From: Charles Grasso ([email protected])
Date: Fri Nov 03 2000 - 10:47:54 PST
Very nice summary... I especially like the software solutions. I have used
those to great effect...
----- Original Message -----
From: <[email protected]>
To: <[email protected]>; <[email protected]>
Sent: Friday, November 03, 2000 6:11 AM
Subject: Re: [SI-LIST] : Unit failing ESD testing
Jeff,
This is a set of guidelines that I put together seven years ago for my
department, on ways to harden electronic devices against Electrostatic
Discharge
(ESD). The three main strategies are:
1. Prevent an ESD arc from occurring.
2. Prevent coupling to ESD-sensitive circuits.
3. Add redundancy (memory, circuits, time) and checking to the design to
permit
detection/correction of random errors.
John Barnes Advisory
Engineer
Lexmark International
DESIGNING ELECTRONIC EQUIPMENT FOR ESD-HARDNESS
Sources of ESD Threats:
� Charged person touches the equipment.
� Charged person touches the table beneath the equipment.
� Moving paper/parts inside the equipment build up a charge.
� Charged furniture touches the equipment.
� Charged furniture touches uncharged/oppositely-charged
furniture near the equipment.
Levels of ESD Threats:
� Voltages up to 20kV (Straus, Capps, Boxleitner, Pepe,
Mardiguian) OR 15kV (Leibowitz) OR 25kV (Paul) OR 35kV (Ott,
Welsher).
� Currents up to 200A (Boxleitner) OR 100A (King).
� Electric fields up to several hundred kV/m (Mardiguian) OR
3 MV/m (King).
� Rise times of 0.2ns to 70ns (frequencies up to 5GHz).
� Initial 0.5-10ns spikes, followed by 100ns to 2000ns pulses.
� May have multiple discharges separated by 10us to 200ms.
� Highest voltages not necessarily the worst-- lower ESD voltages
often have faster edge rates.
Effects of ESD:
� Conducted coupling tends to cause damage.
� Radiated coupling tends to cause upsets/errors.
ESD Testing:
� Test equipment in operating & installation configurations.
� Keep ground strap of ESD simulator perpendicular to the
suspected discharge path or electrically far from the
unit-under-test.
� Test to IEC 801.2 level 3:
- 8kV air-discharge without failure.
- 15kV air-discharge without damage.
- 8kV contact-discharge without damage.
- Check any exposed metal, including connector pins.
- Check ventilation holes, controls, and seams in case.
� Discharge to a horizontal/vertical coupling plane with all
cables attached & again with everything but the power cable
detached. Shows you whether the cables or the basic design/
shielding is weak.
� If a few spots are vulnerable, turn out lights & zap the
product to see path of discharge-- may be able to fix easily.
� With an emulator, change registers/stack-pointers/program
counter to random values to simulate ESD-induced changes and
see what happens to the equipment.
� At critical junctions in the code toggle an unused output pin,
once at point #1, twice at point #2, etc. Can monitor this
pin with an oscilloscope to see what the code is doing.
Overall Strategies:
� Prevent ESD discharges.
� Prevent coupling of ESD into circuits/devices.
� Increase noise immunity of devices/circuits.
HOW TO PREVENT ESD DISCHARGES
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----> -1Enclosure Design to Prevent ESD Discharges:-0 > 1. Insulating barriers prevent conductive coupling, but don't > affect radiated ESD. > 2. Set breakdown level of enclosure > 20kV by thin high- > dielectrics or air gaps > 10-13mm. > 3. Keep operator > 20mm from electronics/ungrounded metal. > 4. Keep ungrounded enclosure metal > 20mm from exposed > electronics (Boxleitner) OR > 8.4mm (Ott) OR > 10mm (Paul) OR > > 9.5-12.7mm (Straus). > 5. Keep grounded un-insulated enclosure metal > 2.2mm from > exposed electronics (Boxleitner) OR > 0.5mm (Ott) OR > 1mm > (Paul). > 6. Position seams/openings to provide clearance from conductive > parts. > 7. Use tongue-in-groove or shiplap joints. > 8. Use mylar tape where clearance is limited. > 9. Cover keyholes with tape extending > 13mm past edge of holes. > 10. Avoid sharp edges/points on metal components. > 11. Recess sensitive signals at connectors, making them difficult > or impractical to touch manually. > > > -1Cabling Design to Prevent ESD Discharges:-0 > 12. Space wires/connector pins by > 2.2mm to prevent arcing. > 13. Cover unused/rarely-used connectors with plastic dust > covers. > > > -1Circuit Board Layout to Prevent ESD Discharges:-0 > 14. Isolate electronic components from the ESD source, > 20mm > away from areas/ungrounded metallic items that the operator > can touch. > 15. Make conductor corners rounded to prevent arcing (heatsinks > or any other metal items close to areas that a person can > touch). > 