The Different ESD Events and their Models – HBM, CDM and MM

Introduction

As previously explained, an Electrostatic Discharge (ESD) is a rapid, spontaneous transfer of an electrostatic charge, and occurs when two objects at different electrostatic potentials approach one another. The ESD Association characterises this Electrostatic Discharge that causes ESD damage into three event types. ESD Models can then help to determine the ESD Sensitivity or Susceptibility of a device.

Per ESD TR20.20, section 3.2 “ESD models have been developed to help distinguish between major types of stress in production and to provide a basis for the determination of relative ESD sensitivities of devices.”

Each model has specific discharge properties such as the rise and fall times of the discharge current waveform and therefore they can be distinguished.

ESD Events and their Models

ESD Event	Description	Model
Discharge to the Device	“A charged object (including a person) coming into contact with an ESDS item.” ANSI/ESD S20.20 – FOREWORD	The model used to simulate when a person comes into contact with an ESDS device is the Human Body Model (HBM). However, the person isn’t always the charged conductive object, when a metal tool or fixture comes into contact with the device the model is known as Machine Model (MM).
Discharge from the Device	“A charged ESDS item making contact with ground or another conductive object at a different potential.” ANSI/ESD S20.20 – FOREWORD	The model used to simulate this event is the Charged Device Model (CDM).
Field-Induced Discharges	“An ESDS item making contact with ground or another conductive object while exposed to an electrostatic field.” ANSI/ESD S20.20 – FOREWORD	This can result in two different Charged Device Model (CDM) events. The first when the ESDS devices makes contact with ground or another conductive object, then the second if the item is removed from the electrostatic field and grounded again.

The ESD Models – HBM Vs CDM Vs MM

Human Body Model (HBM) 

HBM simulates a person becoming charged and discharging from a bare finger to ground through the circuit under test. Humans are considered a primary source of ESD and HBM can be used to describe an ESD event due to the combination of the capacitance of a human body and resistance of skin touching a sensitive component. Typically, you need to pay better attention to personnel grounding to eliminate HBM.

“HBM has been in use for over 100 years. It was first defined to allow measurement and evaluation of explosion hazards for underground mining operations. There are a few different test standards describing the HBM for military and commercial applications, but the differences are in the application of the test, calibration of the system, and other ancillary items. The waveform, as defined by the human body resistance and capacitance, is virtually identical among all the test standards. The most widely used standard is ANSI/ESDA/JEDEC JS-001. The HBM test standard uses a stressing circuit which charges a 100 pF capacitor to a known voltage and discharges through a 1500-ohm resistor as shown in Figure 3. The simulators are verified by measuring various features of the current waveform, some of which are shown in Figure 4. Full details for tester qualification and waveform verification are described in ANSI/ESDA/JEDEC JS-001.”

ESD Handbook ESD TR20.20 section 3.4.1 Human Body Model (HBM)

An operator handling an ESD sensitive device

Charged Device Model (CDM) 

CDM simulates an integrated circuit becoming charged and discharging to a grounded metal surface. CDM can be used to describe an ESD event due to an integrated circuit that is suspended on a vacuum pick and then placed on a metal surface during assembly.
Manual operation and handling is much less likely these days as operations have become more automated. CDM is the most pragmatic discharge model in automated production today. Anytime a sensitive device is lifted from a tray and transported it is most likely generating a charge.

