Circuit protection: Understanding differences in the Human Body Model and IEC 61000-4-2 standards
Circuit protection in electronics systems continues to be a key consideration for increasing safety, reliability, and cost-effectiveness of electronics products. There are many critical factors for designers seeking to implement appropriate levels of circuit protection in their designs. One such factor is proper circuit protection for manufacturing versus real world environments. A finished product capable only of withstanding electrical overstress to manufacturing standards is ill-advised. In real world environments and conditions this is a recipe for circuit failures in the hands of end users.
So, a key consideration for designers is to understand the difference between manufacturing environment and end user environment electrostatic discharge (ESD) testing. Most designers are familiar with classic device-level tests that are applied to integrated circuits (IC). However, the most common misunderstanding occurs between the Human Body Model (HBM), which is according to MIL-STD-883 Method 3055, and HBM ESD testing, which is according to IEC 61000-4-2 standards. These two standards are designed for very different purposes. Only the stringent IEC 61000-4-2 standard is acceptable to indentify real world ESD stress conditions.
These standards and others such as, Machine Model (MM) and Charged Device Model (CDM), have been developed to test the performance and robustness of electronic devices. Unfortunately these standards are also the cause of frequent misunderstanding. This results in their being used interchangeably, which ultimately results in failures of so-call protected systems while in the consumers' hands.
Think of ESD as a miniature lightning event, where an electrical charge is transferred between two surfaces or objects that are at differing potentials. The build-up of a static electrical charge is called tribocharging or triboelectric generation. When a charged object approaches or touches another conductive object, an arc will occur causing an uncontrolled transfer of charges. ESD is a single, fast, high voltage and high current event resulting from the near or direct contact of two objects generating both an electric and magnetic field. ESD is defined by the three standard models: HBM, MM and CDM.
A static charge can accumulate on a device through handling or contact with packaging materials or work surfaces. This frequently occurs when a device moves across a surface or vibrates in a metal tray. The model used to simulate the transfer of charge between two devices is CDM. Similarly, MM refers to the accumulation and transfer of a charge from equipment to equipment. HBM is the most prominent source of ESD, where simple human contact or air discharge can cause component or product failure on fixed or handheld equipment.
Today's ESD Vulnerabilities
There are three important changes in today’s systems that have increased ESD vulnerability. Smaller manufacturing geometries are one. Most ICs have decreased to less than 90nm and the voltage and current levels have decreased. ESD damage can occur due to excessive voltage, high current levels or a combination of both. High voltages cause gate oxide punch-through and high current levels cause junction failures and melting of metalized traces. These decreases in geometries have made it very difficult to provide adequate on-chip ESD protection.
Today we also have a reduction of on-chip protection. This came about from the Industry Council on ESD Target Specifications, which announced a move to reduce the standard level of on-chip protection. This seemed to be done to shift focus to providing adequate protection for the manufacturing environment.
We also have a dynamically changing application environment: laptops, smartphones, digital music players, digital cameras, tablets and other hand-held devices that are used in uncontrolled environments. They require additional ESD protection. In these environments people touch I/O connecter pins - such as USB or HDMI - when plugging in and removing cables. The devices are subject to constant ESD stress levels when this happens.
Portable devices can also build up a charge and will discharge when plugged into a grounded device such as a TV or computer. The simple act of walking across a carpet can generate up to 40kV of this level of discharge through a port. This can cause latent and permanent defects of supposedly-protected ICs (see table below for other examples).