This is the type of conversation we often have with companies about the handling of Class 0 (Zero) ESD devices. The purpose of this blog post is to shed some light on the topic and answer some common questions with regards to Class 0 Devices.
What does the word “Class” signify?
The word Class is used to identify a range of voltages that a component will withstand without being damaged when exposed to an ESD event. Different ranges of voltage are identified for Human Body Model (HBM) and Charged Device Model (CDM) events. Class 0 signifies a range of voltage thresholds of devices – specifically less than 250 volts Human Body Model. For this post we are is going focus on the Human Body Model classifications. Below are the classes for various HBM voltage thresholds from the ESD Association.
- Class 0Z – Voltage Threshold (HBM) – <50v
- Class 0A – Voltage Threshold (HBM) – 50 to <125v
- Class 0B – Voltage Threshold (HBM) – 125 to <250v
- Class 1A – Voltage Threshold (HBM) – 250 to <500v
- Class 1B – Voltage Threshold (HBM) – 500 to <1000v
- Class 1C – Voltage Threshold (HBM) – 1000 to <2000v
- Class 2 – Voltage Threshold (HBM) – 2000 to <4000v
- Class 3A – Voltage Threshold (HBM) – 4000 to <8000v
- Class 3B – Voltage Threshold (HBM) – >8000v
How is the class determined?
The class is determined based on the HBM voltage threshold of the most sensitive component. The ESD Program would be set up to protect that component(s).
Is there a guideline/baseline that can be used to help set the ESD Program requirements for controlling HBM events?
ANSI/ESD S20.20-2014 is set up to protect devices with HBM Voltage Thresholds of 100v or more. If the HMB is 100v or higher then S20.20-2014 can be used as is. If it’s less than 100 then more precautions need to be taken.
Is there a standard for that?
No, however the ESD Association does provide guidance in ESD TR20.20-2016 – Handbook for the Development of an Electrostatic Discharge Control Program for the Protection of Electronic Parts, Assemblies, and Equipment. It notes that if the body resistance to ground of 35 megohm is maintained, less than 100 volts will generate. If the body resistance to ground (Rtg) is in the range of 10 meg ohms it will generate slightly less than 40 volts. The primary way to ground the body is with a wrist strap or a Footwear/Flooring System. The theoretical solution is to match the upper limit alarm of the a wrist strap/footwear tester or continuous monitor to the maximum resistance allowed for the desired HBM voltage threshold. For example, if a facility was handling a <50v device, the upper limit on the tester or monitor would be ~10 meg.
Besides maintaining a lower body Rtg , what other precautions should be considered for handling devices that are less than 100v?
- A <109 mat surface is in spec per S20.20-2014 but 106 or 107 will more efficiently dissipate a charge.
- Carts and shelves should be treated like workstations with similar Rtg or expose ESD susceptible devices must be stored in packaging that provides shielding.
- Ionizers would be necessary to control charges on process essential insulators and isolated conductors.
- Consider making the lowest charged item that can come within 1” of a device 50 volts instead of the 125 volts stated by S20.20-2014.
- Pay attention to things like fingercots and gloves resistance and how they charge. The more conductive the better.
- Garments should meet the S20.20-2014 requirement for: Groundable Static Control Garment (Rtt < 1.0 x 109) or Groundable Static Control Garment System(Rtt < 3.5 x 107)