Standardized Direct Charge Device ESD Test For Magnetoresistive Recording Heads I Tim Cheung (2), Lydia Baril (1), Albert Wallash (1) (1) Maxtor Corporation, 500 McCarthy Blvd, Milpitas, CA 95035 USA Tel.: 408-324-7067, fax: 408-894-3207, e-mail: email@example.com (2) Previously at ReadRite Corporation, 44100 Osgood Rd, Fremont, CA 94359 USA Tel: 805-967-8152, email: firstname.lastname@example.org Abstract – The effects of the Human Body Model (HBM) Electrostatic Discharge (ESD) waveform on Giant Magnetoresistive (GMR) recording heads is well characterized. This information has provided a starting point for understanding ESD damage to magnetic recording heads but the HBM no longer reflect the reality of ESD mechanisms along the production lines. Damage will most likely occur from metal contact to the MR electrodes (metal) rather than from bare fingers. Direct Charged Device Model (DCDM) can be used to simulate metal-to-metal contact discharge. A manual DCDM was characterized using disk capacitors. DCDM manual ESD testing of grounded (suspension is grounded) and floating suspension (suspension is not connected to ground) GMR heads was performed, and a DCDM breakdown voltage ranging from 3V to 9V was measured. Pspice simulation was used to simulate the DCDM discharging events, the results agree well with the DCDM manual discharge data.
I. Introduction characterize the DCDM and understand how the system behaves using disk capacitors, GMR heads, It is well known the GMR heads are sensitive to and Pspice simulations. damage from an electrostatic discharge (ESD) event,
as the areal density increases dramatically in magnetic II. Experimental Setup recording industry, the magnetic recording heads
become more susceptible to electro-overstress (EOS), The experimental setup consisted of the following and electrostatic discharge (ESD) events. HBM no components: longer reflects the reality of ESD damage along the
manufacturing lines. ESD will mostly occur from 1) Manual DCDM test jig direct charging the GMR head capacitances then 2) Tektronix CT-6 current probe followed by metal contact to an input, MR+ or MR-, 3) Tektronix 3GHz TDS694C Digital Oscilloscope of a GMR head. ESD damage to GMR heads during 4) HP Digital Control Power Supply all stages of handling and assembly is a serious 5) Gold plated metal plate problem in magnetic recording head manufacturing. 6) Disk capacitors The GMR sensor will have a voltage if the sensor is
itself charged, or if it is in the electric field of a nearby, charged insulator. Tribocharging during High Speed Power Supply handling of a head gimbal assembly (HGA) with an Oscilloscope insulating Kapton flex circuit has often been the root + G N D cause of severe ESD problems. ESD testing that directly charges and then grounds the MR+ or MR- of 10M CT-6 a GMR head, known as “DCDM” shows that if the Disk GMR sensor at about 4V is grounded, then the current Short W ire Capacitor that flows through the GMR sensor is severe enough Ground Plate to change the sensor resistance. The discharge current
waveform from metal contact and DCDM to a GMR Figure 1a. Schematic representation of the DCDM manual test jig head are similar. The purpose of this paper is to for disk capacitors.
The purpose of using a manual test jig is that it has the ground wire to the top disk plate. During very small parasitic capacitance and inductance. discharge process, the manual contact must be very Figure 1a shows the schematic representation of fast and precise. The discharge current waveforms are DCDM manual test jig. The inductanceless disk detected with a CT-6 current probe and the capacitors are made out of 0.8 mm thick FR-4 double- waveforms are captured from a high-speed digital sided printed circuit board (PCB). One side of the oscilloscope Tektronix TDS 694C 3GHz and 10GS/s. board is gold-plated etched to a circular disk, and the Figure 2 shows that the grounded and floating other is gold-plated ground plane. The capacitance suspension GMR heads were tested on the same depends on the size of a circular disk (the spacing is DCDM manual test jig. A grounded suspension HGA fixed). The larger the circular disk is the higher the means that the suspension contacts the grounded capacitance. Figure 1b shows four different sizes of metal plate. A floating suspension HGA means that disk capacitance, 0.5pF, 2.2pF, 4.4pF, and 11.5pF. the suspension is isolated from the grounded metal
plate. A 10MΩ resistor is connected from a power
supply to the MR+ of a GMR head. Then, the
grounded short wire contacts the MR+ or MR- pad for
discharge the charged GMR head. The CT-6 current
probe captures the discharge current; the high speed
oscilloscope is used to monitor the discharge current,
and an Ohmmeter is used to measure the MR
resistance after each discharge event until the MR Figure 1b. Four disk capacitors 0.5pF, 2