1. ESD Class 0 Protection Stress Levels Their Origin and Application
In this presentation we will explore:
What (energy or voltage) will stress or damage a component?
What do we mean by “Class 0” when referring to the ESD sensitivity of a component?
Are we talking about the same thing?
Are “Class 0” environments sufficient to protect sensitive parts?
José D. Sancho
NASA Workmanship Standards
4/19/2017

2. ESD Event Classification
From EMI to EOS - Speed Classification
EMI caused ESD has short and repetitive pulses with low energy.
EOS is and ESD event with unlimited Current/Time constrains
HBM, MM & CDM model typical events in the manufacturing areas.
NASA Workmanship Standards
4/19/2017

3. Introduction 1What is an ESD Sensitivity Level? 2How is it obtained?
3Why is it important to the user?
4Why different test models?
This presentation is an overview of the different models commonly used to characterize parts sensitivity to ESD events.
(1) An ESD sensitivity level is an nominal level defined by a repeatable set of test models which simulates likely sources of ESD related failures on parts and equipment.
(2) The test procedures used to characterize, determine and classify the ESD sensitivity of components are based on three basic models of ESD events: the human-body model (HBM), machine model (MM) and the charge device model (CDM). The sensitivity levels for different components are obtained by subjecting a statistically significant sample of those components to the simulated conditions specified by each model.
(3) It provides a benchmark to compare the ESD sensitivity of different components.
The different models simulate the different environments which may be encountered by the component during their manufacture and application. The models differ in that the HBM simulate the electrostatic discharge from a person to a device, the MM simulate the electrostatic discharge from a tool or a automatic equipment to a device, whereas the CDM simulates the electrostatic discharge to & from a device due to induced or triboelectric charging.
(4) The ESD models allow us to classify, identify and mitigate ESD related risks.
NASA Workmanship Standards
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4. (differences & interrelation)
Overview
ESD Models Provide a way to characterize the sensitivity of components to ESD
The different ESD models simulate the different environments experienced by electronic components during the manufacturing process.
Parts and assemblies may be exposed to more than one type of ESD event over the manufacturing and test life cycle.
HBM MM
ESD Models:
(differences & interrelation)
CDM
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5. Purpose of ESD Models Models establish Benchmarks for ESD Sensitivity.
Different Models are used to simulate different work environments.
Models provide help to prevent and analyze ESD Failures
None
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4/19/2017

6. ESD Event Test Models
Human Body (HBM): discharging event through the body and the part to ground.
Machine (MM): discharge voltage through automated handling equipment or hand-tools and the part to ground.
Charged Device (CDM): discharge into or out of a part due to charge accumulation within the part itself.
NASA Workmanship Standards
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7. ESD Damage to Die Structure
Damage types can vary depending on event models.
Long, higher Voltage HBM event can look like electrical overstress at die periphery.
Fast, high Current CDM event causes defects in core area which can be latent failures.
Must use advanced FA techniques to locate sites.
A Comparison of Electrostatic Discharge Models and Failure Signatures for CMOS Integrated Circuit Devices, M. Kelly, G. Servais, T. Diep, S. Twerefour, D. Lin, G. Shah, EOS/ESD Symposium 95
NASA Workmanship Standards
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8. ESD Sensitivity Levels
Human Body Model
Machine Model
Charged Device Model
Class 0
<250 V
Class M1
< 100 V
Class C1
< 125 V
Class 1A
250 V to< 500 V
Class M2
100 V to < 200 V
Class C2
125 V to < 250 V
Class 1B
500 V to < 1 kV
Class M3
200 V to < 400 V
Class C3
250 V to < 500 V
Class 1C
1 kV to < 2 kV
Class M4
≥ 400 V
Class C4
500 V to <1 kV
Class 2
2 kV to < 4 kV
---
Class C5
1 kV to < 1.5 kV
Class 3A
4 kV to < 8 kV
Class C6
1.5 kV to < 2 kV
Class 3B
≥ 8 kV
Class C7
≥ 2 kV
There is a “Rule of Thumb” conversion between HBM and MM sensitivities Which is used when the MM sensitivity is not known:
Divide the HBM sensitivity by 30 to obtain the approximate MM sensitivity level.
There is no conversion for the CDM model since the CDM event is “powered” by the device itself (discharging out of the part) and is very short.
For Classes 1B through 3B the use of wrist straps normally provides enough protection to the assembly.
NASA Workmanship Standards
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9. NASA Workmanship Standards 4/19/2017
This graph gives a representation of the currents and time involved in typical electrostatic discharges encountered during manufacture.
HBM is usually a higher voltage discharge due to required voltage to breakdown the skin insulation.
MM voltage is limited by the inductance of the device and the tools or equipment.
In CDM the inductances and capacitances are very small creating very sharp high current pulses, even for moderate voltage discharges.
White Paper 2: A Case for Lowering Component Level CDM ESD Specifications and Requirements, Industry Council on ESD Target Levels, March 2009
NASA Workmanship Standards
4/19/2017

