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Electrostatic Discharge (ESD)
Chapter 18
Electrostatic Discharge (ESD)
Table 18.1
The fundamental aspect of matter that permits the flow of electricity is the existence of free electrons in that matter. The electrical resistance of a material is defined by h o w m a n y free electrons exist in the material. Metals, for example, have m a n y free electrons, so it is relatively easy to generate a current flow in them. Insulators, on the other hand, have few free electrons, so current flow is proportionally lower. An attribute of insulators is that they do not allow redistribution of electrical charge across their entire surface, so they can permit local electrical charge to build up. This is static electricity and leads to electrostatic discharge (ESD) d a m a g e to electronic devices. Triboelectric generation results w h e n e v er sufficient energy has allowed the transfer of free electrons from one insulating material to another by m e a n s of contact or friction. Table 18.1 s h o w s the triboelectric series, which is the electrostatic relationship b e t w e e n the items. Materials at the top e n d of the left colu m n of the table have a greater t e n d e n c y to lose electrons and so are considered "increasingly positive." Figure 18.1 shows what h a p p e n s w h e n w o o l is r u b b e d against hard rubber. Electrons are transferred from the
Triboelectric Series
,
Air Human hands Asbestos Rabbit fur Glass Mica Human hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel Wood Amber
Hard lubber Nickel, copper Brass, silver Gold, platinum Sulfur Acetate, rayon Polyester Celluloid Orlon Saran Polyurethane Polyethylene Polypropylene Polyvinyl chloride (PVC) KEL-F Teflon Silicon
Sealing wax wool to the rod. This occurs because the energy level of the valence electrons is raised to the point w h e r e they b e c o m e free electrons and can escape to the hard r u b b e r rod.
209
210
THE TECHNICIAN'S EMI HANDBOOK
Table 18.3
Means of Static Generation
Means of Static Generation
Electrostatic Voltages
10-20% Humidity 35 kV 12 kV
65-90% Humidity 1.5 kV 250 V
Worker at bench Vinyl envelopes for work instructions
6 kV 7 kV
100 V 600 V
Common poly bag
20 kV
1.2 kV
Work chair padded with polyurethane foam
18 kV
1.5 kV
Walking across carpet Walking over vinyl floor WOOL
HARD RUBBER~~ WOOL
Fig. l & 1 Rubbing hard rubber rod on wool generates static electricity.
Materials can be classified according to the surface resistance in o h m s p e r s q u a r e ( o h m s / s q ) . Table 18.2 shows the resistance of materials in o h m s per square. Metals and other conductors are not triboelectric generators because they can redistribute electrical charge received by rubbing over their entire surface, avoiding the local build up of charge. Insulative materials can be charged by triboelectric generation. The a m o u n t of moisture in the air is the humidity, and it can determine the a m o u n t of electrical charge that an object can hold. The air resistance can control this charge and will "bleed off" electrostatic charge. This explains w h y electrostatic discharge will occur more Table 18.2 Type of Material
Insulative Antistatic Static dissipative Conductive
Resistance o f Materials Ohms per Square
Above 1014 109 to 1014 104 to 109 1 to 104
often in the wintertime w h e n the humidity is low and air resistance is high. The threshold of feeling for electrostatic discharge is around 4,000 volts. Any potential below 4,000 volts cannot be felt, but is still dangerous to electronic equipment. If the static electric spark is heard, then it is in the range of 5,000 to 50,000 volts. It is quite possible for the h u m a n body to have an electrostatic charge of 35,000 volts! Table 18.3 shows the charge that can be built up by various activities. This is c o m p a r e d with various electronic devices' electrostatic voltage tolerance of 30 to 7,000 volts (Table 18.4). Table 18.4
Static Electricity Susceptibility
Device
Range of ESD Threshold
VMOS MOSFET GaAsFET EPROM
30 to 1,800 100 to 200 100 to 300 100 to 1,000
JFET SAW Op-amp CMOS (B-series) Schottky diodes
140 to 7,000 150 to 500 190 to 2,500
Film resistors Bipolar transistors
300 to 3,000
ECL SCR SchottlW TTL
500 680 to 1,000 1,000 to 2,500
Electrostatic Discharge (ESD)
ESD EFFECTS
lic ions will flow into the void created by the ESD, shorting the structure. Far more devastating are soft faults to electronic equipment. These faults do not appear immediately because the void (Figure 18.2B) is not filled with metal--rather, metal ions migrate into the space from the gate structure over time. Statistics indicate that 90 percent of all ESD may be this type. It is far harder to diagnose because it takes from 90 days to 6 months of operation to fill in the void, with nothing to show for it other than (possibly) reduced performance.