16. Recess LED's & cover them with light pipes/tape. > 17. For membrane keyboards extend border to increase path length. > 18. For membrane keyboards use plastic bezel to increase path > length. > > HOW TO PREVENT COUPLING OF ESD INTO CIRCUITS > > -1Enclosure Design to Prevent Coupling of ESD into Circuits:-0 > 19. Conductive barriers prevent conductive coupling & reduce > radiated ESD-- 2mm separation between grounded shield & > circuitry will prevent secondary arcs (up to 20mm separation > needed for ungrounded shield). > 20. Use multipoint grounds in areas that you want ESD current to > flow. > 21. Use single-point grounds in areas that you don't want ESD > current to flow. > 22. Connect metal portions of enclosure to chassis ground. > 23. Keep bonding jumpers short and wide, well away from sensitive > electronics. > 24. Grounded parts should have > 1500V isolation to circuitry. > 25. Ungrounded parts should have > 25000V isolation to circuitry. > 26. Try to put all cable entries close to center of enclosure. > 27. Provide chassis ground within 40mm of each cable entry. > 28. Keep seams & openings > 10-13mm away from grounds & sensitive > signals/devices. > 29. Keep conductive parts (screws) penetrating shields well away > from circuitry. > 30. Radiated noise entering hole depends on longest dimension, so > keep slots/holes < 20mm long. > 31. Use several small openings instead of one big opening. > 32. Space openings apart by their largest dimension. > 33. Perforate shields by deep holes with no conductors inside, > acting as waveguides below cutoff to attenuate noise. > 34. Provide for shielding, to be added if needed. > 35. Tie shields to circuit common. > 36. Tie shields to conductive exit points for ESD charges-- > connectors, switches on battery-operated devices, green wire > ground on line-operated devices. > 37. Put a ground plane underneath cards, parallel and close to > them. Connect grounds of peripheral cables to this ground > plane at their entry point. > 38. Put a secondary shield between the enclosure and circuitry, > tied to circuit ground for I/O shielding & bypassing. > 39. Put grounded conductive shields behind control panels to act > as spark arrestors. > 40. Choose shield materials to minimize corrosion (< 0.75V EMF). > 41. Avoid nicks, cracks, thinning of conductive shields. > 42. Overlap shield seams by > 5x the gap. > 43. Electrically connect seams in shields every 20mm with > fasteners/gaskets to shorten slots. > 44. Connect foil tapes to rest of shield. > > > -1Cabling Design to Prevent Coupling of ESD into Circuits:-0 > 45. Use multipoint grounds where you want ESD currents to flow. > 46. Use single-point grounds where you don't want ESD currents to > flow. > 47. Keep I/O cables well away from ground straps & pieces of the > enclosure that may carry ESD current. > 48. Flat cables should have at least 1 ground next to each signal > line (S-G-S-S-G-S-...S-G-S), with sensitive signals at the > center of the cable. Grounds should be connected at each > connector. If you must have a single-point ground system to > prevent ground loops, directly connect a ground wire to ground > at one connector and use capacitors for low-frequency > isolation at the other connectors. > 49. Use shielded cables with shields tied to chassis ground at > each connector. > 50. Connect cable shields to chassis grounds with high-frequency > connections (360 degrees preferred) within 40mm of each cable > entry point. > 51. Keep unshielded portions of cable < 40mm long. > 52. If you don't have a chassis ground, tie cable shield to logic > ground with a 1-10nF capacitor (3.9nF 1kV is good). > 53. Prefer foil- or foil-and-braid-shielded cables > 54. Keep cable shields > 0.025mm thick. > 55. Either clip off extra lines so they are within the shield, or > parallel them with other lines. > 56. Space wires/connector pins by > 2.2mm to prevent arcing. > 57. Prevent constant current flow through grounding contacts. > 58. Metals in contact should have an electromotive force < 0.75V. > 59. Prefer cathodic materials. > 60. Make anodic (positive) items bigger than the cathodic > (negative) items. > 61. Use common-mode chokes on signals & signal grounds but not on > shields (Boxleitner); on entire cable (Straus, Paul). > 62. Put ferrite beads on ground/power/signal lines, but not on > shield conductors. > 63. Do not pass ESD current through a ferrite bead. > 64. If used, put ferrite beads near receiver end. > 65. Put over-voltage clamping devices close to cable entry points. > > > -1Circuit Board Layout to Prevent Coupling of ESD into Circuits:-0 > 66. Put all connectors in one area. > 67. Keep I/O