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

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.

What Exactly Happens During a Short Circuit in a PCB

The scenario that initiates the short circuit is an unintended connection between traces or component leads. After that, excessive current flows through the joint because the connection provides a shortened, low-resistance path to the return line, bypassing the required route.

A burned-out PCB caused by shorting

This current surge causes localized heating that can damage the surrounding components or melt the board, creating a fire hazard. Once the components start malfunctioning, the entire device will follow suit.

Most modern electronics have fuses to prevent further damage, which can be in the wall plug or inside the device. So, it is difficult to get to the point of melting the board, but you will most likely have some burnt components.

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.

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

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

However, the digital multimeter should have good sensitivity (milliohm) and a buzzer function to alert you when it detects a short. If you measure the resistance between two adjacent wires, the resistance readings should be high.

However, if the resistance is low and the adjacent wires are in separate circuits, there may be internal or external solder bridges between them.

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

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.

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.

A burned (short) PCB

Reasons and Solutions for Short Circuit Boards

Soldering Short Circuit

A soldering short circuit occurs when the spacing between the components is too small, or the pitch between IC pins is too tiny due to a high-density design in a tiny package. In such a case, solder can cause contact between the component pins or mounting pads, creating a short circuit.

A shorted, blown-up IC

This issue mostly occurs in SMT devices, where excessive solder paste can create contact after soldering.

Excessive solder paste comes about when manufacturers use thick stencils, don’t set the size of the holes properly, don’t support the PCB evenly, use a tilted stencil when printing, use a dirty stencil or use a steel mesh with little tension.

Rectifying these issues will help prevent shorting.

Edge collapsing and PCB position offsetting can also cause short-circuiting. So you should use solder paste with moderate viscosity, set the reflow temperature to match the solder paste’s parameters, use advanced laser cutting machines to prepare the stencils, adjust the placement and pressure of the pick-and-place machine, and manually calibrate the printing accuracy.

PCB Component Short Circuit

PCB electronic components are fragile devices, and they can short-circuit due to assembly failure, cleaning issues, and even component failure, which usually occurs when you connect both lead ends to one wire.

A shorted resistor burning while connected to a PCB

Electrochemical Short Circuit

The chemicals the PCB is exposed to during assembly can leave ion residues that corrode the assembled board or create electrochemical migration that shorts the components directly or via the circuitry.

A PCB with corroded copper traces

These ion residues can come from excessive flux, which assemblers use to increase the wettability of lead-free solder, and poor cleaning after assembly.

Poor or zero cleaning after assembly is not as big of an issue immediately but the board’s surface will begin to change color and rot later when in use in the finished product.

ESD Breakdown

Electrostatic discharge occurs when a charged surface, such as the human body, contacts a circuit or electronic components. Induction and friction between electrical appliances can also cause this phenomenon.

If the PCB is not designed to handle static charges, it can short out or become unstable.

Some measures to make the PCB ESD resistant include avalanche diodes, high-voltage capacitors, ferrite beads, CMOS devices, TTL devices, or LC filters.

You can also improve the circuit structure or use multilayer boards, implement the spark gap method, or use decoupling capacitors to reduce the loop area for high-frequency energy.

How To Fix a Short in a Circuit Board

Once you’ve located the source of the problem, you should fix it depending on the damaged part or section and the extent of the damage.

For instance, if the board has a damaged component, you have to desolder and resold a new one.

A severely damaged and unrepairable PCB due to short-circuiting

If you’re dealing with shorted or corroded traces, you’ll have to scrape about a millimeter of the solder mask to expose the copper on the sides of the damaged section, apply some flux, and then melt the solder above the two sides. Use KYNAR or enameled wire to connect the two ends if the space is too wide.

If the damage is extensive, you can isolate the damaged sections using insulating materials, recreate the damaged circuit, or replace the entire board.

How To Prevent Shorting in Circuit Boards

There are five ways to prevent PCB shorting, the first being to implement isolation techniques.

You can use trenches and isolation slots to separate high and low-voltage components/traces physically on the PCB to prevent shorting from the high-voltage difference.

A PCB with a 15-amp surface-mounted fuse

We recommend implementing current-limiting components as well, such as resistors, to connect to the critical traces in series. These components will limit excessive current flow from the line if a fault occurs, which minimizes the risk of shorting.

You should also consider using overload protection devices like fuses and thermal cutoffs to prevent shorting and overheating if the current flow and heat levels exceed their maximum ratings.

On the design aspect, use design-rule checking software to enforce the design constraints, which include the minimum spacing and clearances that prevent unintentional connections.

Automated optical inspection is important as well during and after assembly to detect defects like solder bridges, which cause shorting.

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.