The effects of ESD damage can be devastating to electronic equipment, but it is often misdiagnosed for the following reasons: 1. Failures are analyzed as being due to electrical transients other than electrostatics. 2. Failures are categorized as random, unknown, "manufacturing defects," or other causes. Very few failure modes and effects laboratories are equipped with the scanning electron microscopes needed to see the damage. Figure 18.2 shows two views of a MOSFET transistor, undamaged (Figure 18.2A) and damaged (Figure 18.2B). Hard faults are immediate and catastrophic failures due to ESD and are immediately apparent. The critical part of the MOSFET device is the thin dielectric insulator between the gate and the substrate/drain/source structure. If enough heat is generated by the static charge, metal-
IDENTIFICATION BY CLASS Electronic components can be classified according to electrostatic discharge capability. Class-1 is the most sensitive, class-2 is less sensitive, and class-3 is the least sensitive. Obviously, class-1 includes MOSFETs and JFETs, or anything on the list that has susceptibility less than 1,000 volts. Class-2 devices
GATE
DRAIN " ,-t . . . . . .
,
~,"."::~.",i ~~':~' i >~i,~,~.'~.,,,'.~,;,,~:,-,...~~ . : ,
"~,,~:,
~ ":,;%'.-,~-'
.: ~:: ~' .._ .'-j
i',
;.
" ..
',i
'
,
" '
.. .
|
GATE
UF ~,.,',..:x~,,.7. :'~, ~'4'~
Fig. 18~2 (A) Standard MOSFET; (B) with ESD defect.
211
;r
~):",.*~".'- :,'."":,
~AIN ,
9 .~ .... '~.,-,' ..
,. :"'"
. '", :
....
212
THE TECHNICIAN'S EMI HANDBOOK
have a sensitivity range of 1,000 to 4,000 volts, and class-3 devices have a sensitivity range of 4,000 to 15,000 volts.
No plastic note holders, pen holders, calendar holders
ESD CONTROL PROCEDURES
No u n a p p r o v e d soldering iron
The key to a good program for controlling ESD is found in creating an environment that is protected against generating static electricity. The environment must consider things such as floors, workbenches, materials, equipment, and operating procedures. At the very least, one must equip the technicians and other workers with electrostatic wrist straps, a portable protective work mat, and protection for the parts being placed into the equipment. A manufacturing facility may include an elaborate grounded workbench made of ESD protective materials, humidity controls, conductive flooring and air ionizers, in addition to the above. An electrostatic location meter will help you determine whether or not static has built up. You can sense voltages from 500 to 50,000 volts with these instruments, depending on model. In the discussion below the terms "hard ground" and "soft ground" will be used. The term hard ground means that the item is connected to ground through copper or aluminum wiring; the term soft ground means that it is through a resistance.
No u n a p p r o v e d solder suckers
W O R K AREAS The work area should be controlled-access, which means that people should be kept out unless they are trained. Minimally protect an area at least 1 meter surrounding the work area. The following restrictions should be followed: No tapes (3M Magic, masking), except where installed and removed under controlled ionization No vinyl-covered notebooks or instruction folders No telephones unless designed for the area
No plastic-cased vacuum cleaners or heat guns
PROTECaTv~ FLOORING Floors are very important in protecting against ESD damage. Only use conductive, static dissipative floors, antistatic carpeting, conductive vinyl, or terrazzo floor tiles. When vinyl tiles are used, the adhesive should be conductive also. Use conductive wax on floors, or leave them unwaxed. Static charge builds up on normal wax. Painted or sealed concrete floor, or finished w o o d floors, should be covered with ESD floor mats or treated with a topical antistatic compound, if no floor mats are available. In any event, the floor should be tested periodically to determine if it is still conductive. Use of a g r o u n d e d floor is not terribly useful if the individual worker is not soft g r o u n d e d also. Use conductive shoe covers, conductive shoes, heel grounders, or some similar device to ground the body of the worker. Conductive work stools or chairs should also be used.
