Solder flux is a chemical that helps promote good solder junctions by cleaning the surface of oxidation and enhances the flow of solder connections. After completing the soldering process, it is important to properly clean the surface of the PCB to remove residual solder flux. Residual solder flux has a high impedance that changes over time, humidity, and temperature. Solder flux residue is most problematic in applications involving high impedance circuits. For example, a low power op amp might use 100 kΩ or larger feedback resistors to minimize power consumptions. In this case, the solder flux residue forms parasitic paths on the feedback network that introduce gain error. This error is more significant for larger value feedback resistors as the flux impedance is typically high. Different types of flux require different solvents and cleaning methods, so for best result refer to the solder flux manufacture recommendations. For example, water soluble solder flux is frequently cleaned in an ultrasonic water bath at 60°C. The ultrasonic vibrations help break free the flux material. For assemblies with mechanical devices (such as relays) an ultrasonic clean and immersion in fluid is not practical as this process could damage the mechanical components. In case fluid immersion is not practical, a spray type clean can be used.

Figure 3-16 shows a circuit with solder-flux residue that was not cleaned. The solder-flux residue looks like a clear thick substance resembling a syrup. The circuit impacted by the solder-flux is a bridge sensor amplified by an instrumentation amplifier (see Figure 3-17). Figure 3-18 and Figure 3-19 show how different levels of contamination can affect the amplifier inverting input and output over time. The no-clean results show very large drifting over time. This drift is affected by temperature and humidity. The graphs show qualitatively that flux can have a significant impact on performance and the effect is especially significant on high-impedance circuits.

Figure 3-16 PCB Flux Residue

Figure 3-17 Bridge Amplifier with Flux Contamination

Figure 3-18 Flux Impact on Amplifier Input

Figure 3-19 Flux Impact on Amplifier Output

Some very high impedance applications require and extremely thorough PCB clean as well as special handling requirements. In general, these very sensitive applications use very high-impedance sensors such as a pH sensor (R_S > 100MΩ). These types of applications generally require multiple cleans using different solvents. In ultra-high impedance applications it is important to avoid directly handling the PCB as oil, moisture, and salt from skin can introduce errors. Furthermore, these applications also may require the use of a PCB guard trace to minimize leakage currents. Details on these ultra-low bias current applications are covered in the low leakage design series Part 1, Part 2, and Part 3.