As children, my siblings and I engaged in a favorite game that involved shuffling across the carpet in our fuzziest socks and delivering an electrifying jolt to each other. It was a simple pastime filled with laughter and pranks. However, what was once a lighthearted amusement for us holds much greater significance in the world of electronics manufacturing.
Electrostatic discharge (ESD) events pose a serious threat to the service life of electronic products and can be extremely hazardous to both users and maintenance operators. Under certain conditions, ESD can lead to smoke inhalation, fire, or explosions. Recognizing the potential for grave danger, multiple safety agencies have established standards for ESD testing at both the component and system levels. While testing for worst-case scenarios can be challenging in the realm of electronic devices, manufacturers and developers can rest assured knowing that their products are designed to operate safely in most environments and applications.
Here we will explore the critical importance of ESD testing in safeguarding both people and electronics. By understanding the risks associated with ESD and adhering to rigorous testing standards, manufacturers can ensure the longevity and safe operation of their products.
Components: The First Line of ESD Testing
ESD testing involves the implementation of a setup that generates a high-voltage electrical pulse to assess the susceptibility of the device under test. The objective is to ensure that the device can effectively dissipate or shunt the associated energy without causing damage to its sensitive circuitry. However, ESD testing adds an additional layer of complexity compared to most other forms of electronic testing because it must also account for human interaction with the device during discharge.
In component testing, two equivalent circuits are commonly used:
Human Body Model (HBM)
ESD originating from humans and transferring to devices is one of the most prevalent modes of electrostatic charge transfer. A significant charge can accumulate as a person walks across surfaces or due to friction between clothing and chair fabric, bridging small air gaps. The HBM circuit incorporates a switch that toggles between two series connections. On the charging side of the circuit, a high-voltage supply charges a capacitor (100 pF) through a resistor with a value in the megaohms range. When the switch is activated, the stored electric field in the capacitor discharges through a 1.5k Ω resistor and then passes through the device under test (DUT). It is important to note that HBM testing is performed per-pin basis for device packages, making comprehensive testing impractical for high-pin count components. Instead, pin coverage is selected according to the ANSI/ESDA/JEDEC JS-001 standard.
Charged Device Model (CDM)
ESD can also occur when a device accumulates charge and subsequently comes into contact with a conductive surface at a lower potential. ESD events originating from the device itself can be more potent than those from the human body due to the low resistance between two conductive surfaces, which accelerates the discharge rate and increases the likelihood of a complete discharge. In contrast, human-body ESD can involve multiple discharge events as the gap between the body and the device decreases. The CDM circuit is slightly more intricate than the HBM circuit (detailed information can be found in the ANSI/ESDA/JEDEC JS-002 standard). The device is placed on a plate with its leads facing upward. The plate is charged and then allowed to discharge evenly through all the component pins, repeating the process for five positive and negative discharge cycles.
To facilitate quick grouping and comparison of parts with similar levels of susceptibility to ESD, a component classification system has been established. This system aids in ESD protection, but it is important to note that it provides only a partial picture of compliance and should not be solely relied upon for comprehensive ESD protection.
Component Classification Tables (as listed in ANSI/ESDA/JEDEC standards) |
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| JS-001 | JS-002 | ||
| Classification | Voltage range (V) | Classification | Voltage range (V) |
| 0Z | V < 50 | C0A | V < 125 |
| 0A | 50 ≤ V < 125 | C0B | 125 ≤ V < 250 |
| 0B | 125 ≤ V < 250 | C1 | 250 ≤ V < 500 |
| 1A | 250 ≤ V < 500 | C2A | 500 ≤ V < 750 |
| 1B | 500 ≤ V < 1000 | C2B | 750 ≤ V < 1000 |
| 1C | 1000 ≤ V < 2000 | C3 | ≥ 1000 |
| 2 | 2000 ≤ V < 4000 | ||
| 3A | 4000 ≤ V < 8000 | ||
| 3B | ≥ 8000 | ||
Understanding the intricacies of component-level ESD testing sets the foundation for comprehending the broader scope of ESD protection and the measures necessary to safeguard electronic devices and their users.
Bringing it All Together With Board-Level ESD Testing
Once the ESD capability of individual components has been verified, the focus shifts to testing the Printed Circuit Board Assembly’s (PCBA) ESD compliance. The IEC 61000-4-2 standard outlines the configuration for this testing. It involves placing a 122 cm square, 6 mm thick aluminum plate on the floor and connecting it to earth ground. The discharge return cable of the ESD equipment is grounded to the plate. On top of the plate, a 0.8 m wooden table with identical surface dimensions is placed. The DUT rests on a 0.5 mm thick, 46×30 mm insulator, and a coaxial cable feeds through the horizontal plane to connect to an oscilloscope for recording voltage pulses used to determine test classification levels based on the discharge method.
IEC-61000-4-2 Test Level Classifications |
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| Contact discharge | Air discharge | ||
| Level | Test voltage (kV) | Level | Test voltage (kV) |
| 1 | 2 | 1 | 2 |
| 2 | 4 | 2 | 4 |
| 3 | 6 | 3 | 8 |
| 4 | 8 | 4 | 15 |
To provide effective protection against ESD damage, robust filtering is essential. The key components of an ESD filter circuit include series inductors to block large current spikes, capacitors to clamp high voltage to ground, and transient voltage suppressor (TVS) diodes to suppress fast-rising ESD signals. While specific values and layouts may vary, these components form the fundamental building blocks of an ESD filter circuit.
In addition to implementing robust filtering, careful consideration should be given to the PCB layout to minimize the likelihood and severity of ESD events. Here are some additional steps to help protect a PCB from ESD:
- Maintain ample clearance between antennae and connectors to prevent potential ESD pathways. Similarly, keep signal traces at a distance from the antennae to minimize coupling.
- Ensure excellent signal grounding using continuous ground pours on outer layers, incorporating fencing or shielding techniques to provide short, direct, and reliable return pathways.
- Traces extending from connectors prone to ESD should have greater clearance than standard traces to antennae.
By combining thorough board-level ESD testing and implementing proper filtering and layout techniques, electronics manufacturers can significantly enhance the ESD immunity of their PCBs.
Your Contract Manufacturer Stays Grounded in the Face of ESD
ESD testing plays a crucial role in ensuring the long-term reliability and safe operation of electronic devices. As assemblies become denser and more complex, the potential impact of ESD events becomes even more significant. Implementing ESD prevention best practices may require making tradeoffs in other areas of performance, but it is a necessary step to protect sensitive circuitry.
Navigating the path to device optimization and ESD protection can be daunting, but partnering with an experienced PCB manufacturer can make all the difference. With over four decades of industry experience, the team at VSE is dedicated to building electronics for our customers and continuously refining designs. We work closely with our manufacturing partners to deliver high-performance and high-quality devices that have the potential to save and transform lives in various industries.
When it comes to ESD protection and overall PCB manufacturing, you can trust VSE to stay grounded and ensure the utmost quality and reliability in every product we produce. Contact us today to discuss your specific requirements, and let us help you bring your electronic innovations to life with ESD protection at the forefront.