WORKBENCHES Figure 18.3 shows a workbench set up for ESD protection. It is set on conductive flooring or a conductive mat and is made of conductive materials. There is a protective wrist strap on the bench that grounds the worker. It should have a resistance that limits current flow to 5 mA, the threshold of perception, at the voltages that will be w o r k e d on the bench. For 240 volts, that means the resistance should be higher than 240/0.005 = 48,000 ohms. Two types of ESD wrist strap are used: carbon-impregnated plastic and insulated metal conductor. The carbon-impregnated type
Electrostatic Discharge (ESD)
213
WORKBENCH WRIST STRAP
IONIZER/MONITOR
\
CONDUCTIVE MAT
Fig. 18.3
/
Workbench.
distributes the resistance over its entire length, so it should be insulated to guard against touching hard grounds. The insulated metal conductor type contains a built-in resistor. The strap should include an alligator clip, snap, or other form of quick disconnect to permit the user to disconnect in emergencies or when leaving the work area. There is an ionizer and monitor on the bench in cases where manufacturing or very sensitive work is taking place.
EQUIPMENT The equipment used on the ESD workbench should be designed for the purpose. Soldering equipment should be three-wire types with grounded tips. The resistance of the tip to hard ground should be 20 ohn~s or less so that the voltage buildup will be less than 15 volts. Solder suckers used should be ESD types, which means at least they will have conductive tips. All exposed metal surfaces on test equipment should be hard grounded. However, the test equipment should not be set directly on the workbench's conductive surfaces for fear
of rendering the surface, and the person using the equipment, at hard ground potential. Ground-fault interrupters should be used on all outlets on the workbench.
CLOTHING People handling ESD-sensitive electronics products should be properly attired to do so. They should wear long-sleeved ESD smocks or close-fitting short-sleeve shirts. The use of ESD protective aprons is encouraged. Never use smocks or gloves made of c o m m o n , ordinary plastic, as it is not ESD protective.
ESD-PROTECTIVE MATERIALS Protective materials should be used to protect electronic products containing ESD-sensitive components. Chips should be m o u n t e d on those foam carriers and not removed until they are ready for use. The finished printed circuit cards should be stored in ESD bags, wrappings, or boxes.
Electrostatic Discharge (ESD)
Table 18.1
The fundamental aspect of matter that permits the flow of electricity is the existence of free electrons in that matter. The electrical resistance of a material is defined by h o w m a n y free electrons exist in the material. Metals, for example, have m a n y free electrons, so it is relatively easy to generate a current flow in them. Insulators, on the other hand, have few free electrons, so current flow is proportionally lower. An attribute of insulators is that they do not allow redistribution of electrical charge across their entire surface, so they can permit local electrical charge to build up. This is static electricity and leads to electrostatic discharge (ESD) d a m a g e to electronic devices. Triboelectric generation results w h e n e v er sufficient energy has allowed the transfer of free electrons from one insulating material to another by m e a n s of contact or friction. Table 18.1 s h o w s the triboelectric series, which is the electrostatic relationship b e t w e e n the items. Materials at the top e n d of the left colu m n of the table have a greater t e n d e n c y to lose electrons and so are considered "increasingly positive." Figure 18.1 shows what h a p p e n s w h e n w o o l is r u b b e d against hard rubber. Electrons are transferred from the
Triboelectric Series
,
Air Human hands Asbestos Rabbit fur Glass Mica Human hair Nylon Wool Fur Lead Silk Aluminum Paper Cotton Steel Wood Amber
Hard lubber Nickel, copper Brass, silver Gold, platinum Sulfur Acetate, rayon Polyester Celluloid Orlon Saran Polyurethane Polyethylene Polypropylene Polyvinyl chloride (PVC) KEL-F Teflon Silicon
Sealing wax wool to the rod. This occurs because the energy level of the valence electrons is raised to the point w h e r e they b e c o m e free electrons and can escape to the hard r u b b e r rod.
209
210
THE TECHNICIAN'S EMI HANDBOOK
Table 18.3
Means of Static Generation
Means of Static Generation
Electrostatic Voltages
10-20% Humidity 35 kV 12 kV
65-90% Humidity 1.5 kV 250 V
Worker at bench Vinyl envelopes for work instructions
6 kV 7 kV
100 V 600 V
Common poly bag
20 kV
1.2 kV
Work chair padded with polyurethane foam
18 kV
1.5 kV
Walking across carpet Walking over vinyl floor WOOL
HARD RUBBER~~ WOOL
Fig. l & 1 Rubbing hard rubber rod on wool generates static electricity.
