ESD知識和控制

1、ELECTRO STATIC DISCHARGE AWARENESS AND CONTROL,An extensive course on,Examples : Lightning Zapping,An atom is made up of charges,An atom is made up of charges. In its stable state, the size of the positive charge at the centre (nucleus) of an atom is balanced by the size of all the negative charges

2、of the electrons, making it neutral overall. The electrons whirl around the centre like planets.,Creating a charge imbalance,When an atom loses an electron, it has a charge imbalance. Since this atom has lost an electron, which is a negative charge, the atom is now a positive charge. When it gains a

3、n extra electron, an atom becomes a negative charge.,A discharge,This movement of charges is called a discharge.,When something which has a charge imbalance is brought close to or touches something else, a stream of charges might move, to try to bring the atoms back to their stable balanced conditio

4、n.,This powerful, rapid movement of charges can damage electronic components.,Lets see it again!,WHAT IS ESD?,ELECTRO STATIC DISCHARGE IS DEFINED AS THE TRANSFER OF ELECTRO STATIC CHARGES BETWEEN BODIES AT DIFFERENT POTENTIAL CAUSED BY DIRECT CONTACT OR INDUCED ELECTRO STATIC FIELD.,ESD EVENT,IT IS

5、AN UNPLANNED OCCURANCE OF ELECTRO- STATIC DISCHARGE,What is ESDElectrostatic Discharges (ESD) are the most severe form of Electromagnet Interference (EMI). The human body can build up static charges that range up to 25,000 volts. These build-ups can discharge very rapidly into a electrically grounde

6、d body, or device. Placing a 25,000-volt surge through any electronic device is potentially damaging to it.,Moving people Improper grounding Unshielded cables Poor connections Moving machines Low humidity (hot and dry conditions),The most common causes of ESD are:,Figure 13-14: Power Transmission Sy

7、stem,Figure 13-15: Grounds on IC Chips,Some devices used to remove solder from circuit boards and chips can cause high static discharges that may damage the good devices on the board. The device in question is referred to as a solder-sucker, and is available in antistatic versions for use with MOS d

8、evices.,To avoid damaging static-sensitive devices, the following procedures will help to minimize the chances of destructive static discharges:,Triboelectric generation,Basic Principles (2),When two materials are in intimate contact, they share electrons which are at their surfaces.,When materials

9、are in contact,The triboelectric charge of two materials in contact.,When two materials are in intimate contact, they share electrons which are at their surfaces.,When materials are in contact,It is possible for electrons to be “stolen” from one material by nuclei in the other material, because they

10、 have a stronger force,When these materials are separated, electrons are removed from one material and are transferred to the other material.,When the materials are separated,When the materials are separated,When these materials are separated, electrons are removed from one material and are transfer

11、red to the other material.,This action takes place with all types of materials. With insulators, the charges remain at the points of contact. A charge spreads all over an ungrounded conductor.,The triboelectric charge when two materials placed in contact are then separated.,This action takes place w

12、ith all types of materials. With insulators, the charges remain at the points of contact. A charge spreads all over an ungrounded conductor.,When the materials are separated,The loss and gain of these electrons creates an imbalance of negative and positive charges on the surface of each material.,Wh

13、en the materials are separated,When the surfaces are rough, this intimacy and separation is assisted by rubbing the materials together.,When the materials are separated,The size of the charge (imbalance) depends on the intimacy of the contact, how fast they were separated, the humidity and the kinds

14、 of materials,When the materials are separated,The drier the air (lower relative humidity, RH) the higher the generated charge,The creation of a charge when materials are separated is termed “triboelectric charging”,Charging by Induction,Basic Principles,Charging by induction,The presence of this ve

15、ry strong force causes similar charges on the surfaces of nearby conductors to be repelled.,The charge imbalance on a surface produces an electric field.,Charging by induction,If the conductor is grounded while it is still influenced by the electric field, these repelled charges go to earth, thus ma

16、intaining a charge balance in the areas of the conductor that are not affected by the electric field.,Charging by induction,Walking on carpet,Thoughts and theories,Walking across carpet,Walking across a carpet creates a charge imbalance on the lower surface of shoes.,A strong electric field,This cha

