree C rise in Temp applies here. You have to find your own sweet spot however. Perhaps some SIR testing at different temps is in order if you really want to find that sweet spot for your operation.

Daniel (Baer) Feinberg
President and Founder
Fein-Line Associates
Mr. Feinberg is a 52 year industry veteran and President and founder of Fein-Line Assoc, a consulting group serving the Global Interconnect and EMS industries.

Heating the final rinse water is a waste of energy and is not needed in most modern inline cleaners. If the wash, chemical isolation and power rinse sections did their jobs, the flux and cleaning chemistry are gone. The purpose of the final rinse is to dilute soiled rinse water before stripping the water in the dryer; not to clean the boards.

Steve Stach
Austin American Technology
Founder and President of AAT. Steve holds numerous patents and has authored numerous research papers and articles in cleaning and soldering. Steve is a founding member of the Central Texas Electronics Association and is a past Director of IMAPS. Steve is active on several IPC cleaning committees.

Reader Comment
Hello, all regarding the hot DI water dip process. I am using this process as pre-gold wire bonding clean on the gold surface of the microstrip (RF CIRCUITS). HOT DI about 70C and propanol dip after words. I need some scientific article for this subject.
Shlomo Mordechai, Herley GMI Jerusalm, Israel

There is little advantage to heat the rinse water in a defluxing process. Keep in mind the solubilization of the flux (and other contaminants) is accomplished in the wash process. During a heated wash process, assembly and process residues are placed into solution within the cleaning agent. The purpose of rinse is to remove the wash solution (which contains the contamination) from the assembly.

During the wash process, added cleaning agents reduce the surface tension from 74 dynes to 22 dynes. This is where the cleaning happens. To rinse, pure DI water is used. The surface tension of room-temperature water is 74 dynes. If the water is heated to 150F/65C, the surface tension is only lowered to 65 dynes. The amount of energy required to heat the rinse water is more than any benefit received.

Also, if the rinse water is being recycled via traditional carbon/resin exchange technology, the water can not be heated to temperatures exceeding 120F/49C. Excessive rinse water temperatures will cause premature failure of the ion-exchange resin beds.

Mike Konrad
Aqueous Technologies
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.
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