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Stencil cleaners.

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I am looking at two stencil cleaners, and need some input. ... - Jul 10, 2006 by CK  


CK

#42639

Stencil cleaners. | 10 July, 2006

I am looking at two stencil cleaners, and need some input. Electrovert Aquajet 2000 and EMC 30 Reliant. I am told that these are both spray type cleaners. My only experience with stencil cleaners is a PMR Ultrasonic unit which works well for the no clean flux formula we use. My stencils have a 29"x29" frame. I need to know pros and cons, if the systems are closed loop et. Thanks.

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#42646

Stencil cleaners. | 10 July, 2006

The potential to damage stencils by bending the metal of the fine pitch portion of stencils is a concern about spray cleaners.

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#42748

Stencil cleaners. | 17 July, 2006

Here is something I copied from Circuitnet.com. This appears to answer your question and then some... http://www.circuitnet.com/experts/

Ask the Experts Jul 17, 2006

What type of cleaner method is preferred for lead-free stencils? What type of stencil cleaner is preferred: ultrasonic or high pressure jet? The application is in lead free cleaning of stencils after screen printing.

Ask the Experts Comments:

----------------------------------------------------------

This is a good opportunity to let everyone know that the IPC has been working on a Handbook for Stencil and Misprinted Board Cleaning. The final draft was submitted in early June and hopefully the document (IPC-7526) will be released soon.

The decision process for selecting a system for cleaning lead-free is the same for lead/tin solder paste.

There are several factors to consider when selecting a stencil cleaner. Most experts will agree that the most important factor is the chemistry. The machine used to apply the chemistry is secondary. Washing greasy hands is a good analogy. Using a facial soap is usually not very effective, even with hard scrubbing and hot water. However, if a good hand cleaner is used, mild scrubbing is only required. The �soap� = the chemistry and the �scrubbing� = the machine.

Aside from cleaning efficiency, other factors to consider include:

Can the process potentially damage the stencil? Is the process also used for cleaning misprinted PCBs? What is the environmental impact? User safety? Versatility?

The best article I have seen written on the subject is �SMT Stencil Cleaning: A Decision That Could Impact Production� by Richard S. Clouthier. Mr. Clouthier, of Xerox when the article was published, reviews several different chemistries and cleaning technologies. We keep a copy of this and other pertinent articles on our web site: http://www.smartsonic.com - click on the �Recommended Reading� link.

Can the process potentially damage the stencil? Stencils are heat-sensitive. The epoxy glue that bonds the screen to the frame and metal-etched foil is cured at approximately 60 degrees C (140 F). If a stencil is washed in hot water or dried with hot air, the bond will weaken and eventually fail.

In addition, stainless steel has a very poor memory. Once it expands, it does not like to contract back to the exact same position. Therefore, fine-pitch apertures can become distorted and miss registered if the stencil is consistently exposed to heat.

The Clouthier article illustrates how delicate land mass areas, between fine-pitch apertures, can bend from the impact of high-pressure air or high pressure water jets, so air knives and high pressure water jets should be avoided.

Is the process also used for cleaning misprinted PCBs? The Clouthier article uses the term �Solder Ball Nightmare� when attempting to clean misprinted PCBs with spray jet technology. The sprays will broadcast the solder balls throughout the process chamber and contaminate the populated side �A� of a reflowed board.

Ultrasonic technology will not broadcast the solder balls. The solder balls fall away from the board by gravity. So, if the board is oriented properly (populated side up), the solder balls will not contaminate to populated side.

Studies by GEC Maroni and the EMPF Laboratory are summarized in an article by William Kenyon, �Why Not Ultrasonic Cleaning?� (http://www.smartsonic.com/article.html) Mr. Kenyon stresses the importance of High Ultrasonic Frequency and Low Power Density when cleaning PCBs. Using the analogy of scrubbing with a brush, the Ultrasonic Frequency = the type of brush and the Power Density = the amount of pressure used on the brush. Common ultrasonic frequencies range from 20 kHz up to 68 kHz. Most all ultrasonic stencil cleaners use 40 kHz, which has been shown to be safe and effective for cleaning PCBs and stencils. The frequency is normally modulated +/- a few kHz to eliminate �hot spots� in the cleaning bath (referred to as �sweep� or �variable� frequency).

Low ultrasonic frequency (20 kHz) = a wire scrub brush High ultrasonic frequency (68 kHz) = a soft tooth brush Low power density = holding the brush lightly with one hand High power density = holding the brush strongly with two hands

The Power Densities of stencil cleaners can vary tremendously. Mr. Kenyon�s article indicates approximately 10 watts / liter (amount of ultrasonic energy directed into the bath / amount of wash solution in the bath) as being safe. Many ultrasonic stencil cleaners use power densities of 15 w/l or greater. This is more than 50% higher than the recommended safety limits. The higher power densities are normally dictated by the chemistry used. Less effective chemistries require more scrubbing energy and therefore higher power densities.