devices close to I/O connectors. > 68. Use multipoint grounds where you want ESD currents to flow. > 69. Use single-point grounds where you don't want ESD currents to > flow. > 70. Minimize power-to-ground loop areas by gridding power & > ground, or by using multilayer boards. > 71. Put small bypass caps close to each connector and each module > (at least every 80mm). > 72. Use wide chassis ground lines (length-to-width < 5:1). > 73. Fill in unused areas with ground, connected at least every > 60mm. > 74. Put a chassis-ground guard ring around the periphery of a > board, and around mounting holes, in all layers with frequent > vias to tie the layers together. > 75. Leave solder mask off the guard ring on the top & bottom > layers. > 76. Connect connector housings and metal switch housings to > chassis ground. > 77. For membrane keyboards use a grounded peripheral guard ring. > 78. Keep un-insulated chassis ground > 2.2mm from other traces. > 79. Minimize signal-to-ground loop areas. Signal lines must be > < 13mm of ground. Signal lines over 300mm long must be > paralleled by a ground. > 80. Keep signal lines short, feeding from the center of the net > if possible. > 81. Connect L-C or ferrite bead-C filters on connector signals to > chassis ground. > 82. Keep protective-component leads short. > 83. Build a Faraday cage around sensitive devices & their inputs. > > > > -1Circuit Design to Prevent Coupling of ESD into Circuits:-0 > 84. Use low-impedance circuits wherever possible. > 85. Use differential signals wherever possible. > 86. Isolate signals with optoisolators or transformers. > 87. Tie unused inputs high or low (do not let float). > 88. Do not connect sensitive inputs to ESD-susceptible lines (i.e. > microprocessor reset to a long cable). > 89. Put a ferrite bead between chassis ground and logic ground. > 90. Put a ferrite bead in each power line at entry point to card. > 91. Put a surge suppressor/MOV/capacitor between each power pin & > chassis ground. > 92. Filter each sensitive input within 25mm of device pin: > - Protect reset/interrupt lines. > - Capacitors to chassis ground if possible; otherwise to > logic ground. > - Wire to 100-1000pF capacitor to high-Z input. > - Wire to capacitor to ferrite bead to low-Z input. > - Wire to resistor/ferrite bead to capacitor to high-Z input. > 93. Put common-mode chokes on signals (note: may have adverse > effect if ESD on shield induces noise on signals). > 94. Protect inputs with high-speed clamping suppressors. > 95. Series resistors and shunt diodes to I/O pins (up to 1k
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�-100k� for MOS inputs, around 50� for bipolar inputs).
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> -1Component Selection to Prevent Coupling of ESD into Circuits:-0 > 96. Use low-inductance capacitors for ESD filters (chip > capacitors, ceramic discs, mica, glass). > 97. Use non-inductive resistors for ESD filters (carbon comp, > film). > 98. Use high-frequency ferrite beads for ESD filters, resistive > from 10MHz to 1GHz with a single-turn to minimize capacitive > coupling. > 99. Choose transient suppressors that turn on within 1ns. > > HOW TO INCREASE NOISE IMMUNITY OF DEVICES/CIRCUITS. > > -1Circuit Design to Increase Noise Immunity:-0 > 100. Do not use any circuit/system with an unlimited-duration > wait/disable state. > 101. Put peripheral chip resets under software control. > 102. Avoid edge-triggered logic. > 103. Use strobes to latch data, not edges. > 104. Pull-up or pull-down unused inputs. > 105. Check parity & framing wherever possible. > 106. Double-check critical inputs several microseconds apart > before using them. > 107. Read outputs back & verify that they match the intended > value. > 108. Provide redundant hardware. > 109. Provide a hardware watchdog timer to monitor program flow. > The software in turn monitors the watchdog timer, and resets > it periodically using an edge-trigger. If the watchdog timer > times out, it either resets the processor or forces a > non-maskable interrupt. > > > -1Component Selection to Increase Noise Immunity:-0 > 110. Do not push components to their limits. > 111. Use slow, insensitive components wherever possible. > 112. Use differential I/O instead of single-ended I/O. > 113. Use chips with read-back capability (read-only or read/write > registers). > 114. Use chips with high immunity to ESD (table taken from Capps): > Charles, "ESD: A Threat to Buried Circuits", EMC Technology, > Vol. 6, No 7, November-December 1987, pp39-42): > > -1DEVICE TYPE-0 -1RANGE OF ESD VULNERABILITY > (VOLTS)-0 > VMOS 30-1800 > MOSFET 100-200 > GaAsFET 100-300 > EPROM 