PCB Technical - How to quickly solve the problem of PCB board failure

How to quickly solve the problem of PCB board failure

PCB board production involves a series of complex and precise manufacturing processes. As the circuit boards become more integrated and more complex, the manufacturing process becomes more and more challenging for production personnel, and the probability of defects and failures also increases. Big.

Regardless of the reason, for personal use or commercial applications, PCB defects can cause serious adverse consequences. For example, circuit board failures in important medical equipment may be life-threatening, while problems with smartphones or automotive electronics can interfere with user activities .

What are the common defects of PCB board?

Defects in PCB include solder bridges or different solder joints between component pins, short circuits between copper wires, open circuits, component displacement, and so on. In most cases, manufacturers will conduct extensive testing before launching their products on the market. However, some defects may be overlooked, and the defects will only become obvious after the board is actually used by the user. In addition, due to the environment and other conditions beyond the control of the manufacturer, some defects may occur on site. In addition, some defects occur because they occur outside the scope of the manufacturer's controllable environment or other conditions.

short circuit

The types of short circuits in the production stage are not the same, while other short circuits occur, in the process of soldering or reflow soldering, common short circuits include:

When the space or spacing between copper traces is small, a short circuit will occur

The leads of components that have not been trimmed will cause a short circuit

Floating in the air can lead to short thin wires that can cause short circuits between copper traces

solder bridge

Component failure: A defective component usually shorts its input or output to power or ground.

open the way

When the trace is broken, or the solder is only on the pad and not on the component lead, an open circuit will occur. In this case, there is no adhesion or connection between the component and the PCB. Just like short circuits, these may also occur during the production process or during the welding process and other operations. Vibration or stretching of the circuit board, dropping them or other mechanical deformation factors will destroy the traces or solder joints. Similarly, chemical or moisture can cause solder or metal parts to wear, which can cause component leads to break.

loose or misplaced components

During the reflow soldering process, small parts may float on the molten solder and eventually leave the target solder joint. Possible reasons for the displacement or tilt include the vibration or bounce of the components on the soldered PCB board due to insufficient circuit board support, reflow oven settings, solder paste problems, and human error.

Welding problem

The following are some of the problems caused by poor welding practices:

Disturbed solder joints: The solder moves before solidification due to external disturbances. This is similar to cold solder joints, but the reason is different. It can be corrected by reheating, and the solder joints are not disturbed by the outside when they are cooled.

Cold soldering: This happens when the solder cannot be melted properly, resulting in rough surfaces and unreliable connections. Since excessive solder prevents complete melting, cold solder joints may also occur. The remedy is to reheat the joint and remove the excess solder.

Solder Bridge: This happens when solder crosses and physically connects two leads together. These may form unexpected connections and short circuits, which may cause the components to burn out or burn out the traces when the current is too high.

Pads, pins or leads are insufficiently wetting.

Too much or too little solder.

A pad that is elevated due to overheating or rough soldering.

Fault location and repair technology

Once there are signs of a problem, the next step is to track and determine the location. This needs to follow a logical path until it is possible to identify the defect. Different ways to determine the location of the fault include visual inspection without powering the circuit board, and physical inspection using test equipment. Testing technology relies on high-end testing equipment or the use of basic tools, such as multimeters, on powered or unpowered boards.

Although it is easy to identify visible defects or problems on simple single-sided boards with larger traces, troubleshooting complex multi-layer boards is often a challenge. The degree of difficulty depends on the type of circuit board, the number of layers, the trace spacing, the number of components, the size of the circuit board and other factors.

Although more complex circuit boards usually require special test equipment, basic tools such as multimeters, thermal imaging cameras, magnifiers, and oscilloscopes can identify most problems.

High-end test equipment combines a variety of measurement methods including micro-voltage and other non-contact current tracking technologies, which can accurately and quickly identify short circuits in the load and bare PCB. Some of these devices use current injection and field sensing to identify the exact location without powering the circuit board or removing components. However, the high cost may be beyond the reach of many designers.

Nordson testing equipment

Typical equipment includes automatic flight detection instruments, such as the double-sided robot tester Takaya 9600 and Acculogic FLS 980. There are also automatic optical inspection (AOI) machines, such as Nordson YESTECH FX-942. AOI uses high-resolution cameras to inspect various defects, including shorts, open circuits, missing, incorrect or misaligned components.

Visual and physical inspection

Visual inspection can identify defects such as overlapping traces, short-circuited solder joints, signs of circuit board overheating, and burned components. But this is only within the reach of vision.

Some problems, especially when the circuit board is overheated and difficult to identify with the naked eye. In this case, a magnifying glass can help identify some short circuits, solder bridges, open circuits, cracks in solder joints and circuit board traces, component offsets, and so on.