What is the Environmental Impact? Biodegradable and VOCs (Volatile Organic Compounds) are the two key variables. Many of the popular aqueous chemistries are biodegradable. However, many still have a high VOC content. The latest VOC standard set forth by the South Coast Air Quality Management District�s Clean Air Solvent (CAS) Program is less than 25 grams per liter. Chemistries containing VOCs greater than 25 grams per liter are disallowed by California�s Rule 1171. Other states and countries have or will soon have similar restrictions.

User Safety? Flammable solvents such as alcohol have been the cause of several fires and explosions when used in a stencil cleaner. Static charges, the mechanical spark of a spray nozzle hitting a metal stencil and nearby electrical equipment have been the sources of ignition.

Noxious odors of some cleaning solutions can cause illness and employee discomfort.

Versatility? Can the process clean a broad spectrum of solder pastes or just the current flux type in use? If a new flux is introduced, will the process support the change? Newly introduced chemistries will not have the necessary track record. If a company has needed to introduce new formulations over the years to keep up with current flux technologies, the newest and greatest formulation from that company should be suspect. If the chemistry needs to change, the waste management and possibly the machine will also need to change starting the selection process for new equipment all over again.

If cleaning lead and lead-free solder pastes, are two separate machines required? Traces of lead will be present in the form of fugitive solder balls or lead ions in any stencil cleaner used for cleaning lead solder paste, so the potential of lead contamination onto a lead-free stencil is always present. Solder paste is broadcast throughout and adheres to the side walls and plumbing of the chamber of a spray technology stencil cleaner and ions often flow through micron filters. Lead ions will also be found in the wash solution of an ultrasonic machine. The key element to determine if two separate machines are required is the rinsing ability of the chemistry. Surfactants (detergents) tend to rinse easily and thoroughly. Saponifiers and solvent-based chemistries usually require longer rinse cycles.

In summary, first select a broad spectrum, non-flammable and low VOC chemistry that rinses easily. If using a spray jet technology and are also cleaning misprinted PCBs, be aware of potential solder ball contamination and use low spray pressure for cleaning delicate stencils. If using ultrasonic technology, orient misprinted PCBs properly to prevent solder ball contamination of reflowed components and use the highest ultrasonic frequency and lowest power density to effectively clean. Make sure that the chemistry manufacturer and the machine manufacturer guarantee the complete process else, if it fails to perform properly, support will be difficult.

Bill Schreiber, President Smart Sonic Corporation

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#42921

Stencil cleaners. | 24 July, 2006

Here are some additional comments that have been posted on the CircuitNet web site:

Fist, the answer is the same regardless of the specific paste type, (leaded or non-leaded).

High pressure sprays should never be applied to a stencil. Stencils are relatively delicate and should not be exposed to either high pressures or high temperatures. The application of ultrasonic energy (sound waves) is considered by experts to provide thorough cleaning while avoiding any damage potential.

Mike Konrad, President Aqueous Technologies

________________________________________

A great question with a simple answer. In my experience, ultrasonics do a much better job of cleaning the stencils than high-pressure jets. Ultrasonics impart far more energy into the contamination through the solvent than any high-pressure spray could generate. Some tests have found collapsing cavitation can generate up to 10,000 times the force of gravity, which will certainly knock any solder paste off a stencil. No pump system can produce that kind of energy.

Now, there is always the question of damaging the stencil with ultrasonics. The people who most often ask this question are the guys selling the high-pressure spray systems (**sigh**). It's old news. In the old days, most ultrasonic-equipped vapor degreasers operated at a single-frequency, usually in the 40 KHz range for solvents. It was found that certain components, such as the tiny metal traces holding a BGA pattern into the stencil, could suffer damage to their leads from the harmonic vibrations of the ultrasonic frequencies.

(Remember the collapse of the Tacoma Narrow's bridge, "Galloping Gertie"? It is the same effect only with wind instead of ultrasonics providing the energy.... See this link.

However, in today's world almost all ultrasonic systems using frequency "sweeping" to enhance the cleaning process. Thus, the transducer will start at 25 khz and race up to, say, 90 khz, and then drop back down in random patterns. Since there is no repetitive frequency used, harmonics do not accumulate in the solvent. So the risk is completely eliminated.

Most companies using modern, digitially controlled, sweeping ultrasonics find they can do so without damage to their boards or stencils. Of course, get some expert assistance when specifying a cleaning machine, because too much power can break anything apart, with or without the harmonic build-ups.

More importantly, do not use flammable solvents with ultrasonics. The risk of a sonic transducer going bad, while tiny, is still a risk. In such an event, the transducer would fire a bolt of electricity right into the sonic bath. If the bath were say, alcohol, it would be like hitting the IPA with a bold of lightning, and a pretty spectacular fire would result. So get an ultrasonic tank and use a nonflammable solvent, like Micro Care's BGA Stencil Cleaner, to do the cleaning.

