|Ask the Experts|
October 3, 2019
Ultrasonic Cleaning and Surfactants
I've heard that deionized water is a poor conductor of ultrasonic cavitations. Can we add a small amount of a surfactant to improve the ultrasonic effectiveness?
|Expert Panel Responses|
In short, yes. Deionized water does have a high surface tension, and because the ultrasonic cleaning process relies on cavitation, the higher surface tension means that producing cavities is more difficult. Another consideration is that most electronic assemblies have small spaces into which deionized water cannot easily penetrate.
It is therefore a good idea to use an additive containing surfactant that is both formulated for ultrasonic cleaning and is appropriate to the soils to be removed and the materials to be cleaned. Of course, with electronics it is also necessary to ensure that the frequency and power levels of the cleaning equipment are appropriate for electronics. Guidance on this is available in IPC TM650 Test methods 220.127.116.11 and 18.104.22.168
You are correct. It is less efficient to cavitate DI water and it doesn't take much surfactant to improve the efficiency.
As to which surfactant to use will depend on the contaminate to be cleaned. If you are just removing light particulate matter (dust) and/or light body oils, then a small amount of JOY or Dawn dishwashing liquid should work well. If cleaning a more tenacious contaminant such as water washable solder paste from stencils and/or misprinted PCBs, a surfactant blend such as 440-R SMT Detergent may be needed.
Start with about 1 oz (30 ml) per 5 gallons of wash solution. You may need to rinse with DI water after washing with a surfactant.
Smart Sonic Corporation
You may add a surfactant/detergent. Pay special attention to the data-sheet for additional instructions, temperature & pressure settings. You do this in order to maintain any foam generation to a minimum.
Senior Manufacturing Engineer
Vapor pressure and surface tension are the two most important fluid properties governing the formation of cavitation in ultrasonic cleaning systems. Unhappily, water "strikes out" on both of these characteristics; it has a high vapor pressure AND a high surface tension. This is the reason it's hard to get water to capitate or clean.
In fact, water is not a great cleaner. The inefficiency of water cleaning stems from the chemical properties of water itself. These properties are (a) density, (b) surface tension, (c) viscosity, (d) vapor pressure (how quickly a material evaporates), and (e) the pH or Kb values. For example, denser liquids clean better. Liquids with low surface tension and low viscosity clean better. Liquids that evaporate quickly will cavitate more aggressively than high-vapor pressure liquids. Liquids with higher pH or higher Kb values clean better. As you probably know, water scores poorly on all these criteria. This means there are very few options to improve the effectiveness of ultrasonics cleaning systems.
Because of all this, traditional, single-tank ultrasonic cleaners most often use water with some type of surfactant to enhance the cleaning process. While the surfactants can't change the other characteristics of water, they at least can lower the surface tension to improve the formation of cavitation. Some people call these types of additives a "coupler." So the answer to your question is, yes, adding a surfactant is common and almost essential.
But I have three supplemental thoughts for you to consider:
A. In an effort to improve cleaning results, I also have seen people use alcohol or "white spirits" in ultrasonic tanks to get better cleaning. This is a very dangerous choice. When an ultrasonic transducer fails, it shoots all of it's electrical energy out of the transducer into the cleaning fluid. If this is a flammable liquid you end up with a fireball shooting up from your cleaning system, which is generally considered a bad idea. So only use nonflammable liquids, and the one that's most widely accepted for non-critical applications is water and soap.
B. There rarely is any capability on ultrasonic cleaners to recycle the cleaning fluid. As it becomes contaminated, the cleaning quality deteriorates. Eventually the operator must empty the machine and add new, fresh cleaning liquid. This can be expensive. Additionally, because the cleaning process is constantly (if slowly) deteriorating, the parts never are cleaned consistently. The first batch come out perfectly clean, but the last batch is borderline. This inconsistency is a significant side-effect of ultrasonic / batch cleaning.
C. Lastly, if this is a critical application, and you really need to get these parts properly and consistently cleaned, vapor degreasing might be a better choice for you. Vapor degreasing approaches the problem completely differently.
Vapor degreasing uses two tanks (also called sumps) inside the cleaning system. The tanks have different names: one is the "boil sump" where the cleaning happens, and the other is the "rinse sump." Those tanks are filled with a specialized non-aqueous, nonflammable cleaning fluid, optimized for your contamination. There are many choices for these cleaning fluids, but they are they all boil at relatively low temperatures (typically 40 degrees C instead of the 100 degrees C of water) and they have the right chemical characteristics (surface tension, density, etc.) to deliver fast and easy precision cleaning.
Now there are many nuances to this discussion, more than I can put into this memo. But basically the differences between water cleaning and vapor degreasing are night and day. Aqueous ultrasonic cleaning is cheap, but also slow, simple and limited. Vapor degreasing is fast, safe, economical, consistent, and delivers absolutely perfectly and consistently clean parts, every single time.
How clean do your parts need to be? If you need perfect cleaning, quickly, easily, gently, safely, inexpensively, then you need a vapor degreaser.
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