Posted on Thursday 16th October, 2014

For those of us close to the electronics manufacturing industry, we are well aware that solder flux is a very important part of the soldering process. Flux is necessary to reduce the oxides that tend to form whenever you have hot metals in contact with the air. In electronics, we use a rosin-core solder, a mild no clean solder, a more aggressive lead-free solder, or a water-soluble solder to aid the soldering process and eliminate metal oxides. We take great care regarding flux to make sure that we have selected a product that's not plumbing flux or acid-core flux because we know that will tend to corrode parts and components over time; it’s okay for pipes but not for electronics.

In many cases, we're looking at rosin solder flux for most applications. It comes in a paste form. If the temperature's a little bit colder, it will tend to crystallize and solidify, but it can be melted with a little bit of heat from the soldering iron. Of course, when you heat flux it tends to produce smoke, so it's good to have ventilation or a vapor extractor to be sure that you're not breathing in the caustic vapors created from heating the flux. When you heat a work piece, flux reacts with the metal oxides that are forming with the temperature and exposure to the air. These residual metal oxides tend to interfere with the metal-forming process of soldering. Flux will chemically react with these oxides to make a nice, clean metal from the solder forming process that will form a pristine alloy on the board.

As we know, many solders come with flux in the core. The solder is actually hollow and inside the solder wire there is a central core of flux. As you feed the solder into your work piece, the flux then distributes itself over the surface of the work reducing the formation of metal oxides and preventing them from occurring during soldering. Flux core solder can be helpful in many soldering instances for small, specific solder and repair projects. Additionally, we should talk about water-soluble flux which is a recent and significant improvement in flux. With water-soluble flux, cleaning becomes easier. With rosin-core flux, the flux is not water-soluble. It takes an aggressive solvent cleaner in order to dissolve it and to remove it from the work piece. While easier to clean, water-soluble flux may not be appropriate in all circumstances. It can leave a residual layer that may interfere with high-impedance and high-resistance circuits where you have resistances (ten mega ohms or greater). The water-based flux may actually lower that resistance and cause the circuit to malfunction.

Solder flux is an essential element when soldering making the soldering project much easier and higher quality. Unfortunately leaving even the no-clean fluxes behind on a PCB can cause some serious issues in circuit performance and even disable components. As part of the basics of flux, it’s important that we understand what leaving flux on a PCB can do to your circuit.

In many instances, no clean flux may not need to be cleaned from a circuit to prevent the flux from physically destroying the circuit. Unfortunately, the presence of flux changes the capacitance between traces, conductive pins, and circuits, and can ultimately get into the mechanical workings of electromechanical components and bind them up. Flux that requires cleaning is very acidic and can eat away at traces and pins changing their resistance and even destroying surface mount components if the flux is left in place too long.

One particular location in board rework and repair where the direct presence of residual flux has a significant impact is on or near the input circuits to a differential input. Differential inputs are often balanced and used for very precise measurements. The presence of flux here will not only throw off any measurement, but can cause the readings to drift significantly over time.

Electromechanical components are also significantly impacted by flux. Often these components are soldered in by hand in the first few prototypes. The hand soldering of these components often works just fine, and the circuit moves on to an initial low rate production run where problems start to occur in switches and other “mechanical-based” components. Often what happens is that the change in flux application techniques causes flux residue to build up inside the mechanical component and either prevents it from physically moving or makes the switch work intermittently. Troubleshooting these problems is extremely difficult and can lead to significant time loss during troubleshooting. This leads to quality problems we are trying to prevent.

Often when making prototypes components will be hand soldered in place, circuits will be modified during testing, and parts will be replaced. Each of these operations requires the use of flux which will need to be cleaned. ACL offers a complete line of flux removers for all of these specific applications.

No Cleaning – If the circuit is slow, not susceptible to noise errors or increased DC offsets, then no cleaning could work just fine. However, for most circuits leaving the solder flux on the board will cause errors and make troubleshooting much more difficult.

Hand Cleaning – Better than no cleaning at all. Circuits see faster warm up periods and less current offset issues.

Soaking in an Aerosol Solvent Spray – Similar to hand cleaning, aerosol soak and rinse methods can be very helpful in cleaning out mechanical components where hand cleaning simply can’t reach.

While it is tempting to save the 10 minutes or so and not properly clean, for precision circuits, cleaning is an essential step in the testing and manufacturing process. Keeping a PCB board free of flux build up will save countless days in the testing and troubleshooting process and significantly reduce the stress of tracking down random errors and quality problems. Invest the money and time in an effective cleaner and cleaning process and spend more time building and designing great circuits (rather than countless hours troubleshooting problems). ACL Staticide can provide the products and technical assistance to help you achieve your production goals.

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