Soldering circuit boards leaves a lot of residue that changes how the circuit will behave. Cleaning a circuit board after soldering is important to improve the performance of the circuit and makes the circuit more reliable. While a circuit board can be partially cleaned with alcohol and brushes, the best way to clean a circuit board is to use an ultrasonic cleaner.
Is Cleaning Really Needed?
Everyone has gone straight from soldering to testing a circuit board with good results, so is cleaning really need?
For precision work, absolutely. Even a small amount of solder residue, solder flux, dust, dirt, and grim can cause a measureable and significant change in behavior in a circuit, sometime it can even prevent a circuit from working entirely. Scrubbing a circuit board with a soft brush with alcohol can remove a lot of the surface contaminates, but the residue left is still enough to have an effect on circuit performance. The only way to really clean a circuit board is to use an ultrasonic cleaner.
Problems with Ultrasonic Cleaning
Ultrasonic cleaning involves fully submerging the PCB in a solvent. This means that certain components such as gas sensors and pressure sensors can be damaged or destroyed by the process. Sometimes these components can be soldered in after the rest of the board has been cleaned or they can be installed on a removable module and installed only after the rest of the board has been cleaned.
Ultrasonic cleaning can damage circuit boards. Choosing the wrong solvent can damage certain parts or the circuit board itself.
Also the ultrasonic energy can be high enough to physically damage components. Modern ultrasonic cleaners are designed for cleaning circuit boards greatly minimize this risk by sweeping the ultrasonic frequency to avoid resonances that can damage parts. They also operate at different frequencies than ultrasonic cleaners designed for jewelry (which rarely sweep frequencies).
How Ultrasonic Cleaners Work
Ultrasonic cleaners are simply a better way to achieve a mechanical scrubbing action across every surface of the circuit board. Ultrasonic cleaners create this mechanical scrubbing action by creating strong back and forth motion in a solvent which creates cavitation and strong pressure waves. Adjusting the power of an ultrasonic cleaner is the equivalent of moving from a gentle scrubbing action to a powerful scrubbing action with both hands and putting you back in to it. Too much ultrasonic power will damage a PCB. If an ultrasonic cleaner’s power level cannot be adjusted adding more solvent to the cleaning tank has the same impact as turning down the power.
To make the scrubbing action more effective and remove all of the grim a solvent is used. The solvent must be selected to be compatible with the flux used as well as with the components and PCB board. Good solvents are compatible with every common material used on PCBs. Most solvents are sold as concentrates and can be diluted with water. Most ultrasonic cleaners also heat the solvent to make the scrubbing action more effective.
Once a PCB board has been cleaned with an ultrasonic cleaner, it must be washed with deionized water and fully dried before it can be used. This is more difficult for boards with lots of large surface mount ICs since the water will wick underneath them and take a long time to fully dry out. While pure water is not electrically conductive, just a few contaminates will make it very conductive, so testing a board before it is fully dry will lead to significant damage in most cases. The best way to dry a PCB is to bake it in an oven at a very low temperature so as not to damage components, but warm enough to speed evaporation.
