PCB Short Circuit Causes, Detection and Prevention

A short circuit in a circuit refers to the accidental connection of two or more different circuit paths, causing the current to bypass the intended circuit path. This can lead to serious issues such as circuit overload, overheating, equipment failure, and even fire hazards. While short circuits in sockets and switches are common in daily life and typically occur externally to devices, this article will focus on discussing a type of short circuit that occurs internally within devices, known as PCB short circuits. PCB, or Printed Circuit Board, is an electronic board that integrates components and traces and is installed inside devices. Compared to external short circuits, PCB short circuits pose a more challenging problem as they are harder to detect and repair, and the PCB itself is fragile, making it prone to secondary damage.

circuit board short

Causes and Prevention Measures of PCB Short Circuits

Improper Pad Design

Pads, located on the surface of the circuit board, are metallic areas used to connect electronic components. During assembly, solder is applied to the pads, and upon heating, it forms an electrical and mechanical connection with the component’s pins or solder balls, establishing the circuit connection. Improper pad design often refers to inadequate spacing between adjacent pads, irregular pad shapes, or inappropriate pad sizes, ultimately causing solder bridge two adjacent pads during the soldering process, resulting in a circuit board short.

To avoid this issue, it is essential to plan the pad layout properly during the design phase, ensuring an adequate spacing between pads. If constrained by PCB size limitations, consider changing circular pads to elliptical ones to increase the distance between points. After the design phase, CAD software’s design check and simulation functions can be used to identify and correct potential design issues.

changing circular pads to elliptical ones

Scratching

Despite the PCB surface being covered with solder mask as a protective layer, scratching can still occur during manufacturing and use, leading to short circuit problems. For example, collisions or scratches between boards may happen during the manufacturing process due to busy equipment operation, or during use, dropping devices may cause collisions between the PCB and the casing.

Scratching causes PCB short circuit

To avoid these problems during manufacturing, strict adherence to manufacturing specifications and necessary protective measures, such as properly arranging the production process, ensuring correct equipment operation, and avoiding collisions between equipment, is necessary. After manufacturing, necessary testing and quality control should be conducted to ensure that the PCB surface is not damaged or potentially short-circuited.

As for scratching during use, it is advisable for equipment manufacturers to consider possible usage environments during the design phase and take corresponding protective measures, such as improving the protective performance of the casing or reinforcing the support structure of the PCB. Additionally, drop testing for devices that may experience frequent vibration or drops is an effective preventive measure to help identify potential problems and address them promptly.

Conductive Anodic Filament (CAF)

When an electrolyte medium and humidity conditions exist within the inner layers of the PCB, an electric field and chemical action can create filament-like metallic substances within the insulating material. This phenomenon is known as Conductive Anodic Filament (CAF) formation. These filaments continue to grow and eventually penetrate the insulating layer, bridging two conductors within the inner layers and causing a PCB short.

CAF causes PCB short circuit

To prevent short circuits caused by CAF formation in PCBs, it is recommended to select substrate and dielectric materials with low ion content during the PCB design phase, and adopt reasonable wiring designs to reduce the electric field intensity. During the manufacturing process, attention should be paid to controlling the humidity and temperature of the production environment, and necessary moisture-proof measures should be taken after production.

Signs of Circuit Board Short

When devices exhibit abnormalities, the first reaction is often to suspect a circuit board short. However, loose components or foreign objects entering the device can also cause abnormalities. Therefore, when finding a short on a circuit board, it is essential to first determine whether the device’s abnormality is caused by a short circuit.

Typical signs of short circuits include PCB heating, burnt odors, or smoke generation. These signs can be explained by Ohm’s Law. In a circuit, current tends to flow through the path of least resistance, and a short circuit means there is an unexpected low-resistance path, causing the current to bypass the designed path. According to Ohm’s Law, I = V/R, where voltage V remains constant, and a decrease in resistance R means an increase in current I, generating more heat, which may result in component damage.

However, certain parts ceasing to function or exhibiting abnormalities do not necessarily indicate a short circuit. To further determine the problem, testing with a multimeter or other testing equipment is necessary.

