Scope and authoring bodies
ANSI/ESD S20.20 is published by the ESD Association and is the dominant standard in North American electronics manufacturing.
IEC 61340-5-1 is published by the International Electrotechnical Commission and is the global / European reference.
Both are ESD program standards — they require documented procedures, training, and verification, not just product specs.
Where the requirements diverge
Three meaningful differences. (1) Human Body Model classification: S20.20 treats parts with HBM withstand voltage of < 100 V as "high sensitivity"; IEC 61340-5-1 uses < 100 V as the trigger to require additional process controls. The thresholds align but the language differs.
(2) Wrist-strap test frequency: S20.20 requires daily (or continuous monitoring); IEC 61340-5-1 has the same baseline but allows the program to set frequency by risk.
(3) Audit cycle: S20.20 has explicit annual audit language; IEC 61340-5-1 leaves audit cadence to the program but requires it be documented.
Dual certification cost
If your ESD program is already S20.20-certified, adding IEC 61340-5-1 is largely a documentation exercise. $5,000-15,000 for a certification body audit, depending on facility size and the auditor's travel.
Most internal procedures, training records, and equipment qualifications carry over 1:1; only the program manual cover page and a few clause cross-references need rewriting.
Which to spec on the PO
If your contract manufacturer is in Mexico, EU, or Asia: spec IEC 61340-5-1.
If they are US-based: spec ANSI/ESD S20.20.
If you ship to both: dual-spec ("compliant with ANSI/ESD S20.20 and IEC 61340-5-1").
Avoid generic "ESD-safe" language on POs — it is unenforceable and gives the vendor too much room to claim compliance.