PCB Cleanliness During Assembly — Rush PCB US

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Rush PCB recommends maintaining a clean PCB during assembly. This offers a circuit board the foremost chance of success, apart from ensuring maximum longevity for the product. Although contamination observed on the PCB may be only cosmetic, but it can cause corrosion or functional issues, leading to a failure of the product. For any contaminants on a completed PCB assembly, we take steps to eliminate them, as they are a sign that there is something wrong in the assembly process or the cleaning system. We will describe various discrepancies during the assembly of circuit boards.

Flux Residue on Circuit Boards

All circuit board assembly processes use flux to prepare the metal surfaces for soldering. The flux cleans impurities and removes any oxides. Flux is also the most common contaminant seen on PCBs and is the most discussed. Although there is a no-clean process, and it leaves an acceptable kind of residue, not all customers are willing to accept it. Therefore, for customers who specifically demand a clean PCB, we use a special clean process to remove all flux from the circuit board. We do this either manually, or with an ultrasonic cleaner or an inline wash system.

Foreign Substance Debris on Circuit Boards

The presence of foreign substance debris can cause a PCB failure. If conductive, they can provide a path for electrical current to flow and cause a short circuit. Even when they are not conductive, the debris may dislodge from the board and create problems during the final product assembly. Therefore, any foreign substance debris present on the board is a concern, and we have optimized our assembly and storage processes to eliminate any such potential risks.

Carbonates, Chlorides, and White Residues

A variety of chemical reactions can cause a residue of carbonates, chlorides, and white substances to remain on the PCB after a cleaning process. Water-soluble fluxes and resin, epoxies and circuit board resins, materials from components, and other contaminants may all contribute to the complex chemistry on a circuit board. There are several ways to eliminate or mitigate these types of contaminants.

First, it is necessary to clean the boards immediately following the soldering process. The longer contaminants sit on the board, the harder it gets to clean them. More time means chemicals have a higher chance of reacting and creating such contaminants.

Avoiding the use of excessive flux will also help in lowering the number of contaminants, and so will the switching to a clean process solder paste. Most of these contaminants usually come from no-clean fluxes and are very difficult to remove.

Rush PCB recommends switching to a clean process along with an inline ionic or wash chamber to remove the contaminants from a PCB.

Flux Residues with No-Clean Process

The electronics industry widely uses no-clean flux. This may be due to the cost savings offered by the no-clean process, or because it is not allowed to wash the PCB assembly. However, this kind of flux typically leaves some amount of residue on the board. Too much of this residue may inhibit visual inspection and is not acceptable. However, in most cases, the process is acceptable.

Why Clean PCBs?

Rush PCB recommends a clean PCB as it promotes the functionality and longevity of the circuit board. Moreover, a customer receiving dirty PCBs will certainly not be delighted, and it may reflect on future business.

During the process of manufacturing, a circuit board will pass through several processes, starting from the etching of copper to the application of flux and solder during assembly. All these processes can potentially leave various residues and contaminants on the surface of the board. The residues may have short-term and long-term issues with the board.

In the short term, any visible contaminants and residues can be a cause for concern to customers, even when they may not actually be a problem. An example of this is the no-clean flux residue. Although it may not cause any concerns, it does leave the board looking as if the assembler has not cleaned the board properly.

On the other hand, if the residue from cleanable fluxes is not properly cleaned, the residue could cause concerns during testing. The residue and contaminants may also cause concerns of intermittent functioning in the board, and this may be very difficult to reproduce or find.

Long-term concerns of leaving the residue uncleaned can pose a far larger threat. By not cleaning the PCB assembly thoroughly, contaminants and residues can compromise the finished board’s reliability.

For instance, this can cause ionic contamination. This happens when a finished PCB has iconic residues on its surface. Ionic residues may contain molecules that become conductive in the presence of moisture. This can accelerate the corrosion of copper tracks on the board. They can also promote dendritic growth of metal slivers, which being conductive, may cause unintended shorts between tracks.

Ionic residues can collect on a PCB during manufacturing because of the presence of:

  • Perspiration
  • Acids
  • Plating Chemistries
  • Flux activators
  • Salts

There can be additional long-term issues from inadequate cleaning. These can include poor bonding of conformal coatings, poor contact with connector fingers, cables and sockets, and functional issues with switches and relays mounted on the board. This makes PCB cleaning an important process in the entire manufacturing cycle.

How to Test for PCB Cleanliness

Rush PCB uses an in-line wash and heated deionized water for cleaning circuit boards. We set the water pressure in accordance with the guidelines the solder and equipment manufacturers have provided.

Sometimes we require additional cleaning steps. These may include ultrasonic cleaning or isopropanol alcohol wipedowns, and removal of no-clean flux residues. We undertake these specialized cleanings only when customers provide documentary requests.


To ensure that cleaning of the PCB assembly is adequate, at Rush PCB, we regularly test our cleaning process. We do this with the industry-standard ROSE test. For this, we test the resistivity of the solvent extract. In addition, we also conduct a visual inspection test of the finished PCB assembly to be sure it passes our internal standard of cleanliness.