HUMAN BODY MODEL
The updated standard ANSI/ESDA/JEDEC JS-001-2011, For Electrostatic Discharge Sensitivity Testing Human Body Model (HBM) – Component Level Table 3 has divided the Class 0 classification into two withstand voltage levels with class 0A being less than 125 volt sensitivity, and class 0B being 125 to less than 250 volts.
If handling class 0A items, or less than 125 volts, program improvements are called for. Basically, to control the environment to decrease the probability of ESD damage in class 0A situations, involves increasing ESD protective redundancies by adding EPA ESD control items and ensuring that they are working properly by increasing the frequency of compliance verifications of those ESD control items.
ANSI/ESDA/JEDEC JS-001-2011 Table 3. HBM ESD Component
Classification Levels Classification Voltage Range (V)
0A < 125
0B 125 to < 250
1A 250 to < 500
1B 500 to < 1000
1C 1000 to < 2000
2 2000 to < 4000
3A 4000 to < 8000
3B ≥ 8000
Personnel grounding has historically been the foundation of most ESD control programs. The first standard written by the ESD Association was on wrist straps back in the early 1980’s. Therefore many companies mistakenly believe that operator grounding is no longer an issue, but there are areas of operator grounding where improvement should be considered.
Part of the challenge may be for the first time needing to handle electronics having a HBM Class 0 withstand voltage. Per the ANSI/ESD S20.20 Foreword:
- This standard covers “electrical or electronic parts, assemblies and equipment susceptible to damage by electrostatic discharges greater than or equal to 100 volts Human Body Model (HBM)
- When handling devices susceptible to less than 100 volts HBM, more stringent ESD Control Program Technical Requirements may be required, including adjustment of program Technical Element Recommended Ranges.
While ANSI/ESD S20.20 has set the maximum upper limit of 35 megohms resistance for personnel grounding via a wrist strap system, consider lowering that upper limit within your ESD control plan to 10 megohms. The ESD Handbook ESD TR20.20 Figure 14 “Relationship between Body Voltage and Resistance to Ground graph shows this would typically reduce body voltage from about 100 volts to less than 40 volts.
ESD Handbook ESD TR20.20 Figure 14 Relationship between Body Voltage and Resistance to Ground
In addition, the use of continuous monitors should be evaluated. Wrist Strap continuous monitors will provide the benefit of detecting intermittent fault conditions, such as a coil cord wearing out or an operator having the wristband too loose. These will often not be detected by daily or even twice daily touch-testing. There are new models with technology that besides monitoring the ground of operators and worksurfaces, will also alarm when body voltage exceeds 2.5 volts. Thus, alerting the operator to actions, movements or materials that are causing the operator to become charged.
Wrist Strap and Continuous Monitor
From published article “Now is the Time for ESD Control Programs to be Improved” by Fred Tenzer and Gene Felder. See full article at InCompliance Magazine– September 2012