What Is Electrostatic Discharge (ESD) ?

Introduction to Electrostatic Discharge

Electrostatic discharge (ESD) refers to the sudden flow of electricity between two electrically charged objects caused by contact, an electrical short, or dielectric breakdown. ESD can cause serious damage to electronic equipment and components. The rapid transfer of electrostatic charge can result in a short but high current flow that can melt metal surfaces or puncture insulating materials. ESD is a concern in many industries that rely on electronics and integrated circuits, such as aerospace, automotive, medical devices, and electronics manufacturing.

Key Facts About ESD

• ESD is caused by an imbalance of electric charges between two objects. This imbalance creates an electric field.
• When the two objects make contact or approach very close together, electrons transfer rapidly to balance the charges.
• The transfer of electrons in a short burst creates the ESD event.
• ESD can happen through direct contact or via induced electric fields.
• ESD typically occurs at voltages below 15,000 volts.
• The discharge usually lasts for just a few nanoseconds.
• ESD can generate currents of more than 1 ampere.
• The associated power levels can melt metal surfaces or vaporize materials.

ESD Damage Mechanisms

ESD can damage electronic components and devices in several ways:

• Thermal damage – High currents can cause localized overheating, which can melt metal interconnects or fuses.
• Vaporization – Insulating layers can be “punctured” by ESD, allowing metal migration that creates short circuits.
• Gate oxide breakdown – Thin insulating films inside ICs can be ruptured by ESD.
• Latch-up – Parasitic circuits triggered by ESD can disrupt normal functioning.
• Signal errors – Fast transients can alter logic states or stored data.

ESD Standards

There are several important standards that provide testing methods and ESD control recommendations:

• ANSI/ESD S20.20 – For the Development of an Electrostatic Discharge Control Program
• IEC 61340-5-1 – Protection of Electronic Devices from Electrostatic Phenomena – General Requirements
• JEDEC JESD22-A115 – Electrostatic Discharge Sensitivity Testing Human Body Model (HBM) Component Level
• IPC-A-610 – Acceptability of Electronic Assemblies
• IPC 7711/7721 – Rework, Modification and Repair of Electronic Assemblies

These standards help define appropriate ESD control procedures and requirements for electronic components and assemblies.

How ESD Occurs

ESD is fundamentally caused by the build up and discharge of static electricity. Certain materials like plastics and fabrics tend to gain and hold significant static electric charges more easily. ESD occurs most commonly under conditions of low humidity, which reduces conductivity and the natural dissipation of static charges. There are two primary modes of ESD – directly contact discharge and induced air discharge.

Contact Discharge

Contact discharge occurs when two objects come into direct contact while having substantially different electrostatic potentials. For example, an integrated circuit (IC) slid across a table surface can transfer charge due to rubbing contact. If the acquired charge on the IC is of opposite polarity to a nearby grounded point, a rapid ESD event can occur when they make contact. The discharge may happen through a single pin or lead of the IC package.

Induced Air Discharge

Air discharge ESD happens when an object with high electrostatic charge comes close to a conductive object at a different potential without actually touching it. The strong electric field gradients that form around the charged object induce currents that can arc across the air gap. Sparks are a common example of air discharge ESD. This frequently occurs as someone walks across a carpet on a dry day and then approaches a grounded metal object like a door knob or file cabinet.

Triboelectric Charging

Many common materials can acquire significant static electric charges through a process called triboelectric charging. This involves charge transfer between two surfaces during rubbing contact. Some examples of materials that readily gain charge this way include:

• Plastics – PET, PVC, polystyrene, polyethylene
• Paper products
• Glass
• Synthetic fabrics and textiles

Charge build up is affected by properties like surface roughness, conductivity, and chemical characteristics. Generally, rubbing dissimilar materials produces greater static charging than similar materials.