How to Find Short Circuits in PCB

Visual Inspection

This may be one of the most basic and convenient methods, as no specialized tools are required, and only visual observation is needed. When finding short circuits in PCB, it is recommended to perform visual inspections under natural light rather than using desk lamps, as desk lamp light may be glaring and cause reflection on component pins, leading to overlooking certain issues. In contrast, natural light is softer. If you encounter subtle problems that are difficult to judge, place the PCB under a desk lamp and use a magnifying glass for careful observation.

Eye Check to Find Shorts in PCBs

Infrared Thermal Imaging Detection

If you have a larger budget, it is advisable to equip yourself with an infrared thermal imaging detector, which can more accurately identify the specific location of short circuits on the PCB. It operates based on the principle that a short circuit generates additional heat. By scanning the entire surface of the PCB, the problem can be discovered, even if the short circuit occurs in inner layers.

When using an infrared thermal imaging detector, it is necessary to power the PCB to be tested and keep it in working condition. After a period of time, the surface temperature of the PCB will reach a stable value. Then, use the infrared thermal imaging detector to image the PCB. By observing the thermal image, you can clearly see the thermal distribution on the PCB surface. If bright spots or areas appear on the thermal image, these areas are likely the location of the short circuit problem.

Infrared thermal imaging finds short circuits in PCBs

Power Off Method

In excellent PCB designs, designers incorporate devices such as fuses, inductors, diodes, and zero-ohm resistors in the power branch, which play important roles. The core idea of the power-off method is to analyze the schematic diagram, identify series devices that are easily cut off from the current, and gradually remove these devices to locate the short circuit position.

For example, in a circuit where an inductor and diode are in series in the power path, removing one device, such as inductor L1, can determine whether the short circuit occurs in the DC-DC circuit before this inductor or in the load circuit after this inductor. Through similar step-by-step removal, the location range of the short circuit can be narrowed down, thus solving the problem more quickly.

Power Troubleshooting to Find Shorts in PCBs

Minimum Resistance Method

This is a method of using a multimeter to measure the resistance of the PCB to determine the specific location of the short circuit. Since a short circuit causes the current to bypass the designed path and choose a path with lower resistance, we can infer the possible location of the short circuit by measuring the resistance between different non-ground (non-GND) points on the PCB and ground and finding the point with the lowest resistance.

Further Reading: How to Test Circuit Boards Using a Multimeter

Although the minimum resistance method is effective, it has two main problems:

  • The multimeter’s accuracy requirement is high, typically on the milliohm level.
  • Manual detection of each point is time-consuming when testing high-density PCBs.

To address these issues, the emergence of short circuit testing instruments provides a solution. These instruments can collect small currents, amplify them, and then convert them into voltage signals. Next, a microcontroller performs logical judgment and indicates the point with the minimum impedance using a buzzer or digital display. Unlike traditional methods, this approach does not require additional power-on, hence it is referred to as a non-destructive testing method.

Minimum resistance method to find short circuits in PCB

You Might Be Interested

hard drive pcb replacement
Hard Drive PCB Replacement Guide

The guide outlines the importance of Hard Drive PCBs, providing insights into diagnosis and repair/replacement options. It emphasizes BIOS chip compatibility and cautious PCB replacement

what is a short circuit
Fundamentals of Short Circuit

Short circuits, prevalent in electronics, pose serious risks like fires, damage, and shocks. Identifying causes such as wire damage or component failure is crucial for

pcb soldering defects
PCB Soldering Defects

PCB solder defects, like bridging, fillet issues, virtual soldering, voiding, and tombstoning, arise from factors such as temperature control and component placement. Prevention involves planning,

solder bridge
Basic Guide to PCB Solder Bridges

Solder bridges in PCBs are unintended connections between circuit paths or pads, leading to short circuits and device malfunctions. They stem from design flaws, process

How to Test Circuit Boards Using a Multimeter
How to Test Circuit Boards Using a Multimeter

When electronic devices encounter malfunctions, troubleshooting the internal PCBA board is a common method. These circuit boards are the core of device functionality; however, various

Scroll to Top