When troubleshooting non-functional or poor performance circuits, engineers can often run simulations or other analysis tools to examine the circuit at the schematic level. If these methods don’t solve the problem, even the best engineers can be stumped, frustrated or confused. I’ve been there before. To avoid a similar dead end, here’s a simple but very important tip: Keep it clean!

וואָס טאָן איך מיינען? This means that certain materials used during PCB Assembly or modification can cause serious circuit functionality problems if the PCB is not kept properly clean. One of the most common problems with this phenomenon is flux.

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

A balanced Wheatstone bridge network activated by a 2.5V reference voltage can simulate a high impedance bridge sensor. Differential bridge sensor output VIN+ -VIN – can be connected to the INA333 with gain of 101V/V. Ideally, VIN+ -vin – = 0V because the bridge is in equilibrium. But flux contamination will cause the voltage of the actual bridge sensor to drift slowly with time.

In this test, I also recorded the changes of VIN- and VOUT for one hour after different degrees of cleaning after assembly: As can be seen from Figure 3, flux contamination has a serious impact on the output performance of the bridge sensor. Without cleaning or manual cleaning, the bridge sensor voltage never reaches the expected voltage of about VREF/2, even after a leveling off time of one hour. In addition, the uncleaned circuit board also exhibits a large amount of external noise collection. After being cleaned in an ultrasonic bath and completely dried, the bridge sensor voltage is rock steady.

ווי צו ריין PCB?

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

Uncleaned boards have DC errors, long leveling time, and severe external noise collection;

There was a strange very low frequency noise from the hand-cleaned circuit board. I finally got to 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 summary, improper flux cleaning can cause serious performance degradation, especially in high-precision DC circuits. For all hand-assembled or modified PCBS, be sure to use an ultrasonic bath (or similar method) for final cleaning. After air drying with an air compressor, the PCB is baked and cleaned at a slightly higher temperature to remove any residual moisture. We usually bake it at 70°C for 10 minutes.