Ø Grounding of personnel via the floor/footwear system is recommended where the resistance to ground of the operator is < 35 ΜΩ. Ø Grounding of personnel with wrist straps is recommended where the resistance to ground of the operator is > 35 ΜΩ (without wrist strap). This is often the case for operator working in a sitting position.
Protective Garments
ESD protective garments are recommended to serve as a barrier/shield between the electrostatic field and the sensitive components.
Protective Measures
*Handling Ø Protective Garments
q Special garments
Ø Footwear
q Dissipative footwear avoids charging by walking and drains off charge on human bodies in a controlled way.
ESD
Subset of EOS q Limited Energy q Usually caused by static q Event lasts from nano to microseconds Not easily visible q Gate oxide breakdown q Junction spiking
Ø Wrist
q Use a wrist strap for grounding in order not to be charged! q Test your wrist strap (as worn) every day!
Ø Packing
q Use dissipative material only as filling material! q Avoid “hard discharge” on metallic surface during unpacking. q Unpack the devices and place them on dissipative material to drain off possible charge on the devices ”softly“.
Test wrist straps as worn every day !
Resistance to groundable point: * < RG < 35 MΩ * Lower limit due to personnel safety reasons!
Protection against HBM ESD Events
Time / ns
CDM test setup
Charged Device Model (CDM)
Good simulation of “real world” CDM events Discharge characteristic defined by the device Test performance very costly and time consuming: Ø Time consuming alignment of the DUT Ø Monitoring of the discharge current necessary Ø Expensive test equipment Ø Poor reproducibility
q Maintenance / Line Monitoring
q Board / Component Test
q Power Supply Quality
ESD Models
Ø Human Body Model (HBM) Ø Machine Model (MM) Ø Charged Device Model (CDM)
EOS due to : Ø Electromagnetic pulses (EMP) Ø Electrostatic discharge (ESD) Ø System transients Ø Lighces
EOS
Typically generated by: q q q Power supplies Test equipment Event usually lasts from millisecond to seconds • Typically > 50us pulse
ESD Models and ESD Stress in the Field
Human Body Model (HBM) - test circuit
Peak value of Ip =VESD/(R+RDUT)
Characterisation of the HBM tester parasitics
Point to point resistance test
Point to point resistance: RP < 1012 Ω Should be made of cotton !
Footwear
Test the footwear as worn standing on a metal plate !
Indications : q q q q Blown metal Lines Molten Damage Usually Thermal High power
q Latch up Not always restricted to I/O q May cause functional fails
EOS Causes and Effects
EOS Control
q Work Procedures
v v v v v v v v v v v v v v v v v v Documented test/assembly flows. Correct insertion of devices - use pictorial reminders. Beware of powered insertions and removals. Check connections. Equipment not grounded. Loose connections causing intermittence events. Poor wire management. No AC line monitoring. Hot switching. Incorrect test sequence. Excessive voltages. Wrong stress tests. Probing . Poor board layout. Poor design leading to noise. No overvoltage protection. Insufficient line filtering. Wrong fuse and inadequate protection.
CECC - Europe company standards product standards (chip cards: ISO/IEC 10373)
Ø IEC standard in preparation
Important HBM test parameters
Ø Pin combinations Ø Test voltages Ø Failure criteria Ø Further test parameters
ESD System/Device Level Test
System Level
Device Level
Differences between System and Device Level
Numbers are different !!! Don’t compare apple to orange…
Resistance to ground: * < RG < 108 Ω * Lower limit due to personnel safety reasons!
Wrist Straps
The purpose of the wrist strap is to drain off the operator's static charge.
HBM - Standards
Ø MIL STD 883 D, method 3015.7, 1989 Ø ANSI EOS/ESD-S5.1-1993 (ESD Association) Ø JESD22-A114-1996 (JEDEC)
further standards: EIAJ - Japan
q Inadequate Work Procedures
v Hot switching Insertion / removal of components with power on / not fully powered down. v Inadequate Training.
q Poor board design
Machine Model (MM) - test circuit
Machine Model - current waveform
Current / A
Time / ns
Typical CDM discharge events
CDM - discharge current waveform