Understanding Soldering – Part 4: How to Use Flux When Soldering Electronics

A couple of weeks ago I went through the biennial process of renewing the registration on my wife’s car. After getting the car tested and paying the fee, I was given the two small registration date labels for the license plates. You have to be careful though because if not applied correctly, these labels may detach while you are driving down the road. This can lead to an impromptu road-side meeting with a police officer who will want you to pay some additional fees. The key to preventing this from happening is to make sure that the surface of the license plate is clean and dry before applying the labels.

This same principle applies when electronic components are soldered to a circuit board. If the metal surface of the board isn’t clean and prepared for the solder, you won’t get a good metallurgical bond between the surfaces. And if paying a traffic ticket for not having current license plates on your car is expensive, just wait until you start getting the bills for the component failures on your circuit boards due to bad solder joints. The key to getting a good solder joint is in using a chemical cleaning agent known as flux before and during the soldering process. Here’s a closer look at all of this and how to use flux when soldering electronics.

Defining and Explaining How to Use Flux When Soldering Electronics

Flux is a chemical cleaning agent used before and during the soldering process of electronic components onto circuit boards. Flux is used in both manual hand soldering as well as the different automated processes used by PCB contract manufacturers. The main purpose of the flux is to prepare the metal surfaces for soldering by cleaning and removing any oxides and impurities. Oxides are formed when metal is exposed to air and may prevent the formation of good solder joints. The flux also protects the metal surfaces from re-oxidation during soldering and helps the soldering process by altering the surface tension of the molten solder.

Flux is made up of a base material and an activator which is the chemical that promotes better wetting of the solder by removing oxides from the metal. It also contains other solvents and additives to help with the soldering process as well as inhibiting corrosion. Flux may be solid, pasty, or liquid in form depending on how and where it will be used. For hand soldering, flux can be applied by a flux pen or is usually in the core of the solder wire that most technicians use. For the automated soldering processes used by CMs during printed circuit board manufacturing, there are a couple of different ways that the flux will be applied.

The Application of Different Types of Flux

There are three different categories of fluxes used for soldering electronics according to IPC J-STD-004B. These categories are; Rosin and Rosin Substitutes, Water Soluble, and No-Clean. Within these categories are different types and chemical compositions of the fluxes depending on the needs of the components and boards to be soldered. Depending on the automated soldering process being used by your contract manufacturer, the flux will be applied in these methods:

  • Wave Soldering: The flux used for wave soldering is usually made up of more solvents than flux used for other applications, and will be sprayed on the board prior to it going through the solder wave. Once in place, the flux will clean the components that are to be soldered to remove any oxide layers that have formed. If the board is using a less corrosive type of flux, then the board will have to go through a pre-cleaning before the flux is applied.
  • Solder Reflow: For boards that are going through the solder reflow process, a paste composed of a sticky flux and small spheres of metal solder is used. This solder paste holds the parts in place until the heat of the oven causes the solder particles to reflow. Not only are the metal surfaces cleaned by the flux, but the pasty nature of the flux seals out the air preventing further oxidation. The solder paste flux also contains additives to improve the flow characteristics of the solder as it melts.
  • Selective Soldering: The flux used for selective soldering processes is applied either by spraying it, or by using a more precise drop jet process.

The method that the flux is applied to each of these solder processes is carefully controlled to ensure that the flux is able to do its job without compromising the integrity of the soldering process. For instance, if a solder paste is being used that has a greater concentration of solvents in it then other types of paste, there can be a problem if the flux is heated too fast. The heated solvents may outgas forming a void in the solder joint, and splatter molten solder onto areas of the board that shouldn’t be soldered. For this reason, the solder reflow process is carefully controlled with preheat, temperature soaking, and reflow stages.

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Cleaning Flux from Electronics

Another aspect of flux is the need to clean it off the circuit board after it has done its job. Some fluxes are corrosive, and their residue may continue their activity and harm the circuit board long after it has been manufactured. The three categories of flux mentioned above each have their own cleaning needs:

  • Rosin Based: This flux will need to be cleaned with specific chemical solvents that typically involve fluorocarbons.
  • Water Soluble: There are many cleaning agents that can be used for water-soluble fluxes, such as deionized water and detergents.
  • No-Clean: According to the name, these fluxes require little or no cleaning. Usually, any cleaning is more of a matter of cosmetic appeal than actual contamination. However, residual no-clean flux can reduce the adhesion effectivity of conformal coatings, so some sort of cleaning is still recommended.

For those fluxes that are more corrosive, cleaning is essential. Some processes of circuit board manufacturing, such as shielded areas of the PCB that go through wave soldering, can potentially hide flux residue. This residual flux can cause serious problems for the circuit board as time goes by if it isn’t cleaned. In addition to the corrosive problems of the more active fluxes, however, even the residue of no-clean fluxes can interfere with PCB testing, optical inspection equipment, and some sensitive electronic components. In general, it is best to clean flux residue whenever possible.

What You Can Expect from Your Contract Manufacturer

There are many different categories, types, and compositions of flux out there for soldering, just as there are many different types of solders and soldering processes. To be assured that your PCB design is going to be manufactured correctly with the best combination of materials and processes, you need to work with a CM that has a complete understanding of all of it. Your CM should have years of experience working with these different materials and have the equipment and resources to facilitate these processes.

Tempo‘s Custom PCB Manufacturing Service
  • ISO-9001, IPC-600 and IPC-610 commitment to quality certifications.
  • Accurate quote in less than 1 day.
  • Performs entire turnkey process in as fast as 3 days.
  • Emphasizes DFM to eliminate time-consuming back-and-forth design corrections.
  • Sources components from the most reputable suppliers in the industry to reduce procurement time.
  • Performs multiple automated inspections during assembly to ensure PCB quality for prototyping.
  • Provides support throughout the PCB manufacturing process, beginning with design.
  • Smooth transition from prototyping to production.

At Tempo Automation, we do have the experience, skills, and facilities needed to manufacture your PCB. We have been assembling and soldering circuit boards for a long time, and we will make sure that your design is built to the highest levels of quality.

And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.

If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on how to use flux when soldering electronics, contact us.

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