How to Check if a Circuit Board is Functioning Properly After Soldering?

After soldering a circuit board, when checking whether the circuit board can work properly, it is usually not advisable to power the board directly. Instead, follow the steps below to ensure that there are no issues at each step before powering it on.

Check Wiring Accuracy

Checking the schematic is crucial. The first focus of the check is whether the power supply and network node markings of the chip are correct, and also to ensure that there are no overlapping network nodes. Another focus is the packaging of components, including the model of the package and the pin order (remember: the package should not be viewed from the top, especially for non-pin packages). Check the wiring for correctness, including miswires, missing wires, and extra wires. There are usually two methods for checking wiring:Check the installed lines according to the circuit diagram, inspecting the installed lines one by one in a certain order based on the circuit connections;Check the actual lines against the schematic, centering around the components.Verify the connections of each component pin, checking whether each destination exists in the circuit diagram.To prevent errors, lines that have been checked should typically be marked on the circuit diagram. It is best to use a multimeter in the ohm setting with a buzzer test to directly measure the component pins, which can also help identify poor connections.

Check for Power Supply Short Circuits

Before debugging, do not power on. Measure the input impedance of the power supply with a multimeter; this is a necessary step! If there is a short circuit in the power supply, it can cause damage to the power supply or even more serious consequences. When dealing with the power supply section, a 0-ohm resistor can be used as a debugging method. Do not solder the resistor before powering on; check that the power supply voltage is normal before soldering the resistor onto the PCB to supply power to subsequent units, to avoid damaging the chips of the subsequent units due to abnormal power supply voltage. Add protection circuits in the circuit design, such as using resettable fuses and other components.

Component Installation Status

The main focus is to check polarized components, such as light-emitting diodes, electrolytic capacitors, rectifier diodes, etc., and whether the pins of transistors correspond correctly. For transistors, the pin order may differ among manufacturers for the same function, so it is best to test with a multimeter.First, perform open and short circuit tests to ensure that there will be no short circuits after powering on. If the test points are well set, it can be very efficient. The use of 0-ohm resistors can sometimes benefit high-speed circuit testing. After completing the hardware checks before powering on, you can start the power-on tests.

Power-On Testing

1. Power-On Observation: After powering on, do not rush to measure electrical indicators; instead, observe whether there are any abnormal phenomena in the circuit, such as smoke, unusual smells, or whether the integrated circuit package feels hot to the touch. If any abnormal phenomena occur, immediately turn off the power and troubleshoot before powering on again.
2. Static Debugging:Static debugging generally refers to DC testing conducted without input signals or only with fixed level signals. You can use a multimeter to measure the potential at various points in the circuit, comparing it with theoretical estimated values, and analyzing the circuit principles to determine whether the DC operating state of the circuit is normal, promptly identifying any damaged or borderline components in the circuit.By replacing components or adjusting circuit parameters, ensure that the DC operating state of the circuit meets design requirements.
3. Dynamic Debugging:Dynamic debugging is conducted based on static debugging, where appropriate signals are added to the input of the circuit. Following the signal flow, sequentially check the output signals at each test point. If any abnormal phenomena are found, analyze the causes and troubleshoot before continuing debugging until requirements are met.

During the testing process, do not rely on intuition; always use instruments for observation. When using an oscilloscope, it is best to set the signal input mode to “DC”. By using DC coupling, you can observe both the AC and DC components of the measured signal simultaneously. Through debugging, finally check whether the various indicators of functional blocks and the entire system (such as signal amplitude, waveform shape, phase relationship, gain, input impedance, and output impedance) meet design requirements. If necessary, further propose reasonable corrections to the circuit parameters.

Other Tasks in Electronic Circuit Debugging

1. Determine Test Points: Based on the working principles of the system to be debugged, draft debugging steps and measurement methods, determine test points, and mark their locations on the drawings and boards, creating debugging data record forms, etc.
2. Set Up a Debugging Workbench:The workbench should be equipped with the necessary debugging instruments, arranged for convenient operation and observation.Special Reminder:During production and debugging, ensure that the workbench is clean and tidy.
3. Select Measurement Instruments:For hardware circuits, select measurement instruments that are more accurate than the system being tested;For software debugging, a computer and development device should be equipped.
4. Debugging Sequence:The debugging sequence of electronic circuits generally follows the signal flow direction, using the output signal of the previously debugged circuit as the input signal for the next stage, creating conditions for final overall debugging.
5. Overall Debugging:For digital circuits implemented with programmable logic devices, complete the input, debugging, and downloading of the source files for the programmable logic devices, and connect the programmable logic devices with the analog circuits to form a system for overall debugging and result testing.