Materials can be classified according to the surface resistance in o h m s p e r s q u a r e ( o h m s / s q ) . Table 18.2 shows the resistance of materials in o h m s per square. Metals and other conductors are not triboelectric generators because they can redistribute electrical charge received by rubbing over their entire surface, avoiding the local build up of charge. Insulative materials can be charged by triboelectric generation. The a m o u n t of moisture in the air is the humidity, and it can determine the a m o u n t of electrical charge that an object can hold. The air resistance can control this charge and will "bleed off" electrostatic charge. This explains w h y electrostatic discharge will occur more Table 18.2 Type of Material
Insulative Antistatic Static dissipative Conductive
Resistance o f Materials Ohms per Square
Above 1014 109 to 1014 104 to 109 1 to 104
often in the wintertime w h e n the humidity is low and air resistance is high. The threshold of feeling for electrostatic discharge is around 4,000 volts. Any potential below 4,000 volts cannot be felt, but is still dangerous to electronic equipment. If the static electric spark is heard, then it is in the range of 5,000 to 50,000 volts. It is quite possible for the h u m a n body to have an electrostatic charge of 35,000 volts! Table 18.3 shows the charge that can be built up by various activities. This is c o m p a r e d with various electronic devices' electrostatic voltage tolerance of 30 to 7,000 volts (Table 18.4). Table 18.4
Static Electricity Susceptibility
Device
Range of ESD Threshold
VMOS MOSFET GaAsFET EPROM
30 to 1,800 100 to 200 100 to 300 100 to 1,000
JFET SAW Op-amp CMOS (B-series) Schottky diodes
140 to 7,000 150 to 500 190 to 2,500
Film resistors Bipolar transistors
300 to 3,000
ECL SCR SchottlW TTL
500 680 to 1,000 1,000 to 2,500
Electrostatic Discharge (ESD)
ESD EFFECTS
lic ions will flow into the void created by the ESD, shorting the structure. Far more devastating are soft faults to electronic equipment. These faults do not appear immediately because the void (Figure 18.2B) is not filled with metal--rather, metal ions migrate into the space from the gate structure over time. Statistics indicate that 90 percent of all ESD may be this type. It is far harder to diagnose because it takes from 90 days to 6 months of operation to fill in the void, with nothing to show for it other than (possibly) reduced performance.
The effects of ESD damage can be devastating to electronic equipment, but it is often misdiagnosed for the following reasons: 1. Failures are analyzed as being due to electrical transients other than electrostatics. 2. Failures are categorized as random, unknown, "manufacturing defects," or other causes. Very few failure modes and effects laboratories are equipped with the scanning electron microscopes needed to see the damage. Figure 18.2 shows two views of a MOSFET transistor, undamaged (Figure 18.2A) and damaged (Figure 18.2B). Hard faults are immediate and catastrophic failures due to ESD and are immediately apparent. The critical part of the MOSFET device is the thin dielectric insulator between the gate and the substrate/drain/source structure. If enough heat is generated by the static charge, metal-
IDENTIFICATION BY CLASS Electronic components can be classified according to electrostatic discharge capability. Class-1 is the most sensitive, class-2 is less sensitive, and class-3 is the least sensitive. Obviously, class-1 includes MOSFETs and JFETs, or anything on the list that has susceptibility less than 1,000 volts. Class-2 devices
GATE
DRAIN " ,-t . . . . . .
,
~,"."::~.",i ~~':~' i >~i,~,~.'~.,,,'.~,;,,~:,-,...~~ . : ,
"~,,~:,
~ ":,;%'.-,~-'
.: ~:: ~' .._ .'-j
i',
;.
" ..
',i
'
,
" '
.. .
|
GATE
UF ~,.,',..:x~,,.7. :'~, ~'4'~
Fig. 18~2 (A) Standard MOSFET; (B) with ESD defect.
211
;r
~):",.*~".'- :,'."":,
~AIN ,
9 .~ .... '~.,-,' ..
,. :"'"
. '", :
....
212
THE TECHNICIAN'S EMI HANDBOOK
have a sensitivity range of 1,000 to 4,000 volts, and class-3 devices have a sensitivity range of 4,000 to 15,000 volts.