17、rge imbalance creates a strong electric field that emanates in all directions. The material in the sole of shoe enables a strong field to influence charges within foot.,An induced charge,Since the body is relatively conductive, the charges that are repelled from the lower regions of my foot set up a

18、 charge throughout the rest of my body. But the body still has a balanced charge overall (it has not lost or gained any charges).,Contacting a source of charges,While you are still on the carpet (still influenced by the electric field on the sole of your shoe),you briefly touch a metallic object (a

19、chair, table, door knob, metal stapler, etc.) This enables charges to be provided by the metallic object.,Charging through the air,This movement of charges is thus producing an overall charge imbalance in my body, since I was previously neutral overall.,If the charge imbalance on my body is large en

20、ough, I do not need to touch the object, as the imbalance could allow a movement of charges through the air.,An overall charge imbalance,Therefore, after that brief encounter with the metallic object, my body now has an overall charge imbalance.,Removing that charge,The only way to remove the possib

21、ility of damaging an ESD-sensitive device or assembly is to remove the overall charge imbalance on my body. This is done by connecting my body to the ready source of charges that is at the same reference potential as the item I wish to handle - using a wrist strap or foot strap.,ELECTROSTATIC DISCHA

22、RGE PROCESS,1. Charge is generated on the surface of an insulator. 2. This charge is transfered to a conductor by contact or induction. 3. The charged conductor comes near a metal object (grounded or ungrounded) and a discharge occurs. 4. When a discharge occurs to an ungrounded object, the discharg

23、e current flows through the capacitance between the object and ground.,STATIC GENERATION,THE STATIC CHARGE IS GENERATED BY IMBALANCE IN THE MOLECULAR STRUCTURE BY RUBBING; CAUSING STRIPPING OF NEGATIVELYCHARGED ELECTRONS WITH ONE MATERIAL GETTING PASITIVE CHARGE AND OTHER NEGATIVE,CAUSE FOR STATIC C

24、HARGE,FRICTION SEPERATION INDUCTION,CAUSES FOR ESD GENERATION,WALKING ON SYNTHETIC SURFACE RELATIVE MOVEMENT ON CHAIR TABLE, COMPUTER, PLASTIC PARTS NYLON OR SYNTHETIC CLOTHES CLEANING WITH COMPRESSED AIR CLEANING WITH SOLVENT,ESD,FACTORS: TYPE OF MATERIAL RELATIVE HUMIDITY TYPE OF CLOTHING SPEED AN

25、D MANNER OF WALKING BODY RESISTANCE,RECORDED ESD ACCIDENTS,In 1937, The German flying boat Hinderburg arriving in Lakehurst, New Jersey, caught fire while anchoring at its landing mast. In 1970s, in USA, a space craft launching rocket exploded during the fueling operation, killing 3 engineers. In Ja

26、nuary, 1985, during the assembly of a missile in Germany, the motor case made of Kelvar, got rubbbed against the cushioning in its container. The ESD generated caused the highly flammable propellant to catch fire and the motor exploded, killing 3 people. 3 to 30% of Integrated Circuits manufactured

27、every year die in “infancy” due to ESD.,Device sensitivity to Electrostatic Discharge,At higher magnification the pit in the area between the base and emitter becomes more obvious,Device sensitivity to Electrostatic Discharge,At this magnification see the 1/2 of the width of the material has been de

28、stroyed.,40 50% EQUIPMENTS HAVE FAILED IN THE FIELD DUE TO ESD,Statistics,SENSITIVE CONSTITUENTSFAILURE MODES,MOS STRUCTURESSHORT BIPOLAR JUNCTIONSLEAKAGE FILM RESISTORSVALUE SHIFT METALIZATION STRIPESOPEN FIELD PERFECT STRUCTURESOPERATIONAL PIEZOELECTRIC CRYSTALSDEGRADED CLOSELY SPACED CONDUCTORSDE

29、GRADED,PLASTICS CONDUCTORS FURNITURES VINYL FLOOR COOLING FANS WITH PLASTIC BLADES PRINTERS/COPIERS PAPER NYLON & WOOLLEN GARMENTS COMPRESSED AIR GUN,ESD SOURCES,MAN MADE:,NATURAL:,HUMAN BODY MOVEMENT OF CLOUDS,PACKING HANDLING MATERIALS: COMMON POLYTHENE BAGS, WRAPS,ENVELOPES COMMON BUBBLE PACK FOA

