Chapter 15 Electrostatic Discharge

Transcription

1 Chapter 15 Electrostatic Discharge Electromagnetic Compatibility Engineering by Henry W. Ott

2 Foreword Examples of products using the static electricity principle are electrostatic copiers, dust precipitators, air purifiers, and electrostatic spray painters. However, uncontrolled electrostatic discharge (ESD) has become a hazard to the electronics industry. Since the early 1960s, it has been recognized that many integrated circuits (ICs), metal-oxide semiconductors (MOSs), discrete electrical parts such as film resistors and capacitors, and crystals are susceptible to damage from electrostatic discharge. As electronic devices become smaller, faster, and operate at lower voltages, their susceptibility to ESD will increase. JHLin, AppEMC; Electrostatic Discharge 2

3 Static Generation Static electricity can be created in many different ways - triboelectric charging, induction charging, and piezoelectric effect. But the most common is by contact and subsequent separation of materials - triboelectric charging. Triboelectric means rubbing amber. All that is actually required is that the materials come into contact and are then subsequently separated. The triboelectric series is a listing of materials in order of their affinity for giving up electrons. JHLin, AppEMC; Electrostatic Discharge 3

4 Static Generation (The series is only approximate.) 4

5 Static Generation The degree of separation of the two materials in Table 15-1 does not necessarily indicate the magnitude of the charge created. The magnitude depends not only on the position of the materials in the triboelectric series but also on the surface cleanliness, pressure of the contact, amount of rubbing, surface area in contact, smoothness of surface, and the speed of separation. A charge can also be generated when two pieces of the same material are in contact and subsequently separated. Triboelectric charging also occurs when an insulator is separated from a conductor. Intimate contact is all that is required for electron transfer to occur. JHLin, AppEMC; Electrostatic Discharge 5

6 Static Generation Rubbing tends to increase the pressure of the contact and bring more of the surface in contact and hence increases the charge transfer. Faster separation allows less time for charge reflow, which also increases the charge transfer and the subsequent voltage. 6

7 Static Generation Static electricity is a surface phenomenon. Grounding an insulator will not eliminate the charge. Electrostatic discharge is normally a three-step process as follows: 1. A charge is generated on an insulator. 2. This charge is transferred to a conductor by contact or induction. 3. The charged conductor comes near a metal object and a discharge occurs. A charged insulator by itself is not directly an ESD threat. The danger from a charged insulator comes from its potential for producing a charge, usually by induction, onto a conductor, such as a person, which then is capable of a discharge. JHLin, AppEMC; Electrostatic Discharge 7

8 Static Generation Inductive Charging JHLin, AppEMC; Electrostatic Discharge 8

9 Static Generation Inductive Charging 9

10 Static Generation Inductive Charging The ground connection only has to be momentary, and it can have considerable impedance (100 k or more). JHLin, AppEMC; Electrostatic Discharge 10

11 Static Generation Energy Storage Free-space capacitance of objects r F/m C 111 r (pf) ( r in meters) (15 3) The procedure to estimate the minimum (free-space) capacitance of any object: 1) determine the surface area of the object for which you want to calculate the free-space capacitance; 2) calculate the radius of a sphere having the same surface area; 3) calculate the capacitance from Eq The capacitance between two parallel plates is equal to JHLin, AppEMC; Electrostatic Discharge 11

12 Human Body Model Humans are a prime source of electrostatic discharge. JHLin, AppEMC; Electrostatic Discharge 12

13 Human Body Model If, however, the discharge occurs from a large metal object in contact with the person, such as a chair or a shopping cart, the resistance can be as low as 50. JHLin, AppEMC; Electrostatic Discharge 13

14 Human Body Model Figure 15-6 shows the typical waveshape produced by a 150-pF, 330- human body model discharge into a special 2- test target specified in EN The rise time is 0.7 to 1 ns, and the peak current is 30 A for an 8-kV discharge, and 15 A for a 4-kV discharge. JHLin, AppEMC; Electrostatic Discharge 14

