When troubleshooting nonfunctional or nonperforming circuits, engineers can often run simulations or other analysis tools to consider circuits from a schematic level. If these methods don’t solve the problem, even the best engineers can get stuck, frustrated, or confused. I’ve been through this before. To avoid going down similar dead ends, here’s a simple but important tip: keep it clean!
What do I mean by that? This means that if the PCB is not properly cleaned, certain materials used during PCB assembly or modification can cause serious circuit functional problems. One of the most common problems with this phenomenon is flux.
The following figure shows PCB with excessive amount of solder residue.
Flux is a chemical preparation used to assist in soldering components to PCB. Unfortunately, if not cleaned after welding, the solder will deteriorate the surface insulation resistance of PCB, which will cause serious degradation of circuit performance in the process!
The balanced weston bridge network, activated by a 2.5v reference voltage, can simulate high-impedance bridged sensors. VIN+ -vin – output of differential bridge sensor can be connected to INA333 with gain of 101V/V. In an ideal state, since the bridge is in an equilibrium state, VIN+ -vin – = 0V. However, flux contamination causes the actual bridging sensor voltage to drift slowly over time.
In this test, I also recorded the one-hour change of VIN- and VOUT after different degrees of cleaning after assembly:
Manual cleaning, air drying;
Ultrasonic cleaning, air drying, baking.
Flux contamination has a serious effect on the output performance of bridge sensors. The bridge sensor voltage never reaches the expected voltage of about VREF/2 without cleaning or manual cleaning, even after an hour of stabilization. In addition, the uncleaned circuit board also shows a large number of external noise collection. After ultrasonic bath cleaning and complete drying, the bridge sensor voltage is as stable as a rock.
Looking at the output voltage of INA333, we can continuously see the performance degradation caused by improper cleaning.
DC errors, long stabilization time and serious external noise collection occurred in the uncleaned circuit board.
A strange low frequency noise appeared on the hand-cleaned circuit board. I finally found the root cause – because of the air conditioning cycle inside the test facility!
As expected, properly cleaned and dried boards performed well, with no drift occurring at any point in the test.
In summary, improper solder cleaning can cause serious performance degradation, especially in high-precision DC circuits. For all manually assembled or modified PCBS, be sure to use ultrasonic bath (or similar) for final cleaning. After the air compressor is used to dry, the PCB assembled and cleaned at a slightly higher temperature can be used 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 you spend troubleshooting and help you spend more time designing excellent high-precision circuits!