Electrostatic Induction

Electrostatic induction describes how electric charges can influence the distribution of charges in nearby conductive objects without physical contact. When a charged object is brought near an uncharged conductor, electrons in the conductor will shift position. For example, proximity to a negatively charged object will repel electrons in the conductor, leaving the facing side with a net positive charge. This induced polarization effect can facilitate ESD when the two objects are grounded at different times.

ESD Effects on Electronics

ESD can cause both immediate and latent damage to electronic components and assemblies. Sensitive components need to be handled carefully using ESD protection techniques to prevent harmful discharges. Some key effects of ESD on electronics are highlighted below.

Latch-Up

Latch-up occurs when parasitic circuit elements trigger a high current state that disrupts normal operation. ESD can activate p-n-p-n junctions inherent in CMOS ICs to initiate latch-up and potential thermal runaway. Latch-up may cause the device to stop functioning until power is cycled. Repeated latch-up can degrade reliability over time.

Gate Oxide Breakdown

The thin insulating gate oxides of MOSFET transistors can be ruptured by ESD. Gate oxide puncturing leads to short circuits and improper operation. Oxide breakdown is a common ESD failure mechanism in integrated circuits.

Junction Damage

Forward and reverse biased PN junctions are subject to damage at high ESD currents. Thermal effects can melt silicon and metallization, altering device characteristics. ESD protection circuits help clamp junction voltages during ESD events.

Metallization Burnout

Conductors and interconnects can melt, vaporize, or migrate when subjected to brief, high current ESD pulses. Thin metal semiconductor contacts and wire bonds are especially susceptible. This can create open circuits within ICs.

Passivation Cracking

The insulating passivation layers on chips can crack under ESD stress. Cracks expose the silicon and traces to contaminants and moisture that can lead to corrosion over time and eventual circuit failure.

Dielectric Breakdown

Insulating films between metal layers can breakdown during ESD events, leading to shorts between traces. This damages circuit functionality through parasitic conduction paths.

ESD Sensitive Components

Many types of electronic components and devices are susceptible to ESD induced failure and damage:

Integrated Circuits

• Digital logic (CMOS, TTL)
• Linear devices
• Microprocessors
• ASICs and FPGAs
• EPROMs

Discrete Semiconductors

• BJTs
• MOSFETs
• JFETs
• Diodes
• LEDs
• Thyristors

Passive Devices

• Resistors
• Capacitors
• Inductors
• Crystal oscillators

Optoelectronic Devices

• Photodiodes
• Solar cells
• Image sensors
• Lasers
• LED displays

MEMS Devices

• Accelerometers
• Gyros
• Microactuators

Piezoelectric Devices

• BAW filters
• SAW filters
• Piezoelectric sensors

ESD Safety Control Programs

To adequately protect components and products from ESD damage, manufacturers implement structured ESD control programs. These institutionalize best practices for handling, storage, packaging, and assembly operations.

ESD Control Program Elements

• ESD training for personnel
• ESD protected workstation areas
• Use of wrist straps, footwear straps, mats
• ESD protective packaging and bags
• Grounding of equipment, personnel, worksurfaces
• ESD testing and auditing

ANSI/ESD S20.20 Program Standard

The ANSI/ESD S20.20 standard covers key requirements for developing, implementing, and monitoring an ESD control program. It establishes ESD program fundamentals including:

• Grounding/Equipotential bonding systems
• Personnel grounding
• ESD protected areas (EPAs)
• ESD sensitive devices (ESDS) handling
• Compliance verification

Adhering to S20.20 ensures all necessary aspects of ESD control are addressed. Many electronics companies mandate compliance with this standard.

ESD Control Handbook

The ESD Control Handbook from the ESD Association provides in-depth guidance on all aspects of ESD control in electronics manufacturing and handling. Topics covered include:

• ESD basics and testing
• ESD program management
• Facilities and equipment grounding
• EPA requirements
• Packaging, storage, and handling
• Training techniques
• Compliance verification

ESD Protective Equipment

A variety of specialized equipment helps protect electronics from ESD damage during manufacturing, assembly, test, inspection, and storage operations.