15 Human Body Model 15

16 Human Body Model An actual discharge from a voltage of less than 3500 V will not be felt or sensed by the person involved. Because many electronic devices are sensitive to damage from discharges of only a few hundred volts, component damage can occur from a discharge that is not felt, heard, or seen. At the other extreme, discharges from potentials greater than 25 kv are painful to the person involved. JHLin, AppEMC; Electrostatic Discharge 16

17 靜電放電測試環境設置圖 JHLin, AppEMC; Electrostatic Discharge 17

18 Static Discharge Charge accumulated on an object leaves the object by one of two ways, leakage or arcing. Because it is better to avoid arcing, leakage is the preferred way to discharge an object. Charge can leak off an object through the air, because of humidity. The charge on an object can also be counteracted by using an ionizer to fill the air with positive and negative charged ions. Leakage from a charged conductor can be made to occur by intentionally grounding the object. This ground may be a hard ground (close to 0 ) or a soft ground (a large impedance, a few hundred k s to a few M s) that will limit the current flow. However, grounding a person will not drain the static charge from his or her clothing (nonconductors), or from a plastic object held in the hand, such as a Styrofoam coffee cup. JHLin, AppEMC; Electrostatic Discharge 18

19 Static Discharge When grounding a person, a hard ground should be avoided. The minimum impedance that should be used in grounding a person is 250 k. Grounded wrist straps usually have a 1-M resistance to ground. The higher the resistance, the longer it will take for the charge to bleed off the object. The decay time the time it takes for the charge to be reduced to 37% of its initial value: or 19

20 Static Discharge Decay Time Materials with surface resistivities of 10 9 per square or less can be discharged rapidly by grounding. Static-dissipative materials are preferred to conductive materials because charge dissipation occurs at a slower rate. Antistatic materials are the slowest to dissipate charge. Nevertheless, they are useful because they can dissipate charge faster than it is generated and therefore prevent an object from accumulating a charge. To prevent triboelectric charging, the surface resistivity of a material should not exceed per square. Static-dissipative and antistatic materials are the preferred materials to use in an ESD-sensitive environment, such as a manufacturing line for electronic equipment. Insulators do not dissipate charge but retain whatever charge they have. JHLin, AppEMC; Electrostatic Discharge 20

21 ESD Protection in Equipment Design Effective ESD immunity design requires a three-pronged approach. 1. prevent or minimize the entry of the transient currents by: Effective design of the enclosure Cable shielding Providing transient protection on all conductors of unshielded external cables 2. harden sensitive circuits by: Resets Interrupts Other critical control inputs 3. write transient hardened software capable of detecting, and if possible correcting, errors in the following: Program flow Input/output (I/O) data Memory 21

22 ESD Protection in Equipment Design Energy from a static discharge can be coupled to an electronic circuit in two ways: 1. By direct conduction 2. By field coupling, including a) Capacitive coupling b) Inductive coupling Direct conduction occurs when the discharge current (typically tens of amperes) flows directly through the sensitive circuit. The fast rise time, large voltage, and high current associated with ESD produces intense electric and magnetic fields. These fields, although usually not causing damage, are strong enough to upset the operation of many electronic circuits, even at distances of a meter or more away from the actual discharge. 22

23 ESD Protection in Equipment Design A circuit or system may be protected from a static discharge by any of the following: 1. Eliminating the static buildup on the source 2. Insulating the product to prevent a discharge 3. Providing an alternative path for the discharge current to bypass the sensitive circuits 4. Shielding the circuit against the electric fields produced by the discharge 5. Decreasing loop areas to protect the circuit from the magnetic fields produced by the discharge The first three items in the above list deal with controlling the direct discharge, and the last two items deal with controlling the associated field coupling. ESD-induced effects in electronic systems can be divided into the following three categories: JHLin, AppEMC; Electrostatic Discharge 23

