Why should PCB be cleaned ?

When eliminate the trouble a non-functional or poor performance circuit, engineers can usually run simulations or other analysis tools to take the circuit from a schematic level. If these methods don't solve the problem, even the best engineers may be stumped, frustrated or confused. I have also experienced this kind of pain. To avoid drilling into a similar dead end, I would like to introduce you to a simple and very important tip: keep it clean!

 

What do I mean by saying this? This means that certain materials used in PCB assembly or modification can cause serious circuit functionality problems if the PCB is not properly cleaned. One of the most common problems with this type of phenomenon is flux.
Figure 1 shows the PCB with excess flux remaining.

 

the PCB with excess flux

 

Fiture1

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

 

circuit performance

 

Figure 2

Figure 2 shows the test circuit which used to show the results of flux contamination. A balanced Wheatstone bridge network activated by a 2.5V reference voltage simulates a high-impedance bridge sensor. The differential bridge sensor output, VIN+ - VIN-, can be connected to the INA333 with a gain of 101V/V. Ideally, VIN+ - VIN- = 0V because the bridge is in equilibrium. However, flux contamination can cause the actual bridge sensor voltage to drift slowly follow time.

 

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

1) not cleaned;

2)Manual cleaning and air drying;

3)Ultrasonic cleaning, air drying, baking.

 

PCB Ultrasonic cleaning

 

Figure 3

As can be seen from Figure 3, flux contamination has a severe impact on the output performance of the bridge sensor. In the case of uncleaned or manual cleaning, the bridge sensor voltage never reaches an expected voltage of approximately VREF/2, even after an hour of stabilization time. In addition, uncleaned boards also exhibit a large amount of external noise collection. After cleaning with an ultrasonic bath and completely drying, the bridge sensor voltage is as stable as a meteorite.

 

Figure 4

Observing the output voltage of the INA333, we will continuously see the performance degradation caused by improper cleaning.

1)Uncleaned boards have DC errors, long settling times, and severe external noise collection;

2)  Hand-cleaned boards have strangely low frequency noise. I finally found the root cause — it was because of the air conditioning cycle inside the test facility!

3) As expected, the properly cleaned and dried boards performed exceptionally and did not drift at any point in the test.

 

In summary, improper solder cleaning can cause severe performance degradation, especially in high-precision DC circuits. For all manually assembled or modified PCBs, 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 elevated temperature to remove any residual moisture. We usually bake at 70 ° C for 10 minutes.

 

This simple “keep clean” technique should help you dramatically reduce the time spent troubleshooting and help you spend more time designing high-precision circuits!