In almost all cases, 20 kHz is used. A sweep frequency ensures that the actually frequency moves +/- 5 kHz. Although ultrasonic technology is effective in post-reflow and misprint defluxing, it is not commonly used as fear of damage discourages most assemblers.

While the ultrasonic frequency is an important consideration, research studies have shown the Power Density and ultrasonic exposure time (cycle time) may be of equal or greater importance. There are several published articles and a copy of the IPC-7526 on the subject available for download at:

SmartSonic, click on the "Recommended Reading" button.

The IPC-7526, “Stencil and Misprinted Board Cleaning Handbook” summarizes these cleaning parameters on Page 6, Section 4.3 as follows:

• Ultrasonic Frequency of 40 kHz or higher is effective and should alternate or “sweep” +/- 3 kHz
• Sweep technology eliminates hot spots or focused ultrasonic energy in the wash bath
• The lower the ultrasonic frequency, the more aggressive the scrubbing action (cavitation)
• The Power Density is the amount of electrical energy delivered to the bath via the ultrasonic generators. This energy is measured in watts per liter of wash solution. For example, if the wash bath contained 100 liters and the generator is rated at 1000 watts, the power density would be 10 watts per liter. The same studies indicate that the power density should be around 10 watts per liter or less.

Articles by B.P. Richards et al suggest the ultrasonic exposure time to be less than 10 minutes. Normal ultrasonic cleaning cycles for PCB cleaning range from 1–2 minutes, well within the recommend guideline.

Ultrasonic cleaning is specifically addressed in the J-STD-001D document. You’ll find that ultrasonic cleaning is not recommended for a populated circuit board.

However, if you do choose to use ultrasonic cleaning, check with the component manufacturer to establish if:

1. the component CAN be cleaned with an ultrasonic system and,
2. what frequency range is permissible for the individual component.