Cleaning in an ultrasonic cleaner, bare boards, assembled boards and raw parts will poorly clean even with the most expensive solvent. The energy is to low and does a poor job of cleaning flux and fabrication residues. Low pressure cleaning with saponifier still works the best but followed by steam will clean even the no cleans. We can do a comparison using Ion Chromatography to show the effectiveness.

This is an extremely complex issue with no simple answer. Ultrasonic cleaning systems can have special requirements, and there are a number of good choices from which you can select. But first, a bit of background information.

In general, your solvent must meet the following criteria:

• The solvent must be nonflammable. DO NOT USE FLAMMABLE SOLVENTS IN ULTRASONIC CLEANERS due to the risk of fire.
• It should be ozone-safe and environmentally benign;
• It must have manageable toxicity ratings;
• Unless your cleaning system has refrigeration, the solvent must have a higher boiling point, usually between roughly 100 degree C or higher;
• The component chemistries should not separate when heated (which is called "azeotropic" behavior in the chemical world),
• The chemistry should not foam when agitated, and...
• It should be recyclable and disposable using standard industrial processes.

Basically, you need to consider the contamination you're trying to remove, the compatibility of the materials you're trying to clean with the potential solvents, the through-put you require, and cleanliness standard to which you are operating. These four parameters will define the solvent and the cleaning system you need.

First and foremost, one must understand that "ultrasonic" cleaning is just a mechanical scrubbing action. As with any cleaning application, the most important factor to consider and evaluate is the cleaning "chemistry". A simple analogy would be washing your hands after working on your car. If you use hot water and a "facial soap", you can scrub for hours and not remove all of the contaminant.

However, by using an appropriate "hand cleaner" soap you will need to scrub less to obtain far superior results. The "scrubbing" action equates to the ultrasonic cavitation or spray impingement of a cleaning machine.

Without the right cleaning chemistry, the cleaning machine will not be effective. Additional information regarding the importance of the cleaning chemistry can be found at: http://www.smartsonic.com/ssonicfaq.html#1

The value of ultrasonic technology is its ability to deliver the cleaning chemistry into tight tolerance areas where spray-in-air technology cannot reach.

While Smart Sonic does not manufacture equipment for the final cleaning of PCBs, our ultrasonic stencil cleaners are often used to clean populated double-sided misprinted PCBs. As a result, we have compiled a wealth of published information about the benefits and use of ultrasonic cleaning technology as it relates to cleaning electronic circuitry. These documents are available as free PDF downloads at our web site, http://www.smartsonic.com/ , click on the "Recommended Reading" button.

As a rule-of-thumb, I feel comfortable in summarizing these documents into the following guidelines:

1. The ultrasonic frequency should be 40 kHz or higher (the lower the frequency, the more aggressive the ultrasonic cavitation).
2. The power density should be 10 watts per liter or less (this is the amount of electrical energy transferred into the cleaning bath via the ultrasonic generator, measured in watts per liter of cleaning solution in the tank).
3. The ultrasonic cleaning cycle should be less than 10 minutes (a correctly sized ultrasonic cleaner combined with the appropriate cleaning chemistry should effectively clean PCBs in 3 - 4 minutes).

Additional information regarding ultrasonic frequency and power density may be found at: http://www.smartsonic.com/ssonicfaq.html#13 .

Firstly you need to consider at what stage of PCB manufacture you wish to clean and therefore what contaminants require removal. Contaminants could include flux, solder, surface mount adhesive or other materials made apparent during the assembly process; such as fingerprints, dust, etc. The contaminants involved will play a significant role in the selection of your cleaning material.

Lead free residues, for example, can be more difficult to remove than traditional leaded solder residues due to the high temperatures required for processing. This effectively "bakes" the residues onto the surface of the PCB making them more difficult to remove and so an optimised cleaning process may be required. Selection of the correct cleaning agent must also take into account material compatibility.

Any sensitive metals on the PCB must be evaluated and the correct cleaner/corrosion inhibitor combination chosen for the specific metals present. This stage can be evaluated with the manufacturer of the cleaning product if required. Compatibility with plastics, photoresists and any inks or labels also needs to be checked; most products on the market should not affect standard materials used in PCB production, however.

The time and temperature of cleaning is crucial to obtaining a high level of cleanliness. For ultrasonic cleaning, the material needs to be in one phase at the temperature set for cleaning. Ideally, this temperature should be the lowest possible for efficient cleaning of the PCB within a desirable production time. Cleaning solutions often have an optimum temperature for use and therefore the temperature set must also correspond to that provided by the manufacturer of the product.

There are many manufacturers of ultrasonic equipment who will be able to give more detailed information on the equipment set up and the power output most suitable for your application. This may be crucial to ensure that any delicate joints or components are not damaged during the cleaning process.

Ultrasonics can increase the temperature of the cleaner as the process imparts energy into the system. It is therefore important that the flash point of the cleaner is considered as well as any vapour given off during the cleaning process.

Many types of cleaner are suitable for use in ultrasonics however the main choice is between aqueous technology (non-flammable materials) and solvent based products (flammable). If the latter is chosen then it is imperative that the ultrasonic equipment used is fire/explosion proof and that adequate ventilation is provided. Your emission levels may also have to be monitored.

Finally, with regards to the cleaning agent used, the efficiency of the cleaner for the contaminants present must be reviewed. It is important that after the cleaning stage, no re-deposition of contaminants occurs upon removal from the ultrasonic tank. Therefore the cleaning agent must have sufficient up take of the residues involved.

Alternatively any contamination at the top of the tank may be removed mechanically prior to the PCBs being removed from the solution. Any rinsing or drying stages are also crucial following this process and will be specified within the literature given for the cleaning product.

The cleaning process can be evaluated according to many of industry standards available to ensure that the process gives a high level of cleanliness.

• IPC 610 - Visual Inspection
• IPC TM650 - Surface Insulation Resistance
• J-STD 001 - Ionic Inspection

Please feel free to contact me direct if you would like to discuss your particular application in more detail.