Workstation Equipment

• Wrist Straps – Ground personnel through a coiled wire and resistor. Used when handling ESD sensitive items.
• Footwear Straps – Conductive shoes or heel straps for personnel grounding.
• Floor Mats – Dissipate charges from personnel at workstations.
• Table Mats – Dissipative surfaces for work areas.
• Garments – ESD protective smocks, coats, and finger cots.

Workplace Fixtures

• Shelves and Racks – Metal units for grounded storage.
• Carts and Carriers – Mobile grounded platforms for WIP transport.
• Ionizers – Charge neutralization for workspaces and tools.
• Testers – Devices to check grounding integrity.

Component Handling

• Tweezers – ESD safe versions with conductive tips.
• Packaging – Shielding bags, tape, foam, tubes, trays.
• Grounding – Wrist straps, heel straps, gloves.

EPA Monitoring

• Meters – Handheld and benchtop monitors to check electrostatic levels.
• Instrumentation – Data logging systems for continuous monitoring.
• Testing – Simulators and generators to test ESD susceptibility.

ESD Standards and Models

Human Body Model

The Human Body Model (HBM) represents ESD events caused by human contact. It consists of a 100 pF capacitor discharged through a switching component and 1.5k Ω series resistor into the device under test. This models the capacitance of the human body and skin resistance. HBM testing per JEDEC JESD22-A115 evaluates component sensitivity levels.

Machine Model

The Machine Model represents discharges from equipment through metal conductors or charged insulators. A 200 pF capacitor is discharged directly into the device, producing higher peak currents than HBM. Machine Model testing per IEC 61340-3-1 and ANSI/ESDA S5.2 characterizes robustness for automated handling.

Charged Device Model

The Charged Device Model (CDM) evaluates susceptibility when a device itself becomes charged and then discharges to ground. For CDM testing, the device is charged with a high voltage supply before being discharged through a current limiting resistor. JEDEC JESD22-C101 defines standard CDM testing.

IEC 61000-4-2 Air Discharge

IEC 61000-4-2 simulates ESD events generated through air discharge to product enclosures and housings. It utilizes higher voltages and currents than other ESD models. This technique assesses robustness for end-use conditions.

ESD Failure Analysis

When an ESD-related failure occurs, a structured analysis helps identify the root cause and point of weakness. This drives corrective actions and improvements to prevent similar issues going forward.

Gather Information

• When/where did failure occur?
• What was procedure being performed?
• What equipment was involved?
• Was established ESD protocol followed?

Visually Examine

• Use stereo zoom microscopes to inspect.
• Look for physical damage like burns, pits, cracks, melted regions.
• Note location of damage on device.

Electrical Testing

• Perform in-circuit tests to isolate faults.
• Power-up devices and monitor current draw.
• Logically exercise IC pins and trace connections.

Imaging Techniques

• Take high magnification photos of damage areas.
• Use scanning electron microscopy for detailed images.
• Perform x-rays and thermal imaging.

Physical Analysis

• Cross-section damaged components.
• Use FIB-SEM to see failure origins.
• Execute EDX and EELS to identify contamination.

Simulate ESD

• Reproduce suspected ESD event on samples.
• Observe effects under controlled conditions.

ESD Design Strategies

Circuit designers utilize various strategies to make devices more robust against the effects of ESD:

• Protection diodes – Adds diodes to clamp pin voltages during ESD.
• Guard rings – Routes conductive rings around I/O pins on the silicon die to steer current from ESD strikes.
• Snapback devices – SCRs and BJTs that enter snapback mode to shunt ESD current.
• Rail clamps – Shunts power rails to ground during overvoltage from ESD.
• Internal ESD diodes – Adds parasitic diode structures between pins to bypass ESD current.
• Series resistance – Resistors limit input current to sensitive nodes during ESD.
• Current ballasting – Adds distributed resistance in output drivers to limit current per pin.
• Emitter ballasting – Inserts individual emitter resistors in parallel transistors to balance ESD currents.
• Silicon-Controlled Rectifiers (SCRs) – Turns on during ESD to shortcut input to ground or power rail.
• Device spacing – Separates transistors and routing to reduce risk of shorts from ESD damage.

