The hardware cleanliness significantly affects the measurements. Although not all of the parts on the board have to be cleaned, contamination on the sensitive components result in leakage of bias current, resulting in measurement error.

Though there are traditional cleaning methods, such as brushes or sprays, ultrasonic cleaning is highly effective for cleaning electric components. Ultrasonic cleaning is optimized for removing contaminants and flux from complex geometries and small dimension boards.

The ultrasonic cleaning process uses a tank filled with a solution such as de-ionized (DI) water. Once the components are immersed in the solution, transducers generate high-frequency sound waves that create tiny bubbles. The energy of the micro-burst dislodges and removes flux and other unwanted residuals on the surface of the component.

Figure 3-10 Comparison Between DI Water Versus Tap Water

The power of the bubbles is strong enough to erode typical aluminum foil. The energy of micro-burst is reduced in the tap water. As shown in the pictures, aluminum foil in the tap water has less erosion than that of the de-ionized water. This is because tap water has more particles that reduce the energy of micro-burst.

Figure 3-11 Aluminum Foil Before Ultrasonic

Figure 3-13 Aluminum Foil After Ultrasonic With Tap Water

Figure 3-12 Aluminum Foil After Ultrasonic With De-ionized Water

The duration of the ultrasonic cleaning process depends on the cleanness of the board. Typically, cleaning cycles can range from several minutes to ten minutes at warm temperatures. Some components are more sensitive and can be damaged, such as MLCC capacitors, and care need to be taken to not leave these components in the cleaning process beyond the maximum recommendations.

After the cleaning cycle is complete, the components are rinsed with clean DI water to remove any residual cleaning solution. The components are then carefully dried, either through air drying or by using specialized drying equipment such as an oven with dry air to remove moisture. The temperature depends on the components, though, ultra-low current measurement requires 125°C for several hours.