How to Test For a Short Circuit on a PCB

Finding short circuits on a PCB is not a test that is typically performed with a prototype board. In fact, if your PCB fabrication and assembly house performed in-circuit testing or continuity testing, they will have verified connectivity in your PCB against the netlist generated from your design files. Therefore, unintentional short circuits do not become obvious until later, when you start to bring up your board and examine how it functions.

Short circuits can arise due to unintentional placement of shorts between nets in your design data, or due to defects during fabrication and/or assembly. Some reasons these might arise include:

• Aggressive design choices that violate fabrication house capabilities
• Misrepresentation of capabilities by the fabrication house or assembly house
• Shorts in a netlist marked as intentional in continuity testing, often due to misunderstandings by the fab house
• Poor assembly inspection (such as automated optical inspection) by the assembly house, which fails to find defects

Large enterprises, small startups, and entrepreneurs all should understand the above risks and consider these points when vetting a fabrication/assembly vendor. If boards do happen to come back defective, some short circuit testing can help you narrow down who might be at fault, and you might earn some rework credit with your manufacturer in the process. When you need to prepare boards for production, and review your design while performing short circuit testing, look no further than Altium Designer^®.

How to Find a Short Circuit in a PCB

There are many approaches for testing a PCB for short circuits. This need will generally arise when a board is failing to perform as expected, and will typically be identified through some quick measurements on I/Os or power nets. Some of the main methods for locating short circuits include:

• Probing with a meter
• Visual inspection of components
• Thermal inspection
• X-ray inspection
• Destructive testing

Visual Inspection

Assuming you're past the breadboard design stage and there isn't a built-in circuit breaker, the first step on how to test for a short circuit on PCB is to carefully look at the entire surface of the PCB. If you have one available, use a magnifying glass or low-magnification microscope during the circuit board examination. Starting at the power supply and working forward, look for tin whiskers between pads or solder joints. Any cracks or blobs of solder should get careful attention. Check all your vias. If you specified unplated vias, make sure that’s the case on the board. Poorly plated vias can create a short circuit between layers and leave you with everything tied to ground VCC or both.

If the short is really bad and causes components to reach critical temperatures, you’ll actually see burned spots on the printed circuit board. They can be quite small but will be dramatically discolored brown instead of the normal green solder mask. If you have multiple boards, a burned printed circuit board can help you narrow down the specific location without a power supply to another board to sacrifice in the search. Unfortunately, our board didn’t have any burns on the PCB itself, just the unlucky fingers that checked for overheating ICs.

Some shorts will be internal to the circuit board testing and will not produce burn spots. This also means they won’t be noticeable from the surface layer. This is where you will need some other circuit board tester method to detect a short circuit in a PCB.

Burns can definitely help you locate a short but at a very smoky cost.

Infrared Imaging

If you’re not at a startup that just blew its hardware budget, you might be lucky enough to have access to an infrared camera. Using an infrared camera can help you locate areas where a large amount of heat is being generated. If you do not see a hot spot away from your active components, then you might have a PCB short circuit, even if the short is between internal layers.

The short usually has higher resistance than a normal trace or solder joint since it doesn’t have the benefit of being optimized in your design (unless you’re really bad about ignoring rule checks). That resistance, as well as the naturally high current due to the direct connection between power and ground, means the conductor in the PCB short circuit will heat up. Start with the lowest current you can. Ideally, you’ll see the short before it does any more damage.

The finger test is one way to check whether a particular component is overheating

Short Circuit Testing with a Digital Multimeter

To test a circuit board for a short circuit check, you need to check the resistance between different points in the circuit. If a visual inspection doesn’t reveal any clues as to the location or cause of the short circuit, grab a multimeter and try to track down the physical location on the printed circuit board. The multimeter approach gets mixed reviews in most electronics forums, but tracing your short circuit test points can help you figure out what isn’t the problem.

You’ll need a multimeter with milliohm sensitivity, and it’s easiest if it has a buzz function to alert you when you’re probing a short. Typical DMMs you can buy from a home improvement store or on Amazon will work fine; this is something every engineer should have in their toolbox. The Amprobe DMM shown below is a good example of a basic DMM that can be used for DC resistance measurements and power measurements on a circuit board.

As an example, if you measure the resistance between neighboring traces or pads on a PCB, you should measure high resistance. If you measure a very low resistance between two conductors that should be in separate circuits, these two conductors may be bridged, either internally or externally. Note that adjacent two traces or pads that are bridged with an inductor (such as in an impedance matching network or a discrete filter circuit) will produce a very low resistance reading as an inductor is just a coiled conductor. However, if two conductors are very far apart on the board, and you read very small resistance, then you have a bridge somewhere in the board.

Common Ground vs. Short Circuits

Of particular importance is a short involving a grounded via or ground plane. Multilayer PCBs with an interior ground plane will include a return path via near components, which provides a convenient location to check all other vias and pads on the surface layer of a board. Set one probe on a ground connection, and touch the other probe throughout other conductors on the board.

This same ground connection will be present at other locations on the board, meaning if you touch each probe to two different grounded vias, you will read very small resistance. Pay attention to your layout while you do this as you do not want to mistake a short circuit for a common ground connection. All other exposed conductors that are not tied to the ground should read very high resistance between your common ground connection and the conductor itself. If you read a very low value, and you do not have an inductor between the conductor in question and the ground, then you may have a bad component or a short circuit.

Multimeter probing can help you to track down a short, but they aren’t always sensitive enough to find it.

