Flux Cleaning in PCB Assembly: Importance and Methods
Rush PCB UK recommends the use of flux when soldering components on printed circuit boards. Flux is a synthetic agent capable of removing adulterants from metal surfaces, making them ready for proper wetting by molten solder. The electronic industry commonly uses solder flux for PCB assembly.
Flux has additional functions as well. It melts and covers the metal surfaces during the soldering process, thereby preventing them from being oxidized by air coming into contact with them. Flux also removes adulterants that may be present on the metal surfaces, enhancing their wetting capabilities. However, although flux has several good attributes, the residue it leaves on the PCB surface is a matter of concern.
The residue that flux leaves on the board after completion of soldering looks like white or yellowish powder sticking on the PCB. Unless the assembler cleans the residue properly, it can cause long-term damage to the assembly such as reducing the surface resistivity, leading to current leakage and unpleasant results. Ignoring the flux residue can result in unexpected results:
- A weak ion flow path between copper traces and pads under different conditions.
- The presence of a trace amount of flux is good enough for creating the leakage paths.
The above issues has resulted in solder manufacturers offering no-clean solder that leaves little or no residue.
Why is Flux Necessary?
Solder is the process of anchoring metallic components on a printed circuit board. Molten solder wets the copper pad and the metal lead of the component to form a metallurgical connection. As the solder cools, it solidifies and also anchors the component to its pads. This process requires the metal surface of the board to be as clean as possible. Cleaning the surfaces prior to soldering prepares them for a perfect bonding with solder.
Solder manufacturers offer flux as a synthetic cleaning agent useful during and after the soldering process. Flux ensures the surfaces are ready for soldering by cleaning and removing any pollutant like oil and grease from them. It also establishes a film over the metal surfaces, preventing them from coming into contact with air, as pollutants present in air can lead to oxide formation on the metal surfaces. It is difficult to solder metallic surfaces when there is oxide on them.
Flux also serves as a protection against deoxidizing the metal surfaces undergoing soldering. It contains chemicals and additives that not only hamper oxidization, but also aids the soldering process. Flux is available in various forms like paste, liquid, or solid form depending on its usage.
Types of Flux
The electronic industry uses three types of flux, depending on the application:
Selective Soldering — This is liquid flux. The operator must spritz it on a select portion of the PCB. They do this by a specific process of jet drizzle.
Wave Soldering — This is also liquid flux. The operator must spritz it on the entire board during the preheating stage before the PCB assembly goes for the final soldering. The hot flux removes pollutants and oxidized layers that could hinder the soldering process.
Reflow Soldering — This is a paste type of flux mixed with solder paste. The operator applies the paste through a stencil. The process controls the amount of solder paste deposit. It deposits a small amount of paste on the solder pad of the board. This paste holds the component in place until the heat of the oven melts the solder paste and anchors the component to the board. Prior to the solder melting, the flux cleans the surfaces while also covering the metal surfaces, preventing them from oxidization.
Why is it Necessary to Clean Flux
Flux residue is often present on the soldered elements, especially on the PCB surface. This could cause issues in the long run. Therefore, cleaning the flux residue is an important part of the PCB assembly process. Major benefits of cleaning the flux residue are:
Preventing Corrosion of PCB and Components
The flux residue after soldering is acidic and, therefore, hygroscopic. Allowing it to remain on the PCB surface for a long duration may attract water from the surrounding air, leading to corrosion of the metal surfaces. Cleaning the flux residue prevents this corrosion.
Enhancing the Appearance of the PCB
Presence of flux residue on the PCB not only makes it look unattractive, it can also lead to the misconception of an ineffective quality control procedure. Customers might find the PCB assembly repulsive, and this can also lead to a reduction in business volumes.
Improving the Integrity of the Manufacturer
As a vital aspect of business, integrity helps sustain competitiveness. PCB manufacturers sustain their competitiveness by systematically providing good values and earning customer satisfaction in return. By presenting a clean PCB bereft of flux residue, PCB manufacturers make their products more dependable and durable. This maintains relevance, while upholding the manufacturer’s integrity.
How to Clean Flux Residue From PCBs
Using Acetone or Isopropyl Alcohol — Acetone or Isopropyl Alcohol dissolves flux residue readily. The technique is to use a brush dipped in the liquid and gently remove the flux residue from the board. The operator must be careful and apply gentle strokes with the brush to avoid damaging soldered joints. After removing the flux residue, the operator must wipe the excess liquid using a parched cloth, and allow the board to dry up.
Using Industrial Cleaning Agents — Solder manufacturers offer various types of cleaning agents, especially suited to remove flux residue left by the solder paste they make.
Application of these chemical agents may require spraying or brushing it on the board. In some cases, it may also require immersing the PCB assembly in the cleaning solution.
As the cleaning liquid dissolves the flux residue, the operator can wipe the excess with a soft cloth, and allow the board to dry.
Using No-Clean Flux
Manufacturers also offer no-clean flux that produces very low amounts of flux residue deposits on the PCB surface. It is a combination of some inorganic agents blended with organic resins. The operator must use it with the solvent compositions that the manufacturer specifies, to achieve the best results. With proper use, it is not necessary to clean the quantity of flux residue that remains on the board, and the operator may safely ignore it.
Cleaning the flux residue is a tactical approach that PCB manufacturers use to maintain their competitiveness in the industry. Rush PCB UK recommends cleaning flux residue for sustaining the competitive advantage, as it improves the reliability and aesthetics of the PCB