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Short Circuit Board: PCB Damage Caused by Current Diversion/Low Resistance

Even though designers and engineers have perfected the PCB design and fabrication processes, some faults may occur. One of these faults is shorting, which can cause expensive damage if it burns a big chunk of the board. Therefore, testing new circuit boards is vital before they go into service. Also, you should check those already in use if you suspect short-circuiting. If you don't know where to start, this article covers everything you need to know about a short circuit board. Take a look!

Signs of a Short Circuit

The signs of a short circuit board usually depend on the location of the fault. For instance, you can hear a sudden pop and then notice arcing and broken traces when turning on the power. This sign often indicates a short on the power traces.

An electric short circuit burning out an overheated chip

An electric short circuit burning out an overheated chip

Generally, shorted audio, visual, mechanical, or LED outputs are easy to spot. For example, an LED with a shorted positive lead will not light up.

However, others are more difficult to spot. A good case in point is two shorted input pins. These will not give any hints during power-up testing. The only way you can realize there is a problem is if they affect the reading at the output.

How to Find a Short Circuit in a PCB?

The following are the two methods of spotting shorts on a circuit board.

Visual Inspection

This method involves using a low magnification microscope or magnifying glass to examine the circuit board. Begin at the power supply and check for tin whiskers between solder joints. Cracks/blobs of solder should receive immediate attention. Also, check the vias because poorly plated ones can short between the layers, tying every trace to the Vcc, ground, or both.

 

Tin whiskers

Tin whiskers

Source: Wikimedia Commons

 

You will notice brown spots if the short is extensive and burned at high temperatures. However, if the damage is tiny and internal to the PCB, you will not notice it from the surface layer. In such cases, you need a board tester to detect the fault.

Infrared Imaging

Short circuits usually generate heat due to the resistance and high electrical current flowing between the power source and the ground. You can use an infrared camera to detect these hot spots. If you spot a glowing area away from the active components, that is most likely a short circuit. When locating the fault, begin with a low current to prevent further damage, then gradually increase it if you cannot spot the damage.

A thermal/ infrared camera

A thermal/ infrared camera

How to Test a Circuit for Shorts on an Electronic Board?

Besides using your eyes for visual inspection, you can use the following methods to test for shorts in a PCB.

Short Circuit Testing with a Digital Multimeter

If a visual inspection does not reveal the cause or location of the short circuit, multimeter tests will help you track the problem to the source. The multimeter approach will help measure the resistance between different solder pads or points in the circuit.

A digital multimeter

A digital multimeter

However, the digital multimeter should have good sensitivity (milliohm) and a buzzer function to alert you when it detects a short. The resistance readings should be high if you measure the resistance between two adjacent wires. However, there may be internal or external solder bridges between them if the resistance is low and the adjacent wires are in separate circuits.

Test Short Circuit Board Relative to the Ground

This method is effective at detecting shorts involving ground thru-holes. It is also effective for spotting burns involving connections on the ground plane.

Multilayer circuit boards with internal layers feature return paths through vias and near the components. This area is convenient for checking the board surface layer's solder pads and vias. You can test by placing one of the multimeter's probes on the ground connection and the other on the conductors.

However, it is possible to mistake a short circuit for a typical ground connection, so pay attention to the layout. Ungrounded/exposed conductors should have high resistance. Therefore, the PCB might have a short if the resistance is low and there's no inductor between the ground and conductor.

Shorted Components

You can use a multimeter for resistance measurement if there is a short circuit between layers, such as between two pins or pads. These faults usually occur due to poor fabrication.

PCB components burning due to short-circuiting fire

PCB components burning due to short-circuiting fire

Narrow Down the Location

If you think a short circuit exists between a conductor and the ground or two conductors, check the nearby conductors to narrow down the fault to the exact location.

Testing involves moving the multimeter probe further to the ground, which should alter the resistance. If it increases, the lead is moving away from the shorted location.

How to Find Faulty Components on a PCB

Faulty, counterfeit, or incorrectly installed components can be part of the short circuit issue. Also, some genuine components can deteriorate over time, leading to shorting. For instance, electrolytic capacitors can wear out, and most capacitors bulge after wearing out.

How to Test the PCB Destructively

If you have an X-ray imaging unit, examine the board's interior without damaging it. Destructive testing should be a measure of last resort. Therefore, if you don't have access to the X-ray unit, start removing the parts while running the multimeter tests. This process enables you to check whether the electronic component is defective or if there are internal solder bridges.

An X-ray image of a motherboard

An X-ray image of a motherboard

Source: Wikimedia Commons

Even though it is a lot of work, the process has two advantages. First, it gives straightforward access to pads. Second, it ensures you don't short the damaged component.

If the short circuit's physical location is elusive, cut through the PCB to track the issue. Use a multimeter to do repeated tests on the suspected area until you identify the spot.

What Causes Short Circuits on a PCB?

  • Poor fabrication
  • Solder bridges during reflow soldering in electronic component assembly
  • Component failure over time or new faulty components

A burned (short) PCB

A burned (short) PCB

How Can Designers Reduce the Possibility of PCB Short Circuit Protection?

Most PCB failures caused by shorting occur due to errors in the production process. Therefore, you can avoid these problems by adhering to preventive design procedures.

These procedures primarily involve leaving generous spacing between the copper pads and connections to spread out the components. However, this is not always possible due to design constraints.

The alternative is to use advanced design software to adjust the minimum allowed space and set the clearance rules during the PCB design process.

Summary

There you have it! There are several ways to find, test, and reduce the chances of short occurrence in PCBs. If you encounter any challenges while checking your circuit board or have any comments for us, leave a message, and we'll get back to you in no time.