30、M COMMON PLASTIC TRAYS, PLASTIC TOTE BOXES, VIALS,TYPICAL CHARGE GENERATORS,ASSEMBLY, CLEANING, TEST & REPAIR AREA ITEMS: SPRAY CLEANERS COMMON SOLDER SUCKERS COMMON SOLDER IRONS SOLVENT BRUSHING ( SYNTHETIC BRISTLES) CLEANING, DRYING TEPARATURE CHAMBERS,ESD VOLTAGE MEASUREMENT,ELECTROSTATIC VOLTAGE

31、,MEANS OF STATIC GENERATION 10% RW 40% RW 55% RW,PERSON WALKING ACROSS CARPET,PERSON WALKING ACROSS VINYL FLOOR,WORKER AT A BENCH,CERAMIC DIPS IN PLASTIC TUBE,CERAMIC DIPS IN VINYL SET UP TRAYS,CIRCUIT AS BUBBLE PLASTIC COVER REMOVED,CIRCUIT AS PACKED IN FOAM LINED SHIPPING BOX,35000 15000 7500,1200

32、0 5000 3000,6000 800 400,2000 700 400,11500 4000 2000,25000 20000 7000,21000 11000 5500,REASONS FOR DISBELIEF,ESD DAMAGE : WHEN UNNOTICED?,HUMAN SENSE OF FEELING OF STATIC AT 4000V MUCH MORE HIGHER THAN SENSITIVITY OF COMPONENTS,HARDLY ANY PHYSICAL DAMAGE ESD FAILURES CONSIDERED AS GENERAL COMPONENT

33、 FAILURE FAILURE MAY NOT BE INSTANT. (LATENT FAILURE),GUIDE TO STATIC CONTROL MATERIALS,SURFACE RESISTIVITY-OHMS PER SQUARE,INSULATOR,MATERIALS IN THIS RANGE ARE NOT EFFECTIVE FOR STATIC CONTROL,1012 To 10 15,1010 To 10 12,ANTISTATIC,THESE MATERIALS DO NOT GENERATE AND SUPPORT HIGH STATIC CHARGES. B

34、UT DISCHARGE RATES ARE TOO SLOW FOR MOST INDUSTRIAL APPLICATIONS.,SURFACE RESISTIVITY-OHMS PER SQUARE,GUIDE TO STATIC CONTROL MATERIALS,GUIDE TO STATIC CONTROL MATERIALS,SURFACE RESISTIVITY-OHMS PER SQUARE,STATIC DISSIPATORS ( PARTIALLY CONDUCTIVE),MATERIALS MOST SUITABLE FOR STATIC CONTROL WORK SUR

35、FACES, FLOORING AND FLOOR MATS, SEALING, OVERALLS,106 To 10 9,GUIDE TO STATIC CONTROL MATERIALS,SURFACE RESISTIVITY-OHMS PER SQUARE,CONDUCTIVE,MATERIALS FOR CONTAINERS AND HANDLING EQUIPMENT.BELOW 104 OHMS PER SQUARE EFFECTIVE STATIC SHIELDING IS POSSIBLE,OPERATOR SAFELY WILL BE COMPROMISED IF THE E

36、ARTH PATH THROUGH BENCH MATS, FLOOR MATS AND FLOORING LESS THAN 5 X 104 OHMS AND MAINS POWERED EQUIPMENTS IN USE.,101 To 10 4,OBJECTIVES OF ESD CONTROL,PREVENTING STATIC CHARGE GENERATION DISSIPATING THE DEVELOPED CHARGE NEUTRALISING DEVELOPED CHARGE FOR A CHARGE FREE ZONE,CONTROLLING OF ESD,WORK SU

37、RFACE AIR HUMAN BODY FLOOR,DOS MINIMISE HANDLING. KEEP PARTS IN ORIGINAL PACKING UNTILL READY FOR USE. USE ESD PROTECTIVE CONTAINERS AND BAGS. DISCHARGE STATIC BEFORE HANDLING DEVICE BY TOUCHING NEARBY GROUNDED SURFACE. HANDLE DEVICES BY THE BODY. TOUCH THE ESD PROTECTIVE PACKAGE BEFORE TOUCHING INS

38、IDE OF DEVICE. KEEP A DUST FREE WORK AREA.,DONTS,TOUCH THE LEADS OF DEVICE. SLIDE ES DEVICES OVER ANY SURFACE. PUT MASKING TAPE ON PROGRAMMABLE ICS. STORE OR CARRY SENSITIVE COMPONENTS OR ASSEMBLIES IN PLASTIC BAGS. STORE SENSITIVE COMPONENTS IN THERMOCOLE/PLASTIC FOAM. HANDLE ES DEVICES IN “NON ESD

39、 CONTROLLED” ENVIRONMENT.,ESD MODELS,ESD MODELS HAVE BEEN DEFINED TO DESCRIBE THE EFFECTS ON ESD SENSITIVE DEVICES: HUMAN BODY MODEL CHARGED DEVICE MODEL MACHINE MODEL,Human Body Model (HBM),The basic human body model consists of body capacitance and resistance. The charge is stored in the body capa

40、citance and the discharge occurs through the body resistance. When a charged person handles or comes in close proximity to an ESD sensitive part, the part may be damaged by direct discharge or by an electro static field.,0.1mH,Rb,Vb,Cb,Cb : 50 to 350pF Rb : 150 to 10KOhms Vb : 3 to 20 KV,Human Body

41、Model (HBM),HUMAN BODY CAPACITANCE AND RESISTANCE,o,50-100pF,50pF,100pF,100pF,Rb,o,TYPICAL ESD CURRENT WAVE FORM,. . . . . . . .,. . . . . . . . . . . . . . . . . . . .,t,tf=100nSec,tr=1nSec,40A,I,Machine Model A discharge similar to the HBM event also can occur from a charged conductive object, suc

42、h as a metallic tool or fixture. .,Charged Device Model The transfer of charge from an ESDS device is also an ESD event. A device may become charged, for example, from sliding down the feeder in an automated assembler. If it then contacts the insertion head or another conductive surface, a rapid dis

43、charge may occur from the device to the metal object. This event is known as the Charged Device Model (CDM).,ESD CONTROL DEVICES,ANTISTATIC VINYL TABLE MAT CONDUCTIVE FLOOR MAT ESD LAMINATE ESD WRIST STRAP ESD HEEL STRAP ESD TOE STRAP COMMON POINT GROUND ANTISTATIC BINS/TRAYS PCB STORAGE RACK CONDUC

44、TIVE FOAM,ESD AUDIT,MATERIALS CLASSIFICATION,MATERIAL SURFACE VOLUME CLASS RESISTIVITY RESISTIVITY SHIELDING104 OR LESS102 OR LESS CONDUCTIVE105 OR LESS104 OR LESS DISSIPATIVE105 TO 1010104 TO 109 ANTISTATIC1010 TO 1012- INSULATIVE 1015 1014,ESD AUDIT,PERSONNEL TRAINING FOR ESD PREVENTIVE AWARENESS

45、TRAINING CERTIFICATION TRAINING CONTROL PROGRAM AUDIT,ESD TEST & MEASURING INSTRUMENTS,STATIC CHARGE METER WRIST STRAP AND GROUND TESTER FOOTWEAR TESTER SURFACE RESISTIVITY METER STATIC SENSOR CONTINUOUS WRIST STRAP MONITOR CONTINUOUS WORK STATION MONITOR TEMPARATURE AND HUMIDITY INDICATOR,TESTING E

46、VALUATION,The test evaluation report consists of the following test areas for each ESD item/s:,POINT TO POINT SURFACE RESISTIVITY TOP/BOTTOM POINT TO POINT SURFACE RESISTIVITY IN/OUT SURFACE TO GROUND RESISTANCE PERSON TO GROUND RESISTANCE STATIC CHARGE MEASURING TEST STATIC SHIELDING TEST,TESTING EVALUATION,Antistatic P.E Bags Static Shield Bags Conductive Bags Cond