100 > JFET 140-7000 > Op Amp 190-2500 > CMOS 250-3000 > Schottky Diodes 300-2500 > Film Resistors 300-3000 > Bipolar Transistors 380-7000 > ECL 500-1500 > SCR 680-1000 > Schottky TTL 1000-2500 > > > -1Software Design to Increase Noise Immunity:-0 > 115. Provide protection from program flow errors (skip steps or > jump to unused address). > 116. Place trap/return codes in code tables & unused areas. > 117. Fill unused ROM with NOP's, ending with a jump to an error > routine. > 118. Put jump to error routine in all unused interrupt vectors. > 119. Include a check for excessive count in delay loops. > 120. Check index registers & other important registers before use. > 121. Write a token before entering a routine & check before > returning. > > 122. Verify stack empty when in main program & when tasks finish. > 123. Check stack pointer before a return & periodically in main > program. > 124. If using a real-time executive, verify that task is valid for > that time whenever a task exits/suspends. > 125. When jump to a subroutine, copy the new stack data to a > second stack. On return verify that the main stack & the > copy match. > 126. Include checkpoint variables to verify orderly program flow. > 127. Run watchdog routines periodically to make sure that program > has performed periodic functions & therefore program flow > okay. Main program in turn verifies that watchdog routines > are running. > 128. At regular intervals: > - Re-enable interrupts. > - Reread control/selection inputs. > - Refresh output ports. > - Check/refresh/recover memory. > 129. Validate data when received. > 130. Check data from humans for syntax and validity. > 131. Acknowledge all valid inputs & retransmit data if don't get > acknowledgement. > 132. Check inputs for reasonable/valid values (range/consistency/ > parity/framing/checksum/CRC/error-correcting-code checks). > 133. Sample inputs twice for software filtering. > 134. Re-validate data just before using it. > 135. Store critical data in multiple locations. Periodically > crosscheck these locations & fix mismatched data. > 136. Use parity/checksums/CRC's/ECC's to check blocks of data. > 137. Break large tables into fixed-length records with checksums. > 138. Work on a copy of a record, then overwrite the original. > 139. Use a maximum depth counter when searching linked lists. > 140. Maintain a copy of all output states in memory. > 141. Check outputs by having receiver echo data. > 142. On finding an error, recheck hardware/ROM/RAM, all programs, > and all data. > 143. Consider shutting down if the error rate gets too high. > 144. If you detect an incorrect state, try to re-establish the > previous correct state. If this can not be determined, > switch to the most-likely state that causes the least-serious > problems. Warn any attached units that you have just > performed an error recovery. > 145. To restore the state: > - Reset stack pointers. > - Reset FIFO's. > - Reset counters. > - Prevent transmission of suspect codes. > - Disable interrupts during restoration, then re-enable & > start timers. > - Reset pending interrupts. > - Refresh outputs. > - Send host a code that restoring state. > - Fix whatever failed the sanity check. > 146. Keep errorlog in RAM of the last few fix-ups, that can be > dumped for analysis. > > > REFERENCES > > Boxleitner, Warren, -1Electrostatic Discharge and Electronic-0 > -0Equipment-0, IEEE Press, New York, 1989. > > Boxleitner, Warren, "How to defeat electrostatic discharge", > -1IEEE Spectrum-0, Vol. 26, No. 8, August 1989, pp. 36-40. > > Capps, Charles, "ESD: A Threat to Buried Circuits", -1EMC-0 > -1Technology-0, Vol. 6, No. 7, November-December 1987, pp. 39-42. > > Gerke, Daryl, "Designing Noise Tolerance into Microprocessor > Systems", -1EMC Technology-0, Vol. 5, No. 2, March-April 1986, > pp. 45-52. > > Jarrett, Dick, "Software fault tolerance staves off the errors > that besiege
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�P systems",
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> -1Electronic Design-0, August 9, 1984, > pp. 187-202. > > Mardiguian, Michel, -1Electrostatic Discharge-0, Interference > Control Technologies, Gainesville, VA, 1986. > > Ott, Henry, -1Noise Reduction Techniques in Electronic Systems,-0 > -12nd edition-0, John Wiley & Sons, New York, 1988. > > Paul, Clayton, -1Introduction to Electromagnetic Compatibility-0, > John Wiley & Sons, New York, 1992. > > Straus, Isidor, "Designing for Compliance: Immunity to ESD", > -1Compliance Engineering-0, Vol. 7, No. 2, Winter 1990, pp. 15-26. > > > > esd-hard.txt John Barnes 8/27/93 > >
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