“In principle, there are two variations of CDM. The first considers the situation of a device that is charged (through tribocharging) on its package, lead frame, or other conductive paths followed by a rapid discharge to ground through one pin or connector. The second considers the situation of a device which is placed in an electric field due to the presence of a charged object near the device. The device’s electrostatic potential is increased by this field. This process is sometimes referred to as static induction. The device will discharge if it is grounded while still in the electric field. In both cases, the device will discharge, the failure mode will be the same, and the failure type and location will be the same. The most widely used CDM standards use the static induction approach. In CDM simulators, the device is grounded by a pogo pin contacting one pin or lead of the device. The current through the pogo pin can be measured and recorded which is particularly important as the discharge current determines the ESD threshold, a schematic of this is shown in Figure 5.
Experimental results show that the CDM discharge current is very fast, with rise-times measured often below 100 ps with a “pulse width” (full width half-maximum [FWHM]) of less than 500 ps to1 ns, an example waveform with the key parameters is shown in Figure 6. By comparison, the HBM discharge has a typical rise-time of 2 to 10 ns and durations of hundreds of ns. Until 2014, the most commonly used CDM standards were JEDEC JESD22-C101 or ANSI/ESD STM5.3.1. These have now been superseded by ANSI/ESDA/JEDEC JS-002.”

ESD Handbook ESD TR20.20 section 3.4.2 Charged Device Model (CDM)

Machine Model (MM) 

MM simulates a machine or metal tool discharging through a device to ground. Its failure mode is similar to that of HBM testing. It is no longer required for device qualification because it does not offer any additional information to that of HBM and CDM.

“It was originally thought that the remaining category of threats (discharges from charged metal) would be covered by a test method called the “machine model” (MM). The conceptual model schematic might have been reasonably expected to do this. However, the test method that emerged was a modification of existing HBM testers and did not do this. It has been shown that the standard MM has effects very similar to the standard HBM. The thresholds of MM and HBM correlate within
given technologies and the two produce the same failure mechanism and design fixes. Finally, MM does not include the expected fast leading edge transient expected for a metal-to-metal discharge. If a design meets minimum requirements for HBM and COM it will be reasonably robust to discharges from charged metal objects.

ESD Handbook ESD TR20.20 section 3.2.2 Models to Describe the Threats

So, why does it matter?

Different types of discharge can affect devices in different ways. HBM is a somewhat slow discharge and ranges from 10 to 30 nanoseconds. CDM is a very fast discharge which in turn means the energy has no time to dissipate. The CDM-type damage threshold is often 10 to 20 times lower than the one for an HBM-type discharge. If an HBM-type discharge causes damage at 2000 V, it is not uncommon to have the same component damaged by a 100 to 150 V CDM event.

Per ESD Handbook ESD TR20.20 section 3.2.1: “ESD threats in electronics manufacturing can be classified into three major categories:

• Charged personnel – When one walks across a floor a static charge accumulates on the body. Simple contact of a finger to a device lead of a sensitive device or assembly which is on a different potential, e.g., grounded, allows the rapid transfer of charge to the device.
• Charged (floating) conductor – If conductive elements of production equipment are not reliably connected to ground, these elements may be charged due to triboelectric charging or induction. Then these conductive elements may transfer charge to a device or assembly which is at a different potential.
• Charged device/boards – During handling, devices or boards can acquire a static charge through triboelectric charging or can acquire an elevated electrostatic potential in the field of nearby charged objects. In these conditions, contact with ground or another conducting object at a different electrostatic potential will produce a very fast ESD transient.

This categorization is useful in that each category implies a set of ESD controls to be applied in the workplace. ESD threats from personnel are minimized by grounding personnel through the use of wrist straps and/or footwear/flooring systems. Discharges from conductive objects are avoided by assuring that all conductive parts that might contact devices are adequately and reliably grounded. The occurrence of ESD involving charged devices or boards is minimized by a) preventing charge generation (low-charging materials, ionization) or b) by providing low-current “soft landings” using dissipative materials. Since these preventive measures are seldom perfectly deployed, the overall threat of ESD failure remains and the risk ultimately depends on how well the controls are maintained and the relative sensitivities of the devices being handled.”

Taking Action

Desco Europe recommends reviewing your manufacturing process and determining what model is the most relevant for your facility. Are your components handled directly by hand or by a hand tool such as tweezers or a vacuum pick? Finding the root cause of ESD events is crucial to solving the problem. Desco Europe technology can identify events in areas like SMT line, soldering, printer, and repair stations. We have the instrumentation to identify component sensitivity and measure ESD events as well as ensure compliance within your facility: