We were recently approached by a customer who wanted to know more about the different classifications of ESD products. So, we thought this would be a good opportunity to share more details with you. Be warned – this is a very theoretical post: so, loads of text but not too many pictures. We promise, we’ll have some more images in our next post!
Part of every ESD Control plan is to identify items in your company that are sensitive to ESD. At the same time, you need to recognize the level of their sensitivity. As explained by the ESD Association, how susceptible to ESD a product is depends on the item’s ability to either:
- dissipate the discharge energy or
- withstand the levels of current.
Whilst some items are easily damaged by discharges arising within automated equipment, others may be more susceptible to damages from personnel when being handled.
There are three main classifications based on three different ESD models. There are detailed standards available from the ESD Association:
- Human Body Model or HBM [100 pF @ 1.5 kilohms]: ANSI/ESDA-JEDEC JS-001-2010
- Charge Device Model or CDM [4 pF/30 pF]: ESD DS5.3.1
- Machine Model or MM [200 pF @ 0 ohms]: ESD STM5.2
The two primary models used for ESD events today are the Human Body Model (HBM) and Charged Device Model (CDM).
The Human Body Model (HBM)
The most common model is the HBM. This model simulates discharge occurring between a human (hand/finger) and a conductor (metal rail). For this model, a 100 picofarads (100 x 10-12 Farads) capacitor is discharged through a 1,500 ohms resistor to simulate a human body. The typical rise time of the current pulse (ESD) through a shorting wire averages 6 nanoseconds (6 x 10-9 s) and is larger for a higher resistant load. The peak current through a 500 ohm resistor averages 463 mA for a 1,000 volt pre-charge voltage.
If a device has failed if it does not meet the parameters outlined in the datasheet.
|Class 1A||250 volts to|
|Class 1B||500 volts to <1,000 volts|
|Class 1C||1,000 volts to <2,000 volts|
|Class 2||2,000 volts to <4,000 volts|
|Class 3A||4,000 volts to <8,000 volts|
|Class 3B||≥ 8,000 volts|
ESDS Component Sensitivity Classification for the Human Body Model (Per ESD-STM5.1)
Charged Device Model (CDM)
This is the most neglected one of the three models but it can severely compromise your ESD control programme. Here, it is the ESDS device itself that becomes charged (sliding out of a tube/bag/sorter/etc.) and when contacting a grounded conductor (table top/hand/metal tool) it will discharge to that conductor and may result in damaging ESD. The length of the discharge may be very short (less than 1 nanosecond) – however, the peak current can reach a high amperage.
The model uses a 4 pF or 30 pF verification module which can simulate from 2 to 30 Amps peak current for non-socked and up to 18 amps for socketed devices.
|Class C2||125 volts to|
|Class C3||250 volts to|
|Class C4||500 volts to <1,000 volts|
|Class C5||1,000 volts to <1,500 volts|
|Class C6||1,500 volts to <2,000 volts|
|Class C7||≥ 2,000 volts|
ESDS Component Sensitivity Classification for the Charged Device Model (Per ESD-STM5.3.1)
Machine Model (MM)
This model simulates a machine discharging through a device to ground. When checking components to the Machine Model (MM), the test replicates MM failures and tells you the MM ESD sensitivity levels for your devices. The criteria is 200 pF at nominal 0 ohms.
|Class M2||100 volts to|
|Class M3||200 volts to|
|Class M4||≥ 400 volts|
ESDS Component Sensitivity Classification for the Machine Model (Per ESD-STM5.2)
Each component in your company should be fully classified using HBM and CDM. That means an item may have a Class 2 (HBM) and Class C1 (CDM).
Bear in mind that these classifications are guides only and do not represent the real world. However, they can be used to:
- “Develop and measure suitable on-chip protection.
- Enable comparisons to be made between devices.
- Provide a system of ESD sensitivity classification to assist in the ESD design and monitoring requirements of the manufacturing and assembly environments.
- Have documented test procedures to ensure reliable and repeatable results.” [Source]
- ESD Association, Inc.: Device Sensitivity and Testing
- ANSI/ESDA-JEDEC JS-001-2010: Electrostatic Discharge Sensitivity Testing — Human Body Model
- ESD STM5.2-2009: Electrostatic Discharge Sensitivity Testing — Machine Model
- ESD STM5.3.1-2009: Electrostatic Discharge Sensitivity Testing — Charged Device Model
- ANSI/ESDA/JEDEC JS-002-2014: Joint Standard for Electrostatic Device Sensitivity Testing – Charged Device Model (CDM) – Device Level
- IEC 60749-26: Semiconductor devices – Mechanical and climatic test methods – Part 26: Electrostatic discharge (ESD) sensitivity testing – Human body model (HBM)
- IEC 60749-27: Semiconductor devices – Mechanical and climatic test methods – Part 27: Electrostatic discharge (ESD) sensitivity testing – Machine model (MM)