10. Models Comparisons Courtesy of ESDA ESD Models ZD RC CD LD ΕS in μJ
5τ in ηs
Pave (W)
IPS
Vc = 1000 V
Typ.#
Req #
Typ #
½CV2
5ReC
Ε/τeff
V/Re
HBM
1500
100 50
750
~67
0.67 MM 55 2
200
5.0E-07
160
625
17.5
CDM 42 25 15
1.0E-08
7.5 5
>750
HBM: Deliver the lowest power over the longest time.
Produces the largest heat damage to the part.
Occurs usually where the electric field is strongest.
Only EOS produces greater damage for the voltage used.
MM: Delivers about 10 times the power of HBM over on fifth of the time.
It is repetitive causing multiple failures but less heat damage.
Voltage Ratings tend to be between 10 to 30% of HBM.
CDM: It is very fast and delivers the highest power to the part.
Damage occurs typically at the core of the IC and is hard to detect (very small).
IC manufacturers calculate that about 90% of the failures from the field are due to CDM ESD events.
Reference: Device Technology and FA Overview ESDA Tutorial 2005 by Leo G. Henry:
NASA Workmanship Standards
4/19/2017
Courtesy of ESDA

11. ESD Models vs. Sources of Threats
Examples of Sources of Threats
HBM MM
CDM
Operator √
Work bench
Pick and Place Machine
Automatic Test Equipment
Device package
Mate/De-mate of harnesses
RF Signals
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12. “Class 0” Parts Protection
“Class 0” has become the generic term to define parts which are very sensitive to ESD.
It now encompass parts sensitive to HBM <250v as well as parts damaged by EMI
Sensitivity for these parts needs to be also defined using CDM classifications.
{EPAs as currently implemented at GSFC can protect parts sensitive to ~100 V HBM}
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13. Proper implementation requires training and follow-up
Model Implementation
NASA-HDBK (in Approval Cycle) Guide for Creating an ANSI/ESD S20.20 Implementation Plan
Focus is on HBM: emphasis on operator grounding, dissipative surfaces, reduction of triboelectric charging
For HBM & MM the methods for protective practices and creating protective spaces are highly reproducible and “low tech”
Proper implementation requires training and follow-up
HBM safety methods have brought HBM & MM failures down (now are ~10% of failures encountered industry-wide)
NASA Workmanship Standards
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14. Model Implementation
Recent failures of high speed devices (LVDS, FPGAs) drive users to Class 0 HBM…
…But IC manufacturers calculate that about 90% of the failures from the field are due to CDM ESD events.
CDM-related field returns are associated with low, medium, and high sensitivity devices.
Safety methods for CDM are highly customized because the model is less mature (many unknown variables and variable relationships, rapidly changing characteristics
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15. Class 0 & CDM Class 0 refers to the HBM model
Currently most ESD damage is caused by much shorter pulses best defined in the CDM model.
ESD pulses can be clamped by internal shunts and bypasses at the expense of design complexity and speed.
There is a limit beyond which the device cannot be internally protected.
When we talk about Class 0 parts we are referring to the most ESD sensitive parts in general.
Presently most of the complex ICs fall in this classification.
Class 0 sensitivity is defined as damage to a part caused by an HBM ESD pulse below 250V.
The vast majority of ICs failures are failures best explained by the CDM ESD model.
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16. Limits of Design-in Protection
CDM protection by the design is driven by the peak current from the IC package discharge at the CDM voltage targeted.
The larger the package the higher the peak current of the CDM pulse created.
The smaller the geometry of the circuit the lower the breakdown voltage of the circuit
Present Theoretical Limit ≈ 125v CDM
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17. Protection of Devices Sensitive to Class 0 ESD
“Shalls” related to HBM Class 0 protection:
Dissipative chairs and stools
Conductive or dissipative floors or floor mats
Relative humidity
Ionizers
Smocks
Procedures for Mating and de-mating of harnesses
Soldering iron testing
Signage
A note of caution:
Humidity and Ionization do not protect against CDM type discharges
NASA Workmanship Standards
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18. External ESD Control Measures for Extremely Sensitive Devices
Measure Area Static Charges
Assess Possibilities for Area charge Reduction
Avoid Hard Discharges
Perform tasks close in a deliberate manner, with slow motions and close to dissipative surface.
Place tools on the dissipative mat before use.
Discharge all cables by placing them in the ionization stream or grounding all shields prior to connecting them to a sensitive assembly. (LVDS Xtmr lines)
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19. Charged Board Events
CBE are caused when a board is pulled from the bag and place on a conductive surface
This ESD hazard was often overlooked
During FA the components failure is usually classified as EOS damage.
Recent data reported by several Companies indicates that CBEs are commonly missed in FA
NASA Workmanship Standards
4/19/2017

