一个法国牛人的射频培训资料 ESDDES1A

ESDDES1A.docELECTRONICSANDELECTROSTATIC DISCHARGESAPRIL 2004STATIC ELECTRICITYRemindersOriginally static electricity may be generated by rubbing materials on one another . (scotch tape)The triboelectric serie shows the ability of non conductive materials to acquire electrostatic charges .Dry airBakeliteSilicon waxNylonWoolHuman hairSilkPaper , Cotton , WoodAmberStyrofoam , PolyurethanePolyethyleneRubberRayonPVCTeflonInfluencing parameters:Type of materialRelative humidity and temperature (dry season or place)Body or object capacitanceCapacity of a conductive bodyAny conductive material is able to keep charges (+/-) on it if it is perfectly isolated, the identical quantity of charges with opposite sign will be facing it on the surrounding conductive structure (earth) This constitutes a real capacitor The capacity depends on the size and the positionof the body .The voltage that is present is V = Q/CCharge transfer Conductive to conductive materials (contact)Non conductive with non conductive materials (rubbing)Non conductive with conductive materials (proximity effect)E.S.D. situations (zaps)1/A charged conductive body (grey) gets in contact with a conductive structure connected to earth , a very strong E.S.D. high current discharge in a short time will take place .In fact this total charge transfer will happen as a spark just before contact at a distance depending on the voltage 2/A first charged conductive body (grey)is in presence of second isolated non charged conductive bodyIf contact happens the charges will sharebetween the two conductive bodies .In fact this partial transfer will happen justbefore contact according to the voltage as a spark between them , this is an E.S.D.: a very high current in a very short timeAn E.S.D. will take place each time charge recombination is possible through conductive materials , in fact charged isolating materials are the reason of charge disociation first .150pF330WThe human body has a capacity , but its conductivity is not perfect so the first electrical model of a human body is a capacitor in serie with a resistorE.S.D. COUPLED INTO ELECTRONICSDirect discharge to an electronic componentThis happens during manufacturing and handlingAntistatic precautions must be taken ( see presentation about it)Either high voltage or high current E.S.D. may distroy the components .Direct discharge to electronic equipment housingA mechanical part is touched intentionally (switch) or accidentally (frame)Charged body in presence of a grounded itemCharged body in presence of an ungrounded itemUncharged bodyIn presence of an ungrounded charged itemIndirect dischargeCoupling mechanisms of ESD , transients into the victims circuitryA cmd For IESD: 5A , tr: 1nsAssuming uniform pathdistanceVoltage in 1 cm3cm3V10cm1V30cm0.3VTwo printed traces one cm apart, having 5cm parallel run and located 10cm from an ESD flow , will see a peak transient of 5V . This is more than enough to create an erroneous bit in most logic technologies .If the housing were a homogeneous shield , the skin depth would avoid ESDcurrent from flowing inside of the casing but slots , joints , vents , displays , and cable entries create hudge leakageespecially at these high frequenciesESD currents excite many of these slot antennas especially those which approach l/2 .Then ESD currents will flow inside of the the casing as well as outside .The radiated field will couple to the inner circuits: circuit boards , cables , and wiring It also exists outside , around the ESD source , and couples into the external cables .Common mode pollutionABThe metal or metallized cover hasa certain RF impedance:For tr: 1 ns1mm thick steel plate: 0.4W/ ground shift: 4Vgraphite paint: 10W/ ground shift: 100VE.S.D. SPECIFICATIONSE.S.D. specifications for device sensitivity MIL-STD-883 and 1686Class 1: 1000V sensitivity-Electrical characterisation on each pin before testing-Apply the ESD pulsesAll pins against center of circuit caseEach pin against common-Determine the critical paths and latent failure patternThis is not a burn in testA failure is declared if One parameter is out of normal specOne electrical parameter has changed by more than 10% even still in specE.S.D. spec for equipment susceptibility EN (IEC)61000-4-2 SeveritylevelRelativehumidityContact testAir test1=35%2kV2kV2=10%4kV4kV3=50%6kV6kV4=10%8kV15kVRecommendations for categorizing the severity of ESD effects:ESD causing random but repeatable eventESD causing permanent malfunctionESD causing permanent damage DESIGN FOR E.S.D. IMMUNITYAt component levelSelection of components that already have built-in ESD protection , rather than addition of transient protection devices , anyway they cannot prevent errors if a transient of a few volts is induced by ESD . Addition of protection devices are necessary for specific cases 15 to 20kV direct ESD on keyboards or switches .YESNOUse the forked leads approach to prevent lead inductance from appearing across the line to be protected .High ZLimitation resistorSerie resistors on sensitive , high impedance inputsto limit current below damage levelDecoupling capacitors on power input pins (ceramic capacitor of a few nF) not appearing necessary at initial design .On signal inputs if they do not degrade the performance( lower value)Where transient protectors will not eliminate low level glitches , decouplingcapacitors will .Protection of very low level input may be acheived by using a low capacitance sinal diode directly polarized . It must be rated for the worse ESD conditions: 20A during 