Shorted Components

Checking for a shorted component also involves using a multimeter to measure the resistance. In the case where visual inspection did not reveal excessive solder or metallic flakes between pads, your short may have formed in the internal layers between two pads/pins on a component. It is also possible that a short occurs between pads/pins on a component due to poor fabrication. This is one reason that PCBs should undergo DFM and design rule checks; pads and vias that are placed too close together could be bridged and short unintentionally during fabrication.

Here, you’ll want to measure the resistance between pins on an IC or connector. Neighboring pins are susceptible to shorts due to solder bridging, but these are not the only locations where a short can form. Check that your resistance between pads/pins with respect to each other and the ground connection has low resistance.

Check resistance between the ground pad and the other pins on your connectors and ICs. This is shown here for a USB connector.

Narrow Down the Location

If you think you have located a short between two conductors, or between some conductor and ground, you can narrow down the location by checking nearby conductors. With one lead of the multimeter on the suspect shorted connection, move the other lead to different nearby ground connections and check the resistance. As you move to farther ground connections, you should see the resistance change. If the resistance increases, then you are moving your grounded lead away from the location of the short. This helps you narrow down the exact location of the short, and you can even narrow it down to a specific pair of pads/pins on a component.

X-ray Imaging

This is another option that is not available to everyone, but it is accessible through assembly houses that support assembly of leadless packages. X-ray imaging can be used to identify short circuits beneath BGAs, or possibly from bridging in the interior of the board (such as between a via and a plane). If you think you have narrowed the short circuit down to a BGA, your assembly house could (or should) be willing to inspect the board with their X-ray imaging system.

Normally, leadless components will be examined in X-ray imaging because they can't be examined using automated optical inspection (AOI); the leads are not visible so X-ray inspection is the only available option. Typical packages include:

• BGA packages
• LGA packages
• QFN packages
• Other leadless packages or leadless chip carriers

Consider BGAs as an example: X-ray imaging can show defects beneath a BGA due to shifting of solder balls during assembly; an example of such an image is shown below. It's also possible that there is an intermittent failure or open circuit due to a head-and-pillow defect on the solder ball. This would lead to no output from the component when I/Os or power are monitored. This would appear to be an intermittent defect that arises after the package is powered and heats up.

X-ray image of a component with BGA packaging on a PCB.

There is one possibility that might arise when dealing with a BGA that might have a short circuit. The assembly could have proceeded successfully with no defects (no solder ball shorts or intermittent failures from head-and-pillow defects), but the pinout was incorrect. If you are looking at the BGA pinout for a datasheet, you should make sure the pinout is specified from the top view or the bottom view. If you build the pinout based on the top view, but the datasheet is showing you the bottom view, then the pinout will be backward. This is a perfect recipe for bridging power and ground, signal and ground, or signal and power.

Faulty Components May Fail to a Short Circuit

Faulty components or incorrectly installed components can be part of a short circuit, creating any number of problems in your board. Your components could be faulty, counterfeit, or burned out, which can create a short circuit (or the appearance of a short circuit). This is because some components, once they fail, will fail into a short circuit state and create the appearance of a defect where there was none before. Examples include:

• MOSFETs (due to overheating)
• Capacitors (when drive with overvoltage)
• Diodes (due to overheating)
• Some I/Os on integrated ciruits
• Magnetics (due to wire fusing)

Some components have a tendency to go bad, like electrolytic capacitors. If you have suspect components, check those first. If you aren’t certain, you can usually do a quick Google search for components that you suspect “failed” to find out if it’s a common issue. In the case where you measure very low resistance between two pads/pins (neither of them is ground or power pins), you may have a short circuit due to a burned-out component. This is a clear sign that a capacitor has gone bad. Capacitors will also bulge once they go bad or if the applied voltage exceeds the breakdown threshold.

See the bulge on top of this capacitor? This is a telltale sign that the capacitor has gone bad.

How to Destructively Short Circuit Test a PCB

Destructive short circuit testing is obviously a measure of last resort. In the absence of an X-ray unit, you can start removing components and run your multimeter tests again. This helps in two ways. First, it gives you much easier access to pads, including thermal pads, that may be shorting out. Second, it eliminates the possibility that a faulty component was responsible for the short, allowing you to focus on the conductors. If you manage to narrow down the location of the short circuit to a connection on a component, e.g., between two pads, it may not be obvious whether the component is defective or if there is a short somewhere in the interior of a board. At this point, you may want to remove the components and check the pads on your board. Removing the components allows you to test whether the component itself is defective, or if the pads on the board are bridged internally.

If the location of the short (or possibly multiple shorts) is still elusive, you can cut open the board and try to narrow down the location of the short. If you have an idea of the general location of the short, then you can cut out a section of the board and repeat your multimeter tests in that section. At this point, you can repeat the above tests with a multimeter to check for shorts in specific locations. If your circuit board testing has reached this point, then your short is already particularly elusive. This will at least allow you to narrow the location of your shorts to a particular region of the board, but it is a painstaking effort.

Here are some ideas for inspecting PCBs for short circuits that involve some destructive testing:

• If you narrow down the potential short to a specific via, take a drill and cut out that via, then re-check for shorts between those nets. If you that via was the culprit, you should measure an open circuit
• A PCB layer can be removed with sandpaper or a small grinder, this lets you visually inspect each layer for defects
• Cut the board into sections and re-check net connections; one section should still contain the short circuit. You can continue narrowing down this area until you find the short circuit.

When you need to prepare boards for production review your design while performing short circuit testing, look no further than Altium Designer^®. The OutJob File feature in Altium Designer helps you quickly create production file packages and testing documentation for your PCBA. When you’ve finished your design, and you want to release files to your manufacturer, the Altium 365^™ platform makes it easy to collaborate and share your projects.

We have only scratched the surface of what’s possible with Altium Designer on Altium 365. Start your free trial of Altium Designer + Altium 365 today.