20. Where to Get More Information
WEB searches under “ESD Models”
ESDA publications
Consulting services provide Advice on tough ESD problems and Solutions.
For Example: Leo G. Henry ESD TLP Consultants / Testing
NASA Workmanship Standards
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21. References ANSI/ESD STM5.1-2001 ESD Sensitivity Testing (HBM)
ANSI/ESD STM ESD Sensitivity Testing (MM)
ANSI/ESD STM ESD Sensitivity Testing (CDM)
ANSI/ESD SP ESD Sensitivity Testing (SDM)
ANSI/ESD SP ESD Sensitivity Testing (TPL)
Scott M. Hull, “ESD Failures in Thin-Film Resistors” NASA/Goddard Space Flight Center
White Paper: Industry Council on ESD Target Levels on CDM
none
NASA Workmanship Standards
4/19/2017

22. Thank you Any Questions?
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23. Typical HBM Generated Failures
100X
Typical damage is heat related and very similar to voltage overstress.
This type of failure is usually found at the weakest point in the design and larger than failures caused by the stresses of other Models. In some cases the failure is obvious to the naked eye.
The picture to the right show the damaged caused by an intentional ESD event using an BHM test set on a nichrome resistor. This is a Vicinal illumination view of Transverse cracking typical of relatively high voltage ESD damage.
2000X
Scott M. Hull NASA/GSFC
Courtesy of JPL
NASA Workmanship Standards
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24. Typical MM ESD Stress Failure
This ESD failure is typical of a event caused by Machine Model type ESD event. The damage is localized to one main event with a few small stresses indicated by the small arrow.
Scott M. Hull NASA/GSFC
NASA Workmanship Standards
4/19/2017

25. Typical CDM generated failures
4600x
Notice magnification of the failures two to four times greater than HBM
8600x
Courtesy of JPL
Courtesy of JPL
NASA Workmanship Standards
4/19/2017

26. Typical CDM Generated Failure
Courtesy of Frederick Felt GSFC Part Analysis Lab.
After repeated parallel polishing the small discoloration in the picture was the only visual indication of the voltage breakdown found using GMR scanning.
ESD event (~1 KV) shown at arrow after parallel Polishing
NASA Workmanship Standards
4/19/2017