Precautions in Circuit Debugging

The correctness of debugging results is largely influenced by the accuracy of the test quantities and the precision of the tests. To ensure the results of the tests, it is necessary to minimize testing errors and improve testing accuracy. Therefore, pay attention to the following points: Correctly use the grounding terminal of the testing instruments. When testing with electronic instruments grounded to the chassis, the grounding terminals should be connected to the amplifier’s grounding terminal; otherwise, interference introduced by the instrument’s chassis can change the amplifier’s operating state and lead to errors in the test results. According to this principle, when debugging the emitter bias circuit, if you need to test Vce, do not connect the instrument’s two ends directly to the collector and emitter; instead, measure Vc and Ve separately to ground, and then subtract the two. If using a multimeter powered by dry batteries, since the two input terminals of the meter are floating, it is permissible to connect them directly across the test points.The input impedance of the instrument used for measuring voltage must be much greater than the equivalent impedance at the measurement point. If the input impedance of the testing instrument is low, it will cause shunting during measurement, leading to significant errors in the test results.The bandwidth of the testing instrument must be greater than the bandwidth of the circuit being measured.Correctly select test points. When measuring with the same testing instrument, different measurement points will result in significantly different errors due to the instrument’s internal resistance.The measurement method should be convenient and feasible. When measuring the current of a certain circuit, it is generally preferable to measure voltage rather than current, as measuring voltage does not require altering the circuit. If you need to know the current value of a certain branch, you can measure the voltage across the resistor in that branch and calculate it.During debugging, not only should you observe and measure carefully, but you should also be good at recording. The recorded content should include experimental conditions, observed phenomena, measured data, waveforms, and phase relationships, etc. Only by comparing a large amount of reliable experimental records with theoretical results can you discover issues in circuit design and improve the design scheme.

Troubleshooting During Debugging

Carefully investigate the causes of faults; do not simply disassemble the circuit and reinstall it when encountering a fault that cannot be resolved. If it is a principle issue, even reinstallation will not solve the problem.For a complex system, accurately identifying faults among numerous components and circuits is not easy. The general fault diagnosis process starts from the fault phenomenon, conducts repeated tests, makes analytical judgments, and gradually identifies the fault.

1. Common Fault Phenomena:The amplifier circuit has no input signal but has an output waveform.The amplifier circuit has an input signal but no output waveform, or the waveform is abnormal.The series voltage regulator has no voltage output, or the output voltage is too high and cannot be adjusted, or the output voltage regulation performance deteriorates, resulting in unstable output voltage, etc.The oscillation circuit does not oscillate, and the counter waveform is unstable, etc.
2. Causes of Faults:Faults may occur in finished products after a period of use due to component damage, short circuits, open circuits, or changes in conditions, etc.

Methods for Checking Faults

1. Direct Observation Method: Check whether the selection and use of instruments are correct, whether the power supply voltage levels and polarities meet requirements; whether the pins of polarized components are connected correctly, and whether there are any misconnections, missed connections, or contact issues. Check whether the wiring is reasonable; whether the printed circuit board has short or open circuits, and whether resistors and capacitors are burnt or exploded. Observe whether components are overheating, smoking, whether transformers have burnt smells, whether the filaments of electronic tubes and oscilloscopes are lit, and whether there are any high-voltage sparks.
2. Use a Multimeter to Check Static Operating Points:The power supply system of the electronic circuit, the DC operating states of semiconductor transistors and integrated circuits (including component pins, power supply voltages), and resistance values in the circuit can all be measured with a multimeter.When the measured values differ significantly from normal values, analysis can help identify faults.It is worth noting that static operating points can also be measured using the oscilloscope in “DC” input mode.The advantage of using an oscilloscope is its high internal resistance, allowing simultaneous observation of the DC operating state and the signal waveform at the measurement point, as well as any potential interference signals and noise voltages, which is more conducive to fault analysis.
3. Signal Tracing Method:For various complex circuits, you can input a signal of a certain amplitude and appropriate frequency at the input (for example, for multi-stage amplifiers, you can input a sine wave signal of 1000 Hz at its input), and use an oscilloscope to observe the waveform and amplitude changes stage by stage, from the front stage to the back stage (or vice versa). If any stage is abnormal, the fault lies within that stage.
4. Comparison Method:When suspecting a problem in a certain circuit, you can compare the parameters of that circuit with the parameters of a normal circuit (or theoretical analysis of current, voltage, waveforms, etc.) to identify any abnormal conditions in the circuit, and then analyze and determine the fault point.
5. Component Replacement Method:Sometimes faults are not immediately visible; if you have a device of the same model as the faulty one, you can replace components, parts, or plug-in boards from the faulty device with corresponding parts from the normal device to narrow down the fault range and locate the fault source.
6. Bypass Method:When parasitic oscillation occurs, you can use a capacitor of appropriate value, select an appropriate test point, and temporarily connect the capacitor between the test point and the reference ground. If the oscillation disappears, it indicates that the oscillation is generated nearby or in the preceding circuit. Otherwise, it is in the later stages, and you should move the test point to search.The bypass capacitor should be appropriate, not too large, just enough to effectively eliminate harmful signals.
7. Short Circuit Method:This involves temporarily shorting part of the circuit to locate faults. The short circuit method is most effective for checking open circuit faults.However, be cautious not to use the short circuit method on the power supply (circuit).
8. Open Circuit Method:The open circuit method is most effective for checking short circuit faults. It is also a method for gradually narrowing down the suspected fault points.For example, if a voltage regulator is connected to a faulty circuit causing excessive output current, we can check the fault by sequentially disconnecting a branch of the circuit. If the current returns to normal after disconnecting that branch, the fault occurred in that branch.

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