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5.1 ESD Control Program (ANSI/ESD S20.20)

Electrostatic Discharge (ESD) is the invisible assassin of modern electronics. Unlike a short circuit or a broken trace, ESD damage often manifests as a latent defect — the "walking wounded." A component may pass final test today but fail in the field after 200 thermal cycles due to a compromised gate oxide. Objective is not merely to enforce "cleanroom etiquette" but to maintain the electrostatic potential difference between any two items in the process below the sensitivity threshold of the most fragile component (typically ≤ 100V HBM). If you cannot prove the process was compliant at the moment of assembly, you cannot guarantee the reliability of the product.

Program Management & Scope

The ESD Control Program must be technically robust and auditable. Adherence to ANSI/ESD S20.20 is the baseline, not the gold standard.

Define the Administrative Requirement:

The Quality Management System (QMS) must designate a dedicated ESD Coordinator. This individual is responsible for the verification plan, not just the purchasing of supplies.

  • Compliance Verification Plan (TR53): You must define how and how often you measure the controls. A wrist strap on a wrist is useless if the coil cord is open.
  • Training Registry: No operator, manager, or visitor enters the ESD Protected Area (EPA) without documented training or an escorted protocol.

Technical Elements of the EPA

The ESD Protected Area (EPA) is defined by physics, not yellow tape. It is a volume where all surfaces, operators, and equipment are equipotential.

1. Grounding Systems

The Common Point Ground (CPG) is the reference zero for the entire EPA.

  • AC Equipment Ground: Verify the impedance between the equipment chassis and the 3rd wire electrical ground is < 1.0 Ω.
  • Worksurfaces: Matting must be dissipative, not conductive, to limit discharge current.
    • Resistance to Ground (RTG): 1 x 10^6 Ω ≤ R ≤ 1 x 10^9 Ω.
    • Pro-Tip: Do not daisy-chain mats. Each mat needs its own direct path to the Common Point Ground to prevent additive resistance.

2. Personnel Grounding Strategy

Human operators are the primary generators of static charge. You must couple them to ground. Use the following decision logic to determine the required method:

  • IF Operator is Seated:
    • THEN Wrist Strap is Mandatory. Flooring/Footwear is insufficient due to lifting feet or insulating chair wheels.
    • Control: Continuous Monitor (preferred) or Daily Log required.
    • Limit: < 3.5 x 10^7 Ω.
  • IF Operator is Standing/Walking:
    • THEN ESD Footwear + ESD Flooring System is required.
    • Control: Test upon entry (Left Foot / Right Foot independent test).
    • Limit: Product of System (Person + Shoes + Floor) < 1.0 x 10^9 Ω AND Body Voltage Generation < 100V.

3. Packaging & Material Handling

Protecting the PCBA inside the EPA differs from protecting it during transport.

  • Inside EPA: Materials should be Dissipative (Pink Poly/Black Carbon). This slows the charge transfer.
  • Outside EPA (Transport): Materials must be Shielded (Metal-in/Metal-out bags).
    • Rule: Never transport ESD-sensitive devices (ESDS) outside the EPA in merely "Pink Poly" bags. They offer no shielding against external fields.

4. Ionization

Insulators (plastics, tape, housings) cannot be grounded. If essential insulators are present within 30 cm of ESDS:

  • Action: Deploy Ionizers to neutralize the charge.
  • Verification: Measure Offset Voltage (Balance) and Decay Time.
    • Limit: Offset voltage < ±35V.

Compliance Verification (Auditing)

Do not trust – verify. Verification must follow ESD TR53 test methods.

The Auditor's Cadence:

  • Daily: Operator self-check (Wrist straps/Footwear). Visual check of ground wires.
  • Monthly: Independent audit of worksurfaces, floor resistance, and ionizer balance.
  • Quarterly: Full system audit (RTG of shelving, carts, chairs, and garments).

Pro-Tip: Humidity affects conductivity. If Relative Humidity (RH) drops below 30%, dissipative materials may become insulative. Increase audit frequency during dry winter months or humidify the facility.

Final Checklist

Control Element

Parameter

Critical Limit / Threshold

Frequency (Min)

Personnel (Wrist)

Resistance

< 3.5 x 10^7 Ω

Daily / Continuous

Personnel (Shoe/Floor)

System Resistance

< 1.0 x 10^9 Ω

Daily (Entry)

Worksurface

Resistance to Ground

< 1.0 x 10^9 Ω

Monthly

Ionizers

Offset Voltage (Balance)

< ±35V

Monthly

Mobile Carts

Resistance to Ground

< 1.0 x 10^9 Ω

Quarterly

Fields

Static Field

< 2000V / inch (at 30cm)

Monthly

Packaging

Shielding

Visual check (No holes/tears)

Per Use

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