Electrostatic discharge (ESD) is the sudden flow of electricity between two electrically charged objects. It is one of the major reliability concerns during printed circuit board assembly using surface mount technology (SMT). ESD can potentially damage sensitive electronic components and assemblies if not controlled properly. This article provides a detailed overview on implementing effective ESD control and protection throughout the SMT assembly process.
ESD Damage Mechanisms
ESD events cause extremely fast transient spikes that can lead to multiple forms of damage in semiconductors and electronics.
Key damage mechanisms include:
- Thermal second breakdown due to localized heating
- Dielectric breakdown causing gate oxide punch-through
- Metal melt and fusing
- Charge trapping and carrier injection issues
Even voltages as low as 30V can be destructive for microelectronics. ESD failure may not be immediately apparent but manifests later as latent defects or reduced reliability.
Figure 1. Common ESD failure modes in ICs
Hence ESD prevention during manufacturing is critical.
ESD Sources During SMT Assembly
Electrostatic charges can build up on personnel, equipment, and materials during PCB assembly. Key ESD sources include:
- Electrostatic charge accumulation on clothes and body
- Movement and contact transferring charges
- Improper grounding when handling ESD sensitive devices (ESDs)
- Automation like pick and place machines
- Conveyors, tracks and trolleys
- Test fixtures and handlers
- Reflow ovens
- Wave soldering machines
- Hand tools
- PCBs, metalizers and ionic contaminants
- Packaging of components like tubes, tapes, trays
- Solder materials
- Cleaning chemicals
- Workstation surfaces
- Low humidity conditions
- Cool and ventilated spaces
Without sufficient precautions, these sources generate electrostatic potentials that can discharge to the ESD sensitive electronics being assembled and cause damage.
ESD Standards and Models
To quantify ESD risks, some key standards and models are used:
- HBM – Human Body Model – Charging of human body and discharge to device
- CDM – Charged Device Model – Discharge from charged device
- MM – Machine Model – Charging of equipment like pick-and-place
Typical ESD voltage levels:
|ESD Model||Voltage Range|
|HBM||250V to 4000V|
|MM||200V to 1000V|
Component sensitivity depends on design, size, and technology. An IC rated for 250V HBM can get damaged by a 100V CDM pulse.
Figure 2. Key ESD models used for defining component sensitivity levels.
ESD Control Plan
A comprehensive ESD control plan should be developed encompassing:
- ESD protected areas
- Equipment grounding
- Personnel grounding
- Handling procedures
- Packaging and storage
- Testing methods
- Compliance verification
- Corrective actions
This ensures every process, material, person, and equipment item is addressed from an ESD control perspective. Audits must be conducted periodically to validate effectiveness.
ESD Protected Areas
Demarcated ESD protected areas (EPAs) must be established with following provisions:
- Warning signs to indicate entry into EPA
- Antistatic flooring – dissipative or conductive material with ground points
- Humidity control – Maintain 40-60% RH range
- Air ionization for charge neutralization
- Antistatic workstations
- Personnel grounding equipment
- Usage of only approved ESD safe materials
- Ban on ordinary plastics or cardboards
Figure 3. Typical setup for an ESD Protected Area
- Components stored in conductive packaging
- Use of conductive bins, boxes and holders
- Avoid contact with ordinary plastics
- EPA approved carts for material movement
By controlling the environment, personnel, equipment, and handling processes, the generation and discharge of damaging ESD events can be minimized.
Since people are a major source of electrostatic charges, proper grounding of personnel is essential.
Operators should wear:
- Antistatic garments like apparel, coats and smocks
- Conductive footwear with grounding straps
- Wrist straps with cords for ground connection
- Grounding should have resistance of 0.1 to 1M ohm range
Figure 4. Typical grounding equipment for SMT assembly personnel.
- Daily checks of wrist bands and footwear using testers
- Display visual indicators like LEDs
- Impedance meters to validate resistance
- Ensure continuity from ground point to person
Proper grounding straps and garments should become a habit for every operator entering EPAs. This prevents ESD risk from human contact.
All equipment, machines, workstations, and fixtures in the EPA must also be grounded.
- Single-point machine grounding to facility earth
- Use grounding grids under machinery
- Ground all metal chassis and covers
- Ensure continuity from points to ground
- No painted metal surfaces
- Verify grounding with impedance testing
- Periodic inspections of connectors and cabling
- Use antistatic plastics and coatings
- Ionized or grounded air blowers
- Static neutralizing equipment
- Avoid insulators like ordinary plastics
Proper grounding and equipotential bonding of everything reduces ESD risks and provides a safe path for charges to dissipate without discharge through components.
ESD Safe Handling Procedures
Strict procedures must be defined and followed when handling ESD sensitive components, PCBs and assemblies in the EPA.
Key handling guidelines:
- Always wear grounded wrist strap before handling ESD devices. Check straps periodically.
- Avoid direct contact with component pins/terminals/pads. Handle by edges/sides.
- Use antistatic gloves, finger cots, vacuum pickup tools.
- Only use EPA approved antistatic packaging and containers. No ordinary plastics or cardboards.
- Transport PCBs in shorting or shunting trays, tubes or bags.
- Connect boards and large components to ground during storage and waiting periods.
Figure 5. Example of ESD safe handling using wrist strap and conductive materials.
