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Valkuilen in CDM tester calibratieprocedures
Theo Smedes, Arjan van IJzerloo, Michal Polewski, Jean Luc Lefebvre,
Marcel Dekker
Pitfalls for CDM calibration procedures
Theo Smedes, Arjan van IJzerloo, Michal Polewski, Jean Luc Lefebvre,
Marcel Dekker
2010 EOS/ESD Symposium
Proceedings EOS/ESD Symposium, pp. 341-348, 2010
Objectives
● Demonstrate pitfalls in CDM calibration procedures
● Calibration results depend heavily on
− Selected calibration equipment
− Calibration targets
− Procedures
● Propose improved calibration procedure
● Demonstrate results with new procedure
● Note: all data in presentation is for ESDA/AEC standard, similar arguments hold for JEDEC
Slide 3
Outline
● Introduction
● Analysis
− Experiments
− Waveform analysis
− Calibration targets
● Proposal for Solution
● Results
● Conclusions
Slide 4
What is ESD?• ElectroStatic Discharge:
sudden, short, electric current when charge, stored on insulated object, finds path to object at different electrical potential
• 2 ESD mechanisms for ICs– Charge builds up on person/tool & transfers to IC– Charge builds up on IC and transfers to other object
• The result is a short, high current peak– 100 ps to 100 ns duration– Up to 10’s A peak current
How do you protect an IC against ESD?• Add circuitry such that:
– core of design is not affected by the ESD currents and voltages for any ESD event
– functionality of core is minimally affected• ESD protection network (lightning rods)• ESD Protections must be robust and efficient
– Robust: able to sink required ESD current– Efficient 1: Correct trigger voltage– Efficient 2: Clamp voltage low enough– Efficient 3: Fast enough
How do you assess ESD sensitivity?• Qualify products by stressing according to several standards
• Different standards try to address different events:– Human Body Model: A person discharges to an IC– Charged Device Model: A charged IC discharges to its
environment– System Level ESD: A person discharges through a
tool to equipment
• In all cases:– models only cover a limited part of ‘real life ESD’– discharge waveforms/procedures are described in standards– Product functionality is tested before and after stress
Standards• Several standards have been developed for specification,
benchmarking and data exchange• These standards prescribe pulse forms, test levels, test
sequences, pass/fail criteria etc.• Most important organizations:
– ESDA: Electrostatic Discharge Association– JEDEC Solid State Technology Association– AEC: Automotive Electronics Council– JEITA: Japan Electronics and Information Technology
Industries Association• Most standards define classification,
but do not prescribe required levels
Charged Device Model
• Four main CDM standards are in place: JEDEC, AEC, ESDA, JEITA
– ESDA is best developed standard– JEDEC is the most commonly used standard– AEC is used for automotive applications
• Unfortunately significant differences– Parts tested with different standards can
deliver a factor of 2 difference in the CDM voltage passing levels for the same part
• This method exists for all standards• Differences are: insulator, calibration modules,
discharge head impedance, …
Field Induced, Non-socketed CDM
CDM Waveforms & Calibration• Discharge specified calibration target• Measure discharge waveform for specified voltage• Compare with specifications for peak current, etc.• If necessary: tune tester by changing .ini file:
– programmed vs. applied voltage– linearity
Insulator
Copper disc
Introduction• Product qualified in 3 different labs• Pass/fail levels differed considerably from lab-to-lab:
• All labs were within limits of theESDA and AEC CDM standards
• Labs had identical ESD controlprocedures
• All labs used same tester typeSlide 12
Lab Pass Level (V)
Fail Level (V)
A 750 1000B 500 750C 250 500
1 GHzLab Spec MeasA 5.4B 4.7C 5.2
4.5 (±20%)
Measured waveforms• 500 V discharge on small ESDA module• >3 GHz scopes
• Large differences, in line with pass/fail of product• Not in line with 1 GHz measurements
Slide 13
-4-202468
10
0E+00 1E-09 2E-09 3E-09Time (s)
Cur
rent
(A) Lab A
Lab BLab C
Fourier analysis (1)• Fourier Transform shows waveforms measured with
high bandwidth scope contain > 1 GHz components• A simple low pass filter can be calculated such that:
filtered 6GHz spectrum = measured 1 GHz spectrum
Slide 14
0
0.2
0.4
0.6
0.8
0E+00 2E+09 4E+09 6E+09Frequency (Hz)
Am
plitu
de
6 GHz spectrum,measured1 GHz spectrum,measured1 GHz spectrum,filtered from 6 GHz
0
0.2
0.4
0.6
0.8
0E+00 2E+09 4E+09 6E+09Frequency (Hz)
Am
plitu
de
6 GHz spectrum,measured1 GHz spectrum,measured1 GHz spectrum,filtered from 6 GHz
Fourier analysis (2)• This filter is characteristics for the 1 GHz procedure• Inverse Transform gives calculated 1 GHz waveform• Good match is obtained• This procedure was repeated for each lab
Slide 15
-4-202468
0E+00 1E-09 2E-09 3E-09Time (s)
Cur
rent
(A) 6 GHz measured
1 GHz measuredfiltered
Comparison of the labs (1)• Predicted filter characteristics for each lab • Labs have totally different characteristics
– Characteristic frequency– Role-off
Slide 16
0.01
0.1
1
1E+07 1E+08 1E+09 1E+10Frequency (Hz)
Am
plitu
deLab ALab BLab C
Comparison of the labs (2)• Frequency response of 2 scopes measured• Scope 1 agrees with the filter calculated for lab A• Scope 2 agrees with the filter calculated for lab B• Lab A uses scope 1, lab B does NOT use scope 2.
