How To Quickly Discover And Solve Problems In PCB

- Aug 06, 2018-

This article will discuss some of the main problems in PCB manufacturing.

Human error

Most of the defects in PCB manufacturing are caused by human error. In most cases, the wrong manufacturing process, the wrong placement of components and the unprofessional production and manufacturing regulations are responsible for up to 64% of avoidable product defects.

Due to the following reasons, the possibility of defects increases with the complexity of the circuit and the number of production processes:

Tightly encapsulated components

multilayer

Fine line

Surface welded assembly

Power supply and ground connection

Although every manufacturer or assembler wishes to produce a PCB board without defects, there are several design and production process problems that cause PCB board problems.Typical problems and results include the following points:

Poor welding can lead to short circuit, open circuit, cold soldering point, etc

The misalignment of the plate layer will lead to poor contact and poor overall performance

Poor insulation of copper trace leads to arc between trace and trace

There is a risk of short circuit if the copper trace is too close to the path

The insufficient thickness of the circuit board will lead to bending and fracture

The bad quality of the board is also affected by the environment

Due to the construction of PCB itself, it is easy to cause PCB board damage when in an adverse environment.Other factors, such as extreme temperature or variable temperature, excessive humidity, high strength vibration and so on, are all factors that lead to the reduction or even scrapping of the board performance.For example, changes in the ambient temperature can cause the deformation of a plate.So it will break the soldering point, bend the shape of the plate, or it may break the copper mark on the plate.

On the other hand, moisture in the air can lead to oxidation, corrosion and rust of metal surfaces, such as exposed copper tracks, solder joints, pads and component leads.Accumulation of dirt on the surface of components and circuit boards, dust or debris will also reduce air flow and cooling of components, causing PCB to overheat and performance degradation.

Finally, vibration, falling, striking or bending the PCB can deform and cause the presence of the crack, while high current or overvoltage can cause the PCB plate to be punctured or cause rapid aging of components and pathways.

What are the common defects of PCB?

Defects in PCB include welding Bridges or different soldering points between component pins, short circuit between copper wires, open circuit, element shifting, and so on.In most cases, manufacturers test a lot of products before they go on sale.However, some defects may be ignored, only after the board is actually used by users will the defects be highlighted.In addition, there are defects on the ground due to the environment and other conditions beyond the control of the manufacturer.In addition, some defects occur outside the manufacturer's controlled environment or other conditions.

A short circuit

The types of short circuits that occur in the production stage are different, while in other cases, short circuits occur during welding or reflow welding. Common short circuits include:

Short circuit occurs when the space or spacing between copper tracks is small

Unclipped component leads cause short circuit

Floating in the air can lead to short thin wire will cause short circuit between copper trace lines

Welding bridge

Component failure: a defective component usually shortens its input or output to a power source or ground.

Open circuit

Open circuit occurs when the trace breaks, or when the solder is only on the solder plate and not on the element lead.In this case, there is no adhesion or connection between the element and the PCB.Just like a short circuit, this can happen during production or during welding and other operations.Vibrating or stretching the circuit boards, dropping them or other mechanical deformation factors can damage the track or solder joints.Also, chemical or moisture causes the solder or metal parts to wear out, leading to component leads breaking.

The looseness or dislocation of components

During reflow soldering, the widget may float on the fused solder and eventually escape from the target solder.Possible causes of displacement or tilt include vibration or bouncing of components on a PCB plate due to insufficient support of the circuit board, setting of reflow furnace, soldering paste problem, human error, etc.

Welding problem

The following are some problems caused by poor welding practices:

Interfered solder joints: external disturbances cause the solder to move before solidification.This is similar to the cold soldering point, but for different reasons, can be corrected by reheating, and ensures that the soldering point is free from external interference during cooling.

Cold welding: this occurs when the solder is not properly fused, resulting in rough surfaces and unreliable connections.Cold solder joints may also occur because excess solder prevents complete melting.The remedy is to reheat the joint and remove the excess solder.

Soldering bridge: this happens when soldering tin crosses and physically connects two leads.These can lead to unexpected connections and short circuits that can cause components to burn down or break wires when the current is too high.

Insufficient wetting of pads, pins or leads.

Fault location and maintenance technology

Once there are signs of problems, the next step is to track and locate them.This requires following a logical path until it is possible to identify the defect.Different ways of locating the fault include visual inspection without the need to power the circuit board and physical inspection using the test equipment.Test technology relies on high-end test equipment or using basic tools, such as multimeters, on electricity or non-electricity panels. 

While it is easy to identify visible defects or problems on a single panel with a large trace, troubleshooting complex multilayer panels is often a challenge.The degree of difficulty depends on the type of circuit board, the number of layers, the distance between trace lines, the number of components, the size of the circuit board, and other factors.

Although more complex circuit boards often require special testing equipment, basic tools such as multimeters, thermographs, magnifiers and oscilloscopes can identify most problems.

The high-end test equipment combines a variety of measurement methods including microvoltage and other non-contact current tracking technologies to accurately and quickly identify short circuits in load and bare PCB.Some of these devices use current injection and field induction to identify the exact location without having to power the circuit board or remove the components.However, the high cost may be beyond the reach of many designers.

Typical devices include automatic flight detection instruments, such as the two-sided robot tester Takaya9600 and Acculogic FLS 980.There are also automatic optical detection (AOI) machines, such as estech fx-942.AOI USES high-resolution cameras to check for defects, including short circuits, open circuits, missing, incorrect or unaligned components.

Visual and physical testing

Visual inspection can identify defects such as overlap marks, solder short circuit, circuit board overheating signs and burned components.But this is only within the reach of the eye.

Some problems, especially when the circuit board is carrying too much heat and it's hard to see with the naked eye.In this case, the magnifying glass can help identify short circuits, welded Bridges, open circuits, soldering points and circuit board wire cracks, element deflection, and so on.

In addition, the multimeter can determine whether there is short circuit or break in the copper trace line on the board.Using continuity tests, the short circuit resistance is very low, usually less than 5 ohms.Similarly, open circuits produce very high resistance values.

PCB

Use multimeter to detect PCB board defects

When low resistance is detected between component pins, it is best to take the component out of the PCB circuit for special detection.If the resistance is still low, this component is the culprit, otherwise further investigation is needed.Be careful when removing the soldering, so as not to damage the copper pad on the PCB or pull out the components to be tested directly from the PCB.

Visual inspection is only applicable to the visual inspection of the circuit board, it may not be applicable to the internal inspection of the circuit board.If there are no visible defects in the appearance, you need to charge the circuit board and perform more detailed tests to detect whether the circuit board is normal.

Locate the PCB short circuit problem point

The above detection method is limited and is performed without electricity to 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 hard-to-detect defects, such as circuit boards that have been charged.This involves using tools such as voltmeters to measure voltage drops on copper tracks, or infrared cameras to identify hot spots.

Low voltage measurement

The technology involves controlling current flow through short circuits and finding the current direction.Because the copper trace on the circuit board also has resistance, the voltage generated through different parts of the copper trace is also different.The amount of voltage depends on the length, width and thickness of the copper trace, because the resistance value caused by these factors is different and the corresponding voltage value is different.

Setting a safe current that is useful for testing is important, but its value should not exceed the wire or device safety threshold.A typical setting provides a 2-volt power supply voltage with a current of up to 100 milliamps.This would provide a usable total power of about 200mW, which would not be enough to damage any component other than a very sensitive one.In some cases, a low voltage may be used with a current up to 1 amperage or higher (for example, 0.4v), but be careful to limit the current to a safe value that does not burn the copper trace.

Using a voltmeter, the voltage difference between two ends of the copper wire can be easily measured.Placing two probes of the voltmeter between various parts of the length of the copper trace indicates 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 route, you find that the voltage is getting smaller and smaller and closer to the short circuit.The short circuit voltage drop will be zero or very low and no current will pass beyond this point.

Millivolt tests require sensitive voltmeters that can measure low voltages in the microvolt and millivolt ranges.For example, when a current of 1 amperage passes through a copper track line with a 1 milliohm resistance, a millivolt voltage is generated, and a sensitive voltmeter should be able to measure and display that voltage.A typical meter is a Fluke 87-v digital multimeter.It has a 5-digit digital display and 10 microvolt resolution.

Measure the heating area of the circuit board with your finger

Since the short circuit will cause the local temperature of the circuit board to rise, it is easier for the tester to find the problem point of short circuit to power the circuit and find the area with heat.If you can't feel a slight hot spot with your finger, use a thermal imager to identify the area that is partially heated.However, be careful to use a short-circuited power source by using a voltage or copper trace that does not damage the element until it overheats until it is turned on.Also, be careful not to burn or shock.

Repair short/open lines

After identifying short circuit or open circuit points on the PCB, the next step is to isolate the problem.While this is easy to do on the outside surface of a circuit board, it is a challenge for the inside.Possible solutions include drilling through a hole or cutting an appropriate external copper trace.

If there is a bridge or short circuit between two soldering points, pass the tip of the hot soldering iron between two pins or wires to remove these soldering points or short circuits.In addition, use tin wire or suction gun to remove excess solder.

The repair opening depends on the nature of the fault and the size of the copper trace.On a wider copper track, you can scrape off both sides of the opening and then weld the jumper between them.However, this may not be suitable for narrow track lines or circuit boards with strict environmental requirements. The best way to do this is to lay the wires between the corresponding pads.When finished, hold the wire in place with some glue.

By cleaning and carefully rewelding the joint, it is easy to repair the opening on the component pin.On the other hand, short circuit or circuit break caused by component failure requires replacement of faulty or aging components.

The last

The PCB board often has problems after use due to design or production defects.In most cases, the board will be at a low performance level or not working at all.In case of a problem, it is important to determine and fix the fault to ensure the continuous use of the equipment.The success of the fix depends on the ability to identify the fault and its location.

Most manufacturers have all the testing equipment and tools to identify and correct circuit board failures.However, for some designers or professionals with limited resources, buying all of these tools is unlikely.Fortunately, electronics professionals can identify and fix a significant number of problems with basic tools and patience by using low-cost technology.