Electrostatic discharge (ESD) can stop a design in its tracks. Although ESD protection is important for several applications, it can be especially crucial for smaller designs like mobile devices. ESD poses little threat of harm to the user, but a small amount of static electricity can ruin semiconductors.
To avoid these malfunctions in manufactured devices, standardized ESD tests were made. These tests were designed to reproduce as many situations that cause ESD events as possible and are divided into different standards based on the ESD event that the test is simulating. These standards are further divided into sensitivity classification levels based on the amount of voltage that runs through the device being tested.
In this article, we'll get familiar with the common device-level testing models for electrosensitive-devices.
Human Body Model (HBM) Testing
The Human Body Model test is the oldest and most commonly conducted method. An HBM test simulates the electrical discharge caused by a person with some residual charge built up from something like dragging their feet across carpet and then touching a component with their fingertip.
The test is modeled by a 100pF capacitor discharging through a switching component and a 1.5kW series resistor into the component.
A typical HBM circuit. Courtesy of the EOS/ESD Association
HBM Sensitivity Classification Levels
|Class 0||< 250 volts|
|Class 1A||250 volts to < 500 volts|
|Class 1B||500 volts to < 1,000 volts|
|Class 1C||1000 volts to < 2,000 volts|
|Class 2||2000 volts to < 4,000 volts|
|Class 3A||4000 volts to < 8000 volts|
|Class 3B||> = 8000 volts|
Charged Device Model (CDM) Testing
A CDM test concerns a device building charge by making friction with a conductive surface and coming into contact with a grounded conductor. These are most common in automated manufacturing environments. An example of a CDM event would be a device sliding around a feeder in an assembly line. Without CDM testing, many devices would be destroyed before their construction is complete.
A CMD test involves placing a device on a field plate with its leads' point up, charging it, and then discharging it.
A typical CDM test. Courtesy of the EOS/ESD Association
CDM Sensitivity Classification Levels
|Class C1||<125 volts|
|Class C2||125 volts to < 250 volts|
|Class C3||250 volts to < 500 volts|
|Class C4||500 volts to < 1,000 volts|
|Class C5||1,000 volts to < 1,500 volts|
|Class C6||1,500 volts to < 2,000 volts|
|Class C7||=>2,000 volts|
Machine Model (MM) Testing
An MM test simulates a device coming into contact with a metallic item like a tool or loose cables. MM testing is far less common than HBM testing because the circumstances that the circuits simulate are more extreme and less likely to occur.
A common ESD model for an MM test is a 200pF capacitor discharged directly into a component with no DC resistor in series with the output circuitry.
A typical MM circuit. Courtesy of the EOS/ESD Association
MM Sensitivity Classification Levels
|Class M1||< 100 volts|
|Class M2||100 volts to < 200 volts|
|Class M3||200 volts to < 400 volts|
|Class M4||> or = 400 volts|
Component-Level ESD Testing
Like consumer devices, components also need to be tested for possible ESD events. The shrinking of integrated circuits allows for smaller product designs and, like with their consumer-end counterparts, the risk of an ESD event increases as semiconductor size decreases.
Over time, chip makers have had to place more and more emphasis on ESD protection. There are also many devices made especially for ESD protection, such as transient voltage suppressors and ESD protection diodes.
If you'd like to learn more about ESD protection, check out this webinar on protecting portable applications from ESD, EMI, and surge.
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