ST AN3960 Application note

AN3960

Application note

ESD considerations for touch sensing applications

Introduction

Electrostatic discharge (ESD) is not a new phenomenon. It is often used to describe high voltage that may produce permanent damage. ESD can be destructive and may leave a sytem in an unknown state from which recovery is impossible. Fortunately, it can be prevented by several methods. Some of these methods are cheap whilst some modify the behavior of the equipment. The ideal situation is to balance both of these factors to obtain a robust application which is not too expensive and which is unlikely to behave erratically.

This document describes ESD, its causes and risks. Several models and standards relating to ESD simulation are outlined. Typical ESD protection techniques are explained. Test results are presented for the STM8T142-EVAL evaluation board which was tested against ESD events using some of the protection methods detailed in this application note.

October 2011

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Contents	AN3960

Contents

1	What is ESD ? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	1.1	Causes of ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	5
2	Risks of ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		5
	2.1	Simulation and testing of electronic devices using models . . . . . . . . . . . .	6

2.1.1 Human body model (HBM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1.2 Machine model (MM) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

2.2 Standards overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 JS-001-2010 international standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.2 SP723 EIAJ IC121 standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.3 IEC61000-4-2 international standard . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.4 MIL-STD-883H . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.5 ESD standard summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 2.2.6 Test results of ESD standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3	Protecting against ESD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .		10
	3.1	Dielectric overlays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	10
	3.2	Spark gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	11
	3.3	Ground rings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	12
	3.4	Adding resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	13
	3.5	Adding diodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	14
	3.6	ESD protection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
	3.7	Firmware . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	15
4	STM8T142-EVAL evaluation board: ESD tests . . . . . . . . . . . . . . . . . . .		16

4.1 Test setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 4.2 Test results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

5	Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	19
6	Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .	20

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AN3960	List of tables

List of tables

Table 1. Test conditions for ESD standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Table 2. Dielectric overlay materials and their dielectric strength. . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Table 3. ESD protection devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Table 4. ESD discharges for tested STM8T142-EVAL evaluation board . . . . . . . . . . . . . . . . . . . . . 18 Table 5. Document revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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List of figures	AN3960

List of figures

Figure 1. Electrostatic discharge test (ESD generator and DUT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Figure 2. IEC61000-4-2 ESD current waveform (RD = 330 W/CD = 150 pF). . . . . . . . . . . . . . . . . . . . 8 Figure 3. PCB with spark gap . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Figure 4. Ground ring. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Figure 5. Test set up . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Figure 6. STM8T142-EVAL evaluation board modifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

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AN3960	What is ESD ?

1 What is ESD ?

ESD is the sudden and momentary electric current that flows between two objects at different electrical potentials.

ESD immunity is a category of electromagnetic compatibility (EMC) - the branch of electrical sciences which studies the unintentional generation, propagation and reception of electromagnetic energy with reference to its unwanted effects.

EMC describes the ability of a piece of equipment or a system to function satisfactorily in its electromagnetic environment without introducing intolerable electromagnetic disturbances to anything in that environment.

1.1Causes of ESD

One of the causes of ESD events is static electricity. Static electricity is often generated through the separation of electric charges when two materials are brought into contact and then separated, for example, rubbing a plastic comb against dry hair, removing some types of plastic packaging. In these cases, the friction between two materials creates a difference of electrical potential that can lead to an ESD stress.

Another cause of ESD damage is through electrostatic induction. This occurs when an electrically charged object is placed near a conductive object isolated from ground. The presence of the charged object creates an electrostatic field that causes electrical charges on the surface of the other object to redistribute. Even though the net electrostatic charge of the object has not changed, it now has regions of excess positive and negative charges. An ESD stress may occur when the object comes into contact with a conductive path. For example, charged regions on the surfaces of styrofoam cups or plastic bags can induce potential on nearby ESD sensitive components via electrostatic induction and an ESD stress may occur if the component is touched with a metallic tool.

2 Risks of ESD

ESD is a serious issue in solid state electronics, such as integrated circuits (ICs). ICs are made from semiconductor materials such as silicon and insulating materials like silicon dioxide. Either of these materials can suffer permanent damage when subjected to high voltages.

The damaging effects of ESD poses unacceptable risks in many areas of technology and it is necessary to control such interference and reduce the risks to acceptable levels through the:

●Simulation and testing of electronic devices using models

●Definition of standards

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Risks of ESD	AN3960

2.1Simulation and testing of electronic devices using models

Several models describe how to simulate an ESD stress. The schematic circuit of Figure 1, shows how to generate an ESD event to a device under test (DUT). It is the basis of these models.

Figure 1. Electrostatic discharge test (ESD generator and DUT)

2	2	$
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(6 SUPPLYU P6$
		AI

1.Legend: R1 = resistor 1, RD = discharge resistor, CD = discharge capacitor, HV = high voltage, and VD = discharge voltage. R1, RD, and CD are defined according to a standard.

2.The charge and discharge switches are not closed simultaneously.

2.1.1Human body model (HBM)

For testing the susceptibility of electronic devices to ESD stress from human contact, an ESD simulator with a special output circuit called the human body model (HBM) is often used.

This model simulates the discharge which might occur when a human touches an electronic device (either a system or a component).

The HBM consists of a capacitor in series with a resistor (see Figure 1). The capacitor is charged to a specified voltage from an external source, and then suddenly discharged through the resistor into an electronic terminal of the DUT.

2.1.2Machine model (MM)

This model simulates what happens when a machine becomes electrostatically charged and subsequently discharges into an electronic device when it comes in contact with it.

The MM test circuit consists of charging up a 200 pF capacitor to a certain voltage and then discharging this capacitor directly into the DUT.

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AN3960	Risks of ESD

2.2Standards overview

Standards exist for the following reasons:

●To reproduce well-defined tests in terms of their setup (bench size, type of isolating area) and conditions (such as temperature and pressure)

●To eliminate misunderstandings between manufacturers and purchasers

●To facilitate interchangeability and improvement of products

●To assist the purchaser in selecting and obtaining the appropriate product for his particular needs.

None of these reasons are paramount. Each depends on the needs of the customer who must also discuss with his purchaser.

The subsections below provide an overview of the more important ESD standards.

2.2.1JS-001-2010 international standard

The ESD association and JEDEC solid state technology association have established a joint standard procedure for testing, evaluating, and classifying components and microcircuits according to their susceptibility to damage or degradation by exposure to a defined HBM ESD (1.5 kΩ, 100 pF and 8 kV).

2.2.2SP723 EIAJ IC121 standard

The SP723 EIAJ IC121 MM standard is for ensuring that the ESD capability is typically greater than 2 kV (from 200 pF) with no serial resistor. For this standard, RD and CD of Figure 1 are respectively 0 Ω and 200 pF.

2.2.3IEC61000-4-2 international standard

The IEC61000-4-2 standard for ESD protection is ±15 kV for air and ±8 kV for contact. The typical waveform of the output current of the ESD generator is described in Figure 2. For this standard, RD and CD of Figure 1 are respectively 330 Ω and 150 pF. This standard is more accurate for performing tests at system level rather than at electronic device level.

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