100ns (the diode barrier voltage being 0.6V it will show an infinite resistance for normal signals )Avoiding the use of a too fast technology (small bandwith) help acheiving good immunity .Pull-up/pull-down resistors help decreasing the vulnerability of high impedance inputs .SMC bring a significant reduction of ESD pick up area . A factor k size reduction induces a factor k of ESD effect reduction E.S.D. protection at PCB levelRule: Check all the runs to avoid large signal-to-zero volt loops or Vcc-to-zero volt loopsA/ never let a signal or a Vcc trace travel without a close ground return (trace or preferably plane) . Ground planes or the largest possible copper lands to act as noiseless ground and shield . Doublesided boards with maximum unetched copper on one side are better than single-sided boards , multilayers are better .B/ never run critical signals (clocks, resets, watchdogs.) near board edges because of capacitive pick up from nearby ESD , even for PCB with ground planes0V 5V clkNOdecoupling capacitors traces are too longthey offer pick up loops to ESD radiationsThe ground ring cannot be equipotential at 100MHz or more , it makes another noisy loop A vulnarable clock trace is at the edge of the boardprone to capacitive couplingUse ground plane or enlarge the copper wire as much as possibleNo signal or Vcc wire without close ground or planeNo sensitive signals at the edge of the board(improvement at low cost)Compartment shield may be used to protect critical areas where a ground plane is notsufficient , it must be the most directly grounded to the chassis to prevent it from reradiating on the PCB or contaminating the 0V with shield currentsI/O cables should be decoupled at their entry pointinto the enclosure , but it is not always possible ,and the cables plug directly on the PC board .Since they are ESD pick up antennas they can ruin thebest PCB design . Incoming lines should be filteredby discrete ceramic capacitors with ultra short leadsor capacitor arrays , or capacitor and miniature ferrites close to point of entry.Filtering connectors are very efficient (but expensive)Keyboards:They cause large openings in the cover creating a privileged entry port for ESD radiations.They have a high probability of being hit by direct discharge , with the arc directly reaching the sensor circuits .Insulating layer with conductive patterns underneathESD shielded membrane keyboard: The interlayer of thin steel is multi point grounded to the chassis so that the ESD current does not penetrate the inner part of the keyboard .Other solutions use thick outer membrane of polyester that cannot be pierced by arc .Add series resistors on the signal traces .Cables:Avoid long run of cables along cover seams , hinges , and bonding wires .Flat cables must alternate 0V return with each signal wireStart with 0V at the edges . CM ferrite sleeves may improve the situation .Protection by the box design and shieldingThe E.S.D. spectrum about 300MHz obliges the designer to consider the possible leakage of any seam and slots exceeding a few centimeters , it means the same precautions as for VHF equipments or sensitive radio frequency device .Shielding means:1/ Absorption which increases with: ThicknessConductivity sr (1/resistivity)Permeability mrfrequency2/ Reflection which increases with:surface conductivityWave impedancecurrentResidualcurrentfieldcurrentfieldfieldResidual currentIndirect dischargedirect dischargeThickness mmCopper0.01 0.1 1Aluminium0.01 0.1 1Zinc0.01 0.1 1Steel0.01 0.1 1Nickelsr=0.20.01 1Copper paintsr=0.040.0530MHz7 70 7005.2 52 5204 40 4003 28 2803 317100MHz13 130 10009.5 95 9507 72 7205 50 5006 5813300MHz22 220 100017 170 100012 125 10009 88 88010 9822Absorption loss in dBBUT housings are not continuous metal cubes , they have slots , seams , apertures which leakAt low frequencies , what counts is the metal nature , thickness , conductivity and permeabilityAt high frequencies , the metal itself provides very good shielding , which is never seen because of discontinuitiesA slot in a shield may be compared to a slot antenna , which behaves like a dipole .1GHz100MHz10MHz1MHz20dB50dB100dB1cm3cmShielding effectiveness with slotsFor a 3ns rise time which equivalent freqency bandwidth is 1/ptr = 100MHza 1cm slot shows a leakage of 45dBHoles and seams are most critical in areas which are likely to receive an ESDhave sensitive components and wiring just behindHow to maintain shield integrity with metal housingsAll metal parts should be bounded together . A floating item may reradiate , it may become a capacitive coupler to the inside electronic . It is not the grounding which is important , it is the equipotentiality with the other parts of the enveloppe .Capacitive coupling especially near edges and protruding areas .If they were connected to the rest of the casing theywould not cause capacitive couplingCover seams and slots should be bonded according to the design objective , that is to say taking into account the sensitivity of the area to be protected compared with the threat , for a 40dB goal , seams and slots should be broken down to 1cm at least . This means frequent screws or welding points or an EMC conductive gasket . According to the type of devices and protection level , several solutions are proposed .Minimum shielding(20dB): straps of braid or copper foil . Distance 90%: good shield integrity , permits perfect bonding at the endsMind the bonding impedance , 360 contact with each housing that the cable penetrates .The noise coupled inside a coax cable depends on