What are the common defects of circuit boards/circuit boards?
Defects in PCB circuit boards include solder bridges or different solder joints between component pins, short circuits between copper wires, open circuits, component displacement, and more. In most cases, manufacturers conduct extensive testing before releasing a product to the market. But some defects may go unnoticed. The defects will only be highlighted after the board is actually used by the user. In addition, due to environmental and other conditions beyond the control of the manufacturer, some defects will occur in the field. In addition, some defects are due to environmental or other conditions outside the control of the manufacturer.
short circuit
The types of short circuits that occur in the PCB production stage are different, and in other cases, short circuits occur during soldering or reflow soldering. Common short circuits include: 1. When the space or spacing between copper traces is small, a short circuit will occur;
2. Untrimmed component leads will cause short circuits;
3. Floating conductive short thin wires in the air will cause short circuits between copper traces.
solder bridge
Component failure: Defective components usually have their inputs or outputs shorted to power or ground.
open circuit
An open circuit occurs when a trace is broken, or when the solder is only on the pad and not on the component lead. In this case, there is no adhesion or connection between the component and the PCB. Just like short circuits, these can also occur during production or during welding and other operations. Vibrating or stretching boards, dropping them or other mechanical deformation factors can damage traces or solder joints. Likewise, chemicals or moisture can cause solder or metal parts to wear away, causing component leads to break.
Loose or misaligned electronic components
Small components can float on the molten solder during reflow and eventually break away from the intended solder joint. Possible reasons for shifting or tilting include vibration or bouncing of components on the soldered PCB due to insufficient board support, reflow oven setup, soldering problems, human error, etc.
soldering problem
Here are some problems that can arise due to poor soldering practices:
Disturbed Solder Joint: Movement of the solder before it solidifies due to external disturbances. This is similar to cold solder joints but the reason is different. It can be corrected by reheating. And ensure that the solder joints are not disturbed by the outside world while cooling.
Cold Soldering: This occurs when the solder does not melt properly, resulting in rough surfaces and unreliable connections. Cold solder joints can also occur due to excess solder preventing complete melting. The remedy is to reheat the joint and remove excess solder.
Solder Bridge: This occurs when solder crosses over and physically connects two leads together. These have the potential to form unintended connections and short circuits that can burn components or blow traces if the current is too high.
Insufficient pad, pin or lead wetting.
Too much or too little solder.
A raised pad 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 locate it. This needs to follow a logical path until it is possible to pinpoint the defect. Different ways to determine the location of a fault include visual inspection without powering the board, and physical inspection using test equipment. Testing techniques rely on high-end test equipment or use basic tools such as multimeters etc. on powered or unpowered boards.
While 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. Ease depends on the type of board, number of layers, trace spacing, number of components, board size, and other factors.
Although more complex boards often require special test equipment; basic tools like multimeters, thermal cameras, magnifying glasses, and oscilloscopes can identify most problems.
High-end test equipment combines a variety of measurement methods including microvoltage and other non-contact current tracking techniques to accurately and quickly identify short circuits in loads and bare PCBs. Some of these devices use current injection and field sensing to identify the exact location without powering the board or removing components. However, the high cost may be out of the reach of many designers.
Nordson testing equipment
Typical equipment includes automatic flight detection instruments, such as the double-sided robot tester Takaya9600 and AcculogicFLS980. There are also automated optical inspection (AOI) machines, such as the Nordson YESTECHFX-942. AOI employs high-resolution cameras to inspect a variety of defects, including shorts, opens, missing, incorrect or misaligned components.
Visual and Physical Inspection
Visual inspection can identify defects such as overlapping traces, shorted solder joints, signs of board overheating, and burnt components. But that's only as far as the eye can reach.
Some problems, especially when the board is carrying too much heat, are difficult to identify with the naked eye. In this case, a magnifying glass can help identify some shorts, solder bridges, opens, cracks in solder joints and board traces, component offsets, etc.
Also, a multimeter can determine if there is a short or an open in the copper traces on the board. Using a continuity test, the short circuit resistance value will be very low, usually less than 5 ohms. Likewise, an open circuit will produce a very high resistance value.
Detecting circuit board defects with a multimeter
When low resistance is detected between the pins of an electronic component, the best way is to remove the component from the PCB circuit and conduct a special inspection. If the resistance is still low, then this component is the culprit, otherwise further investigation is required. Be careful when desoldering, so as not to damage the copper pad on the PCB or pull out the components to be tested directly from the PCB circuit board.
Visual inspection is only suitable for visual inspection of circuit boards, it may not be suitable for internal layer inspection of circuit boards. If there are no obvious defects in the appearance, you need to power up the board and perform more detailed tests to detect whether the board is normal.
Locate PCB short circuit problem
The above detection method is limited, and because the detection is performed without powering on the circuit board. Only a limited number of problem points can be detected. That said, it is easier to pinpoint the exact location of hard-to-find defects, such as shorts on a powered-on board. This involves using tools such as a voltmeter to measure the voltage drop across copper traces, or an infrared camera to identify hot problem spots.
low voltage measurement
This technique involves controlling the amount of current passing through a short circuit and finding out where it is going. Since the copper traces on the circuit board also have resistance, there is no resistance through the copper traces.