ESD Risk Management in Manufacturing

Several strategies can reduce ESD risks during manufacturing operations:

• Assign an ESD program coordinator
• Follow ANSI/ESD S20.20 requirements
• Provide comprehensive ESD awareness training
• Establish ESD protected areas (EPAs)
• Use wrist straps, footwear straps, and mats at EPAs
• Apply ESD protective packaging for WIP and storage
• Ground all equipment, carts, and fixtures
• Utilize ionization to neutralize charges
• Monitor EPAs for electrostatic discharge events
• Perform regular testing to qualify EPAs
• Conduct internal ESD audits and tracking
• Include ESD requirements in supplier agreements
• Label ESD sensitive components and assemblies
• Document ESD protocols and procedures

ESD Risk Analysis

• Identify activities that can generate electrostatic charge
• Determine which materials and processes are high risk
• Evaluate component and product ESD sensitivity levels
• Assess which personnel roles and operations pose highest ESD risks
• Maintain records of ESD events and associated failures
• Periodically analyze ESD risks based on data and experience

ESD Mitigation Strategies

• Substitute high charging materials with dissipative or grounded versions
• Increase humidification levels in EPAs
• Reduce unnecessary movement of items
• Use static neutralizing ionization blowers
• Improve EPA layouts to limit charge buildup
• Implement more frequent grounding procedures
• Enforce rigorous wrist strap testing and auditing

ESD Protection in Device Packaging

Packaging and handling procedures safeguard electronic components and assemblies during transport and storage. Key aspects include:

Shielding Bags

Metallized polymer bags encapsulate devices to isolate them from electrostatic charges. Bags meet shielding effectiveness requirements of standards such as MIL-STD-129 and ANSI/ESD S541.

Conductive Foam and Film

Components are cushioned and separated from charges using conductive polyethylene foam or plastic film inserts and liners inside packaging. This also prevents device charging.

Anti-Static Shipping Tubes

Rigid conductive tubes provide protection for larger items such as circuit boards during handling and shipping. The tubes connect electrically to the contents.

Groundable Materials

ESD safe packaging utilizes static-dissipative and electrically conductive materials. This prevents charge accumulation on packages.

Warning Labels

Labels indicating ESD sensitive contents remind personnel to follow proper ESD handling procedures when interacting with the packaged components.

Moisture Barrier Bags

Separate moisture barrier bags are used to prevent humidity related corrosion during longer term component storage. Desiccants are also added in some cases.

Stackable Configuration

Packaging allows charged devices to be stacked vertically without contacting each other. This reduces ESD risks during transportation stacking.

ESD prevention in PCB assembly

Printed circuit board (PCB) manufacturing and assembly is an ESD-critical application. Bare PCBs contain highly sensitive components and traces. Proper ESD protocols must be followed throughout PCB population, soldering, test, inspection, and repair.

ESD Protection Guidelines

• Use wrist straps, heel straps, and conductive flooring in assembly areas
• Keep workstation grounds properly connected
• Utilize ionizers to neutralize electrostatic charges
• Handle PCBs by the edges only
• Store PCBs in anti-static bags or boxes when not being processed
• Avoid touching component pins or landings on bare boards
• Place PCBs PCB-side down onto grounded assembly worksurfaces
• Keep assembly materials (e.g. solder) in grounded containers
• Ensure all equipment like soldering irons are grounded
• Exercise caution when removing PCBs from bags
• Avoid rapid motions that may generate charge
• Only assemble PCBs in designated ESD-controlled areas

ESD Standards for PCB Assembly

• IPC-A-610 Acceptability of Electronic Assemblies