How to Test for a Short Circuit on a PCB

February 13, 2018 CircuitStudio

Fire in the microchip

Every engineer has that “worst case scenario,” story that they survived, but they kept on designing. During the worst week of my professional life, we had a shipment of overdue PCBs come in. These PCBs were supposed to have been installed in hardware and deployed at customer sites more than a month earlier. We were beyond a little mortified.

Needless to say, there was a slight rush to ship these new boards. Once we powered them up for testing, you could smell the ozone coming off the boards. One of the most expensive components heated up so hot that it actually burned a couple people doing a “touch test.” We didn’t have the time, we thought, to get a test batch of boards, and just ordered the fully populated PCBs.

Obvious frustrations aside, explaining major hardware failures to your boss while sucking on burned fingers is one of the worst meetings you can have. The best thing you can do is offer a plan forward. If you’re ever in that situation, here’s how to find a short circuit on a PCB.

How to Check for Short Circuits in a PCB

Here are some important steps you can take to check for short circuits in a PCB:

 

 

Step 1: How to find a short circuit in a PCB

Visual inspection

The first step is to carefully look at the entire surface of the PCB. If you have one available, use a magnifying glass or low magnification microscope. 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 PCB 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 board 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 method to detect a short circuit in a PCB.

How to detect a short circuit in a PCB for a hard drive visually

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 are not seeing 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 is usually higher resistance than a normal trace or solder joint since it didn’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.

How to find faulty components on a PCB: the finger test

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

 

 

 

Step 2: How to test a circuit for short on an electronic board

Outside of the first step in using your trusty eyes to check the board, there are a few other ways you can check to find the potential cause of PCB short circuits.

Testing with a Digital Multimeter

To test a circuit board for a short circuit, you need to check the resistance between different points in the circuit. If 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 test points can help you figure out what isn’t the problem.

You’ll need a very good multimeter with milliohm sensitivity, and it’s easiest if it has a buzz function to alert you when you’re probing a short. 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, it is possible that these two conductors are 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.

Testing Relative to Ground

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 through a 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 on 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 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 ground, then you may have a bad component or a short circuit.

How to check short circuit in a PCB with a multimeter

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 bridge and short unintentionally during fabrication.

Here, you’ll want to measure the resistance between pins on an IC or connector. Neighboring pins are particularly susceptible to shorts, 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.

How to check short circuit in a PCB on component pads

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.

 

 

 

Step 3: How to find faulty components on a PCB

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 or counterfeit, creating a short or the appearance of a short.

Bad Components

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 are ground or power pins), then 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.

How to check for faulty components on a PCB

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

 

 

 

Step 4: How to destructively test a PCB

Destructive testing is obviously a measure of last resort. If you have access to an X-ray imaging unit, then you can examine the interior of a board without destroying it.

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 you’ve reached this point, then your short is already particularly elusive. This will at least allow you to narrow the location of your short to a particular region of the board.

When we examined our boards for shorts, which did not include infrared testing because we were a broke startup, all we could figure out was that the short was in one half of the board. So, we cut the board into quarters and tested each section. Going back to the multimeter confirmed that most sections did not have VCC and ground tied together. But that single quarter of the board was a little black hole of mystery, and we never got any closer to the short. We did change manufacturers and get test boards on our next production round, and our boards just happened to function properly.

 

 

If you want to avoid the soul-crushing anxiety of searching out short circuits, make sure you have robust in-circuit test rule checking, for errors, design issues, and manufacturer tolerances. A solid design software like Altium’s CircuitStudio®   can do most of that for you, as well as providing the unified design environment you need to accomplish your designs with as little headache and singed fingers as possible.

If you’re still interested in finding potential short locations, or you want to discuss how a proper PCB design software can help that consider talking to an expert at Altium today.

 

Sign up and try Altium 19 today.

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