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Removing Flux From PCBs Using an Ultrasonic Cleaner

Flux residue is a necessary evil when it comes to printed circuit boards (PCBs). This substance is required to ensure that solder bonds properly to the copper pads on a PCB, but it can be difficult to remove once the soldering process is complete.

An ultrasonic cleaner for electronics is one of the best methods for removing flux from PCBs, and it can also be used to clean solder paste, fingerprint oils, marking inks, and other contaminants from a range of sensitive electronic components.

PCB Cleaning

Why Is It Important To Clean Flux From PCBs?

Flux is a substance used in soldering to help wet the surfaces of metals that are being joined. It also helps to protect these surfaces from oxidation during the heating process. Once the soldering is complete, it's important to remove any residual flux from the PCB as it can cause a number of problems.

Residual flux can be corrosive, which can damage sensitive components on the PCB over time. In addition, it can attract dirt and other contaminants, which can lead to electrical shorts and other problems. For these reasons, it's important to ensure that all traces of flux are removed from PCBs after soldering.


Cleaning Examples

Printer Heads
Substrate: Electronics
Contaminant: Printing inks
Process: BT 60H, 150°F, 5 minutes, Brulin 815GD 10%

PC Board
Substrate: PC board with various types of solder conditions
Contaminant: Solder flux residue
Process: BT 60H, 140°F, 2 minutes, Brulin 815GD 10%

Why Use an Ultrasonic Cleaner?

There are a few different ways to clean flux from PCBs, but using an ultrasonic cleaner is generally considered to be the most reliable method. One alternative is spray washing, but to remove stubborn contaminants, you’d need to use some type of abrasive such as a brush. Even soft bristles can damage sensitive components. Ultrasonic cleaners are effective, thorough, and gentle, making them an ideal choice for cleaning delicate electronic components.

An ultrasonic cleaner uses high-frequency sound waves to create microscopic bubbles in a liquid. These bubbles gently agitate the surface of the PCB and help to loosen and remove contaminants. Once the PCB is placed in the solution, the ultrasonic waves will break up the flux and allow it to be easily rinsed away. Because the PCB is fully submerged in the cleaning solution, the ultrasonic waves reach every crevice. This makes this cleaning method ideal for components with surfaces that are inaccessible for cleaning by other methods such as those with complex geometries and blind holes.

Many electrical assemblies have delicate parts that are susceptible to damage, but the ultrasonic cleaning process is gentle enough to not damage these components, yet effective enough to remove even stubborn flux residues. In addition, ultrasonic cleaning is fast and cost-effective, making it ideal for use in production environments.

How To Clean Flux With an Ultrasonic Cleaner?

Using an ultrasonic cleaner for electronics is quick and simple. Here are the basic steps:

  1. Fill the cleaning tank with a suitable solvent. Water is often used as it's readily available and relatively inexpensive. However, you may wish to use alcohol or a mildly acidic or alkaline ultrasonic cleaner solution for electronics. Just be sure to check that the detergent does not contain metal cations which could leave a conductive residue. Chelating agents and halides should also be avoided.
  2. Degas the solution to remove trapped air that can slow the cleaning process.
  3. Place the PCBs in a basket or directly in the tank, as applicable, completely submerging them in the cleaning solution. Ensure that boards are not in contact with each other.
  4. Switch on the ultrasonic cleaner and allow the PCBs to clean for several minutes. The exact cleaning time will vary depending on the type of ultrasonic cleaner being used and the amount of flux to be removed.
  5. Remove the PCBs from the cleaning tank and inspect them visually to check that all flux has been removed.
  6. Rinse them with deionized water to remove any residual detergent. This can be done manually or by using an ultrasonic cleaner rinse cycle.
  7. Allow the PCBs to dry completely before continuing with the assembly process. When searching for the best ultrasonic cleaner for electronics, you may want to look for one that has an in-built drying cycle to speed up this part of the process.

As long as they are used correctly, ultrasonic cleaners offer a quick and effective method to clean flux from PCBs.


Are There Any Potential Risks?

There are a few potential risks associated with using an ultrasonic cleaner to clean flux from PCBs, but these can be easily avoided.

  • Using the wrong solvent: If the wrong solvent is used, it could potentially damage the PCB. When using an ultrasonic cleaner, it’s important to choose a cleaning solution that is specifically designed for use with electronics. 
  • Lack of proper rinsing and drying: If the PCBs are not rinsed properly after cleaning, residual solvent could cause problems with the function of the board. And if they are not dried properly after rinsing, residual water will result in corrosion.
  • Not following instructions: If the cleaning process is not carried out carefully, it's possible to damage delicate components on the PCB. It’s important to follow the manufacturer’s instructions carefully when using an ultrasonic cleaner.


With a little care and attention, you can clean your PCBs effectively without damaging them.