24 ESD Protection in Equipment Design 1. Hard errors - cause actual damage to the system hardware. 2. Soft errors - affect system operation but do not cause physical damage. 3. Transient upset - does not cause an error, but the effect is perceptible The European Union s criteria for an ESD failure (Performance Criteria B) is as follows: The apparatus shall continue to operate as intended after the test. No degradation of performance or loss of function is allowed. During the test, degradation of performance is however allowed. No change of actual operating state or stored data is allowed. In other words, transient upset is allowed, but no soft or hard errors are allowed. The first step in designing equipment to be immune to ESD is to prevent the direct discharge from flowing through the susceptible circuitry. This can be accomplished either by insulating the circuit or by providing an alternative path for the discharge current. JHLin, AppEMC; Electrostatic Discharge 24

25 ESD Protection in Equipment Design If insulation is used, it must be complete, because a spark can enter through an extremely narrow air gap. In the case of a product in a metallic enclosure, the enclosure can be used as an alternative path for the ESD current. To divert the ESD current effectively from sensitive circuits, all metallic components of the enclosure must be bonded together. See Fig The basic principle of ESD bonding and grounding is to use lowinductance multipoint bonding where ESD current is desired and singlepoint bonding where ESD current flow is not wanted. JHLin, AppEMC; Electrostatic Discharge 25

26 JHLin, AppEMC; Electrostatic Discharge 26

27 Preventing ESD Entry The three most common points of ESD entry are the enclosure, cables, and keyboards or control panels. Metallic Enclosures The major advantage: it can be used as an alternative path for the ESD current. The major disadvantage: it encourages a discharge to occur. See Fig Discontinuities in the enclosure (e.g., seams or holes) can cause differential voltages to appear on the enclosure as well as allowing ESD induced fields to couple to the inside of the enclosure (Fig. 15-8). These enclosure voltages and fields can then couple to the circuit and affect its operation. The enclosure should be as continuous as possible with a minimum number of seams and apertures. To minimize ESD field coupling any apertures in the enclosure should h l h f ( ) 27

28 Preventing ESD Entry Metallic Enclosures JHLin, AppEMC; Electrostatic Discharge 28

29 Preventing ESD Entry Metallic Enclosures The situation shown in Fig is not a practical configuration, because the circuit has no connection to anything outside of the enclosure. 29

30 Preventing ESD Entry Metallic Enclosures A similar effect occurs if the enclosure is ungrounded. The enclosure may rise closer to the full potential of the discharge source. Therefore, it is desirable to ground all metallic enclosures for ESD protection. The secondary arc can be prevented by 1. providing sufficient space between all metal parts and the circuit. 2. or by connecting the circuit to the metallic enclosure, thus keeping it at the same potential as the enclosure. The spacing should be sufficient to withstand about 2000 V for a grounded enclosure and 15,000 V for an ungrounded enclosure. The breakdown voltage for air is approximately 3000 V/mm (75,000 V/in) at standard temperature and pressure (STP). V breakdown P T( K) JHLin, AppEMC; Electrostatic Discharge 30

31 Preventing ESD Entry Metallic Enclosures The safe clearance distance to prevent an arc is usually considered to be about one third of this or 1 mm/kv. JHLin, AppEMC; Electrostatic Discharge 31

32 Preventing ESD Entry Metallic Enclosures Even without a secondary arc, the strong electric field produced between the metallic enclosure and the circuit can cause problems. JHLin, AppEMC; Electrostatic Discharge 32

33 Preventing ESD Entry Metallic Enclosures If the circuit is connected to the enclosure, this connection should be a lowinductance connection made in the I/O area of the printed circuit board (PCB). JHLin, AppEMC; Electrostatic Discharge 33

34 Preventing ESD Entry Metallic Enclosures Because the circuit common is connected to the enclosure, the circuit potential rises with the enclosure, and no potential difference exists between points on the circuit or between the circuit and the enclosure. What, however, has happened to the high voltage potential on the enclosure? It is transferred as a common-mode voltage to the interface cables and applied to whatever is at the other end of the cables. The classic ESD problem indicates that a discharge is applied to box A and damage is done to the circuit in box B, or vice versa. JHLin, AppEMC; Electrostatic Discharge 34