|Ask the Experts
July 19, 2018 - Updated
June 21, 2009 - Originally Posted
Ultrasonic Cleaning Causing Damage
Currently we're using ultrasonic cleaning to remove "no-clean" solder paste and it works fine. However, the use of an ultrasonic system causes damage to one ultra sensitive SMT component. Can you recommend a method we could use to prevent damage to this component caused by the ultrasonic cleaning process?
|Expert Panel Responses
Without knowing what you qualify as "damage" and also not knowing the type of ultrasonic cleaner and chemistry you are using, if is difficult to address the question. However, we maintain a selection of published articles on the subject on our web site: www.SmartSonic.com/article.html
Research by B.P. Richards et al at GEC Marconi and the EMPF Laboratory suggests four parameters for the safe cleaning of PCBs using ultrasonic technology:
Most ultrasonic cleaning systems are made to be used with generic cleaning chemistries and therefore need to incorporate the highest power density that is economically feasible.
Comparing the ultrasonic frequency and power density to a manual "scrub brush" application, the frequency is similar to the type ob brush used. 20 kHz would be equal to a wire brush and 60 kHz would be equal to a toothbrush. 40 kHz is somewhere in between.
The power density is equal to the amount of exertion placed of the brush. High power density (> 10 watts per liter) is like scrubbing hard using two hands. Low power density (< 10 watts per liter) is like scrubbing lightly using only one hand.
Therefore, a combination of low frequency and high power density provides the most aggressive ultrasonic cleaning action and high frequency and low power density provides a more gentle cleaning action.
Most ultrasonic cleaning systems incorporate a high power density because they are designed to be used with non specific cleaning chemistries. Some chemistries are less efficient than others and require a heavy duty scrubbing action.
If you wish to safely clean PCBs using ultrasonic technology, I recommend identifying a high efficient chemistry that will clean effectively using low power density ultrasonics. The wash cyle will usually be only 2 or 3 minutes.
Smart Sonic Corporation
These days, the cleaning of no cleans is only surprising to the marketing people at solder companies. It is a daily occurrence to clean so called "no clean" materials and to do so in standard equipment with exceptional results. The most common process today is an aqueous spray system using a modern cleaning agent that should be non-hazardous and designed to clean "no cleans" be they lead bearing or lead free. This can be a batch system (as your ultrasonic is) or if your throughputs are high, may be an in line system. Another approach is to use a semi-aqueous technology. While not as popular as aqueous, this is a terrific technology option and may work in your current equipment with some minor modifications (we have experience in this area). Again, there are many legacy technologies offered alongside newer materials that have been developed for lead free and lead bearing no clean materials. Both may be good options for you. Using ultrasonics on assembled boards is a bit unusual in the US, though not uncommon in other parts of the world. With the process adjustment you are considering, you are moving toward the main stream.
Generally a spray-in-air (like diswasher type batch cleaning process where top and bottom spray bars rotate as they spray the cleaning solution on the substrates) or alternatively a spray-under-immersion type process (where the spray bars with nozzles are completely submersed in the solution, recirculation and spraying action takes place in the cleaning solution) should be to work gently on these sensitive parts. If there is already an ultrasonic tank, one can easily retrofit this tank with small recirculation pump anda single spray manifold with nozzles on it.
Application Technology Manager
We use a semi-aqueous cleanser that works very well at removing "no-clean" residues when we are required to do so. Best alternative is to use water soluable flux/paste.
Capital Equipment Operations Manager
Specialty Coating Systems
We use saponifier and steam to effectively clean no clean.
President/Senior Technical Consultant
In the past, there have been concerns about the possibility of ultrasonic energy damaging specific types of components such as wire-bonded IC's and crystals. Modern ultrasonic stencil cleaners utilize a lower power model (watt density) than older ultrasonic cleaning systems. Additionally, the sweep frequencies of modern stencil cleaning systems prevent damage to sensitive components. One should be careful when considering spray-in-air technology for misprinted cleaning applications. Too much power and you increase the chance of directing solder paste (more specifically solder balls) into a via and under components. Too little energy, and you leave paste on the assembly.
This is an old problem. The wire bonds inside the package if gold, can transmit the ultrasonic energy and when a node is on the wire it will neck down and become a latent failure or become a catastrophic failure that is detected right away. This is what happens when the PWB the microcircuit is attached to is in contact with the ultrasonic cleaner sides or the basket holding the PWB. It was originally discovered by General Dynamics and a government alert was posted to stop defense contractors from doing this. Use of a sweep frequency ultrasonic bath seemed to mitigate this but the jury was out on it. The Ultrasonic equipment manufacturers did not like this discovery. My advice is don't use ultrasonic cleaning with microcircuits on PWBs unless you are sure the cleaning does not affect the thin gold wires used inside the microcircuit are still 100% okay. They could be necked down and over heating and waiting to fail in the field.Dr. David Ciambrone, Ciambrone and Associates, USA
Damage to cavity packages with crystals or wire bonds is a thing of the past. It was well proven by our GEC Research project. I have cleaned cavity parts as a demonstration with the lids removed with no damage under normal conditions. SMART Sonics website feature our old GEC papers and reports on the testing conducted at the time. On the other part of the topic the key thing is to get you chemistry and flux residues and make sure they are soluble. Then test you cleaner to see its capability to clean at the stand off heights you require. This is even before you move to contamination testing or SIR. Simple tests in manufacture to loo at capability are key. Finely don't forget that most sites have been no clean for so many years engineers often have never evaluated components parts for cleaning compatibility which should be confirmed on a product BOM.Bob Willis, Bob Willis, UK
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