The necessity analysis of PCB cleaning

When troubleshooting a non-functional or poorly performing circuit, engineers can often run simulations or other analytical tools to consider the circuit at the schematic level. If these methods don’t solve the problem, even the best engineer may be stumped, frustrated, or confused.

To avoid a similar dead end, let me introduce you to a simple but very important tip: Keep it clean!

Flux is a chemical agent used to assist in soldering components to the PCB. But unfortunately, if it is not removed after welding, the flux will degrade the surface insulation resistance of the PCB, which will cause serious degradation of the circuit performance in the process!

This picture as above is the test circuit I used to show the results of flux contamination. A balanced Wheatstone bridging network activated by a 2.5V reference voltage simulates high-impedance bridging sensors. Differential bridge sensor output VIN+ – VIN- can be connected to an INA333 with a gain of 101V/V. In an ideal state, since the bridge is in equilibrium, VIN+ – VIN- = 0V. However, flux contamination will cause the actual bridge sensor voltage to drift slowly over time.

In this test, after assembly, I also recorded the changes of VIN- and VOUT for one hour after different degrees of cleaning:

Not clean;

Hand cleaning, air drying;

Ultrasonic cleaning, air drying, baking.

As can be seen from above picture, flux contamination has a serious impact on the output performance of the bridged sensor. When not cleaned or manually cleaned, the bridge sensor voltage never reached the expected voltage of about VREF/2, even after one hour of stabilizing time. In addition, the uncleaned circuit board also exhibits a large amount of external noise collection. After being cleaned with an ultrasonic bath and completely dried, the bridge sensor voltage is rock stable.

Looking at the output voltage of the INA333 from above picture, we continuously see performance degradation caused by improper cleaning.

Uncleaned circuit boards have DC errors, long stabilizing times, and serious external noise collection.

The hand-cleaned circuit board is experiencing strange very low frequency noise. I finally found the root cause – it was the air conditioning cycle inside the test facility!

As expected, the boards that were properly cleaned and dried performed well, with no drift at any point during the test.

In short, improper flux cleaning can cause serious performance degradation, especially in high-precision DC circuits. For all PCBS that have been manually assembled or modified, be sure to use an ultrasonic bath (or similar) for final cleaning. After air drying with an air compressor, the assembled and cleaned PCB is baked at a slightly higher temperature to remove any residual moisture. We usually bake at 70°C for 10 minutes.

This simple “keep it clean” trick should help you significantly reduce the time you spend troubleshooting, helping you spend more time designing great high-precision circuits!