In addition, a multimeter can determine whether there is a short circuit or open circuit in the copper traces on the board. Using the continuity test, the short-circuit resistance value will be very low, usually less than 5 ohms. Similarly, an open circuit will produce a very high resistance value.

Use a multimeter to detect PCB board defects

When low resistance is detected between component pins, the best way is to remove the components from the PCB circuit for special testing. If the resistance is still low, then this component is the culprit, otherwise further investigation is needed. Care should be taken when desoldering, so as not to damage the copper pads on the PCB or directly pull out the components to be tested from the PCB.

Visual inspection is only suitable for the appearance inspection of the circuit board, it may not be suitable for the inner layer inspection of the circuit board. If there are no obvious defects in the appearance, you need to power on the circuit board and perform more detailed tests to detect whether the circuit board is normal.

Locate the PCB short circuit problem

The above detection method has limitations, and it is because the detection is performed without powering on the circuit board. Only a limited number of problem points can be detected. In other words, it is easier to find the exact location of defects that are difficult to find, such as a short circuit on a powered-on circuit board. This involves using tools such as a voltmeter to measure the voltage drop on copper traces, or using an infrared camera to identify heating problems.

low voltage measurement

This technology involves controlling the amount of current passing through a short circuit and finding out where the current flows. Since the copper traces on the circuit board also have resistance, the voltages generated by different parts of the copper traces are also different. The amount of voltage depends on the length, width, and thickness of the copper traces. Because these factors cause different resistance values, the corresponding voltage values are also different.

It is very important to set a useful safety current for testing, but its value cannot exceed the wire or equipment safety threshold. A typical setting provides a supply voltage of 2 volts with a maximum current of approximately 100 mA. This will provide a total usable power of about 200mW, which is not enough to damage any components except very sensitive components. Sometimes, you can also use a low voltage with (such as 0.4 volt) current as high as 1 ampere or higher, but care should be taken to limit the current to a safe value that will not burn the copper traces.

Using a voltmeter, you can easily measure the voltage difference between the two ends of the copper trace. Placing the two probes of the voltmeter between the various parts of the copper trace length will indicate the voltage difference and its positive and negative polarity, thus indicating the direction of current flow. When you measure the voltage between different parts along the short-circuit line, you find that the voltage value is getting smaller and smaller, and it is getting closer and closer to the short-circuit. The short-circuit voltage drop will be zero or very low, and no current will flow beyond this point.

The millivolt test requires a sensitive voltmeter, which can measure low voltages in the microvolt and millivolt range. For example, when a current of 1 ampere passes through a copper trace with a resistance of 1 milliohm, a voltage of 1 millivolt will be generated. A sensitive voltmeter should be able to measure and display this voltage value. The typical instrument is the Fluke 87-V digital multimeter. It has a 5-digit digital display and a resolution of 10 microvolts.

Use your finger to sense the heating area of the circuit board

Because the short circuit will cause the local temperature of the circuit board to rise, supplying power to the circuit and finding the area with heat can help reduce the detector to find the problem of the short circuit more easily. If you can't feel the subtle hot spots with your fingers, you can use a thermal imager to identify the locally heated areas. However, by using a voltage that will not damage the component or the copper trace is overheated until it is turned on, be sure to use a power supply that short-circuits the copper trace. In addition, please be careful to avoid burns or electric shocks.

repair short/open lines

After the short circuit or open circuit point is identified on the PCB, the next step is to isolate the problem. Although it is easy to do this on the outer surface of the circuit board, it is a challenge for the inner layer. Possible solutions include drilling through holes or cutting appropriate external copper traces.

If there are solder bridges or short circuits between two solder joints, please pass the tip of the hot soldering iron between the two pins or wires to remove these solder joints or short circuits. In addition, use tin wire or a sniffer to remove excess solder.

Repairing the open circuit depends on the nature of the fault and the size of the copper trace. On wider copper traces, you can scrape off both sides of the open circuit, and then solder a jumper wire between the two. However, this may not be suitable for narrow traces or circuit boards with strict environmental requirements. The best way is to route between the corresponding pads. When finished, use some glue to fix the wires in place.

By cleaning and carefully re-soldering the connector, it is easy to repair the open circuit on the component pin. On the other hand, short circuits or open circuits caused by component failures require replacement of faulty or aging components.

PCB boards often have problems after use due to design or production defects. In most cases, the board will be at a low performance level or not work at all. When a problem occurs, identifying and fixing the fault is essential to ensure the continued use of the equipment. The success of the repair depends on the ability to identify the fault and its location.

Most PCB manufacturers have all the test equipment and tools to identify and correct circuit board failures. However, for some designers or professionals with limited resources, it is impossible to purchase all these tools. Fortunately, electronics professionals can use low-cost technology to identify and fix a considerable number of problems with basic tools and patience.