No plastic note holders, pen holders, calendar holders
ESD CONTROL PROCEDURES
No u n a p p r o v e d soldering iron
The key to a good program for controlling ESD is found in creating an environment that is protected against generating static electricity. The environment must consider things such as floors, workbenches, materials, equipment, and operating procedures. At the very least, one must equip the technicians and other workers with electrostatic wrist straps, a portable protective work mat, and protection for the parts being placed into the equipment. A manufacturing facility may include an elaborate grounded workbench made of ESD protective materials, humidity controls, conductive flooring and air ionizers, in addition to the above. An electrostatic location meter will help you determine whether or not static has built up. You can sense voltages from 500 to 50,000 volts with these instruments, depending on model. In the discussion below the terms "hard ground" and "soft ground" will be used. The term hard ground means that the item is connected to ground through copper or aluminum wiring; the term soft ground means that it is through a resistance.
No u n a p p r o v e d solder suckers
W O R K AREAS The work area should be controlled-access, which means that people should be kept out unless they are trained. Minimally protect an area at least 1 meter surrounding the work area. The following restrictions should be followed: No tapes (3M Magic, masking), except where installed and removed under controlled ionization No vinyl-covered notebooks or instruction folders No telephones unless designed for the area
No plastic-cased vacuum cleaners or heat guns
PROTECaTv~ FLOORING Floors are very important in protecting against ESD damage. Only use conductive, static dissipative floors, antistatic carpeting, conductive vinyl, or terrazzo floor tiles. When vinyl tiles are used, the adhesive should be conductive also. Use conductive wax on floors, or leave them unwaxed. Static charge builds up on normal wax. Painted or sealed concrete floor, or finished w o o d floors, should be covered with ESD floor mats or treated with a topical antistatic compound, if no floor mats are available. In any event, the floor should be tested periodically to determine if it is still conductive. Use of a g r o u n d e d floor is not terribly useful if the individual worker is not soft g r o u n d e d also. Use conductive shoe covers, conductive shoes, heel grounders, or some similar device to ground the body of the worker. Conductive work stools or chairs should also be used.
WORKBENCHES Figure 18.3 shows a workbench set up for ESD protection. It is set on conductive flooring or a conductive mat and is made of conductive materials. There is a protective wrist strap on the bench that grounds the worker. It should have a resistance that limits current flow to 5 mA, the threshold of perception, at the voltages that will be w o r k e d on the bench. For 240 volts, that means the resistance should be higher than 240/0.005 = 48,000 ohms. Two types of ESD wrist strap are used: carbon-impregnated plastic and insulated metal conductor. The carbon-impregnated type
Electrostatic Discharge (ESD)
213
WORKBENCH WRIST STRAP
IONIZER/MONITOR
\
CONDUCTIVE MAT
Fig. 18.3
/
Workbench.
distributes the resistance over its entire length, so it should be insulated to guard against touching hard grounds. The insulated metal conductor type contains a built-in resistor. The strap should include an alligator clip, snap, or other form of quick disconnect to permit the user to disconnect in emergencies or when leaving the work area. There is an ionizer and monitor on the bench in cases where manufacturing or very sensitive work is taking place.
EQUIPMENT The equipment used on the ESD workbench should be designed for the purpose. Soldering equipment should be three-wire types with grounded tips. The resistance of the tip to hard ground should be 20 ohn~s or less so that the voltage buildup will be less than 15 volts. Solder suckers used should be ESD types, which means at least they will have conductive tips. All exposed metal surfaces on test equipment should be hard grounded. However, the test equipment should not be set directly on the workbench's conductive surfaces for fear
of rendering the surface, and the person using the equipment, at hard ground potential. Ground-fault interrupters should be used on all outlets on the workbench.
CLOTHING People handling ESD-sensitive electronics products should be properly attired to do so. They should wear long-sleeved ESD smocks or close-fitting short-sleeve shirts. The use of ESD protective aprons is encouraged. Never use smocks or gloves made of c o m m o n , ordinary plastic, as it is not ESD protective.
ESD-PROTECTIVE MATERIALS Protective materials should be used to protect electronic products containing ESD-sensitive components. Chips should be m o u n t e d on those foam carriers and not removed until they are ready for use. The finished printed circuit cards should be stored in ESD bags, wrappings, or boxes.