By controlling human handling steps and using only approved EPA materials, the risks of directly inducing ESD events can be minimized.
ESD Labels and Signage
Clear visual indicators must be provided on ESD sensitive items.
Typical ESD warning labels:
- ESD warning symbol
- Sensitivity level (e.g. HBM rating)
- Caution note on proper handling
Signs to be displayed:
- EPA area boundaries
- Grounding points
- Warnings on tools and equipment
- ESD safe workstations
- Antistatic material storage bins
- Identify ESD safe materials with green color
This improves awareness on ESD control requirements.
ESD Safe Packaging and Storage
Components must be stored and transported in static controlled packaging within the EPA.
Key features of ESD packaging:
- Made of conductive or static dissipative materials
- Resistance in 108 to 1011 ohm-cm range
- Uses antistatic plastics like polycarbonate, PET/PEN
- Proper shielding from electric fields
Types of ESD packaging:
- Conductive and anti-static bags, tubes and trays
- Use of metalized shielding bags
- Tubes/reels with built-in shorting contacts
- Boxes and storage cabinets with ESD coatings
- ESD warning symbols
- Rated voltage sensitivity level
- First in first out (FIFO) stock rotation
- No contact with ordinary plastics
- Shelves have electrical grounding
Proper ESD packaging and storage ensures components are well protected before they enter the production process.
ESD Control in SMT Processes
Additional ESD precautions must be exercised during PCB assembly steps like solder pasting, component placement, reflow and test.
Solder Paste Printing
- Use antistatic brushes, squeegee blades
- Printer grounded to EPA common point
- PCBs transported on shorting beds
- Avoid ordinary plastics coming in contact
- Use antistatic plastic component tapes
- Place boards in grounded tracks or conveyors
- Include ionization bars over conveyors
- Pick and place machine properly grounded
- Use proper machine grounding
- Avoid insulators inside chambers
- Adjust conveyor height to avoid tipping
- Include ionization at entrance and exit
- Test fixtures must be grounded
- Use fixture shielding and isolation
- Program simulation before live testing
- Personnel wear wrist strap when handling boards
- Use antistatic spray atomization
- Ensure even and immediate spreading
- Proper curing in grounded oven
- Ground all test fixtures and handlers
- Follow strict handler ESD certification
- Conduct daily checks and audits
Packaging and Shipment
- Use EPA approved ESD packing materials
- Shorting of conductors/pads during bagging
- Affix ESD caution labels on packaging
ESD Control Program Management
An organizational-wide ESD control program must be implemented covering:
- ESD Coordinator responsibility
- Written control procedures and compliance checklist
- ESD training for staff and operators
- Scheduled audits and corrective actions
- Tool and equipment certification
- ESD warning signage and visual controls
- Measurement equipment for validation
- Maintaining compliance with standards like ANSI/ESD S20.20
With strong program oversight and vigilance, an effective ESD control framework can be instituted.
Testing and Validation Methods
Various methods are used to validate and monitor ESD control effectiveness:
- Voltage Probes – Check potentials on conductors
- Resistance Measurements – Grounding continuity
- Environmental Monitors – Humidity, temperature, ionization
- Simulated ESD events on test vehicles
- TLP, HMM, Field Induced testing
- Component level ESD stress tests
- ESD sensitivity testing per JS-001
- HBM and CDM qualification levels
- Failure analysis – SEM, deprocessing to confirm
Audits and Tracking
- Routine ESD audit checklists
- Material inspections – packaging, labels
- Process monitoring like ionizer bar output
- Incident reporting and corrective action
Implementing robust ESD control during SMT assembly involves strategic measures encompassing environment, people, equipment, materials, processes and compliance management. By assessing all potential sources of ESD in assembly, defining protected areas, following grounding disciplines, handling devices safely, using only certified ESD safe material, and continuous monitoring and improvements, ESD risks can be minimized. This results in improved quality, yield and reliability of electronic assemblies. Strong leadership commitment supplemented by training and audits is key to an effective ESD control program.
Frequently Asked Questions
What are some early indicators of ESD damage in assemblies?
Subtle signs of ESD damage include unexpected leaks, higher initial failures, latent defects, reliability issues like early breakdowns. Failures may trickle in over time. Robust ESD control practices during manufacturing are essential to avoid such field issues later.
How frequently should grounding equipment be tested?
Daily testing of wrist straps and footwear using impedance testers is recommended. Workstations and equipment grounding should be tested weekly. This ensures any deterioration of connections or straps is caught early.
What level of ESD control is needed for handling 1kV HBM sensitive devices?
1kV HBM sensitivity requires stringent ESD precautions like fully grounded personnel using heel straps, ESD garments, rigorous material control, ionization systems, isolated machine interfaces and handling only using safe pods or gloves. Work surfaces must be static dissipative.
How low can humidity affect ESD charge levels?
At very low humidity around 15%, electrostatic voltages can soar into the 10kV range. 15-20% is considered the critical level requiring extensive precautions. Humidity should be maintained between 40-60% RH for effective ESD control.
What is the recommended frequency of ESD audits?
Daily self-checks and testing is necessary by operators and equipment owners. But formal third-party ESD audits should occur monthly to inspect overall process compliance, material control, validations, labeling, signage, training records, etc. This ensures diligence and aids continuous improvements.