The attached article, by a fellow named Kenyon, pretty well explains the process.

Best regards,

Mike Jones, Vice President Micro Care

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CK

#42930

Stencil cleaners. | 25 July, 2006

Thanks for the input. After reading that article, I think I will stick to ultrasonic.

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#42933

Stencil cleaners. | 25 July, 2006

Here is an excellent technical explanation of stencil cleaners and their differences (spray, ultrasonic, chemistry type) that I found in the archives:

http://www.smtnet.com//forums/index.cfm?fuseaction=view_thread&CFApp=1&Thread_ID=3026&#Message12326

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#42948

Stencil cleaners. | 26 July, 2006

Samir, why do you bring back such idiotic rubbish between two gentlemen that just happen to fall into their presidency? It is not rellavant to the questions at hand. I agree that ultra sonics can clean better but do take longer IMO. As for stencil damage, I do not see how a sprayer can damage a stencil. I have actually stood on some of my stencils and they do not break. Granted I only eat 3 bananas a day since I am fasting. Still you would need a a very high pressure to dmage a stencil. Sorry Davef, I do not mean any dissagreement. You are "da man" around here and are still top dog in ALL our hearts.

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Drew

#42949

Stencil cleaners. | 26 July, 2006

I enjoyed that little exchange between the presidents! It sounds like the guy from the ultrasonic company is more interested in selling his chemistry versus selling a stencil cleaner. The other guy had a little of the sneaky salesman talk but seemed a little more impartial.

This message was posted Add this forum to your site! Click to learn more. the Electronics Forum @

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#42954

Stencil cleaners. | 26 July, 2006

Acually, Dr. Shocker, DaveF is correct. The high power spray can bend the metal between the aperatures on fine pitch parts. I'm sure it's not common, but I have seen this happen. I have not, however, stood on any stencils. Sounds like fun.

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RDR

#42959

Stencil cleaners. | 26 July, 2006

Come on guys, Mumtaz does not even know what a stencil is.

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#42963

Stencil cleaners. | 26 July, 2006

Yes he does. He prints his own T-Shirts.

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#42989

Stencil cleaners. | 27 July, 2006

Yes I do know what stencils are. We have many here and use them for glue and paste. You can stand on them, trust me.

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Rob

#42992

Stencil cleaners. | 27 July, 2006

....and for making little foil hats to keep the voices out.

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Kevin W. Parker

#43944

Stencil cleaners. | 14 September, 2006

I am a tool and process engineer responsible for the cleaning system at Siemens VDO in Huntsville, AL. I have been over this area for more than 8 years. We use EMC Cyberclean 3000RF spray in air cleaners with Zestron SC202 chemistry. At this facility we have BGA, Fine Pitch, 0603 and 0205 components. The facility runs 3 shifts, produces 20,000 to 25,000 completed assemblies a day. In all of my experience, I have never seen a stencil damaged by the force of the spray. We have washed over 10,000 stencils during this time and most of the damage comes from handling not washing. Both the ultrasonic and the spray in air work. The reason we went with spray in air is the time factor and the ease of use. The ultrasonics take longer and it requires more effort on the operators part. If you choose the correct chemistry and the set up your process correctly, both will work. We also use ultrasonics to clean misprinted PCB's (unpopulated only). This system requires a separate rinse and dry system EMC also.This works fine. We use the spray in air systems to clean populated PCB's. Oh, by the way we use the same spray in air cleaners to wash adhesive stencils too.

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Kevin W. Parker

#43945

Stencil cleaners. | 14 September, 2006

They also make cool lampshades.

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aj

#43958

Stencil cleaners. | 15 September, 2006

Kevin,

Dont be so hard on yourself, "I am a Tool"....

aj...

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#43963

Stencil cleaners. | 15 September, 2006

We're all tool unless we own our own companies. Even then you're just the customer's tool then. At least every tool has a purpose. Some maybe rusty or outdated, but all have a purpose.

I also agree with Kevin though, I too have never seen a stencil washer bend up a stencil. Granted, I guess given some time and "tools", I'm sure I could get too, but never under normal conditions.

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#44001

Stencil cleaners. | 17 September, 2006

Hi, CK,

I am kenneth from SMT resources Pte Ltd.

Our company sell Steacil Cleaner CONTEK - CT-2000,(Full preumatic).

Advantages: -More safety compare with using Electric. -Very easy to use. -Lower prices with high performance. -Able to clean stencil and PCB cleanly

if interested, please contact me with email below: kenneth.lee@smtresources.com So i can let you know more about our Steancil Cleaner.

Kenneth

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#44006

Stencil cleaners. | 18 September, 2006

Speaking of tools...

| V

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