Slide 17
Scope 2 is a brand/type mentioned in standard
0.01
0.1
1
1E+07 1E+08 1E+09 1E+10Frequency (Hz)
Am
plitu
deLab ALab BScope 1Scope 2
0.01
0.1
1
1E+07 1E+08 1E+09 1E+10Frequency (Hz)
Am
plitu
deLab ALab BScope 1Scope 2
0.01
0.1
1
1E+07 1E+08 1E+09 1E+10Frequency (Hz)
Am
plitu
deLab ALab BScope 1Scope 2
Discussion (1)• Not all 1 GHz scopes are equal• Scope frequency response affects results significantly
• So even if labs have identical 1 GHz peak current, >3 GHz peak currents may be significantly different
• Consequently actual stress experienced by products is significantly different
• Real stress: scope frequency response is irrelevant• Thus: use highest bandwidth to bring tester on spec.
Slide 18
Calibration modules (1)• Capacitance measurements• Differences between modules exist• Modules are not always flat
Slide 19
Module Small Large#1 #2 #3 #4
C(pF), w/o pressure 4.12 2.9 2.92 26.12C(pF), w/I pressure 4.43 4.38 4.4 31.6
Max gap (µm) <1 47 60 14FR-4 thickness (µm) 704 744 747 716
• Differences between labs exist• Only 1 of used modules is in spec. • All modules were original• Target peak current can be compensated for
difference between true capacitance and specified capacitance
Lab A Lab BC (pF)
SpecificationC (pF)
Measured Ratio C (pF) Measured Ratio
Small 4±5% 4.63 1.16 4.37 1.09Large 30±5% 33.5 1.13 30.7 1.02
Calibration modules (2)
Slide 20
Repeatability issues• Graph shows standard deviation of Ipeak for 2 shifts• After a product qualification, 30 samples of 64 pins,
the Ipeak showed a 30%-60% decrease• After pogo pin cleaning normal values were found
Slide 21
0
2
4
6
8
-2000 -1000 0 1000 2000V_CDM (V)
StD
ev I_
peak
(%) Shift 1
Shift 2
Discussion (2)• Capacitance value impacts calibration current value• Calibration setting impacts stress current for product
– C to be measured in same situation as stressing
• Standards state: out-of-spec modules can not be used– But almost no modules are in spec
• Calibration can be compensated for C value– 4.63 pF: target current should be 16% higher than
current specified for the ideal 4 pF targetSlide 22
Discussion (3)• Demonstrated 2 mechanisms leading to systematic
differences between results of different testers • Tolerances in the standards are not intended to cover
these systematic differences– Tolerances cover random variations due to arc
properties, alignment issues, etc.
• Both problems can be solved in calibration procedure• We propose a new procedure on the next sheet
– in agreement with the intentions of the standards– solves the issues discussed
Slide 23
Proposal• Allow only > 3.5 GHz oscilloscope
– both for calibrations and verifications• Remove 1 GHz table from standards
• Define detailed capacitance measurement method for modules on charge plate
• Compensate target peak current for capacitance Δbetween actual and ideal calibration module
• Use 3.5 GHz table of existing ESDA standard• Define software filter for reporting 1 GHz data
Slide 24
Confirmation of new procedure (1)• Known product re-evaluated after re-calibration
– 3 samples per stress level• All labs have identical pass/fail level• Earlier result was too optimistic
Slide 25
0
1
2
3
0 250 500 750V_CDM (V)
#fai
ls
Lab A, oldLab A, newLab BLab C
0
1
2
3
0 250 500 750V_CDM (V)
#fai
ls
Lab A, oldLab A, newLab BLab C
: 350 V: 500 V
Confirmation of new procedure (2)• Same failure mode found for all• Plotted against peak current all fails are at same level
– Also old qualification! • Redesign easily solved this problem
Slide 26
Classical CDMfailure: X-domain
0
1
2
3
0 2 4 6 8 10I_CDM (A)
#fai
ls
Lab A, oldLab A, newLab BLab C
Conclusions• Existing calibration procedure leads to inconsistent
product qualification results– specification of behavior of oscilloscopes– calibration module specification
• Discharge surface contamination also causes inconsistency
• An improved procedure is proposed:– does not require new equipment or modules– demonstrated to yield consistent results
• Revision of the CDM standards should be consideredSlide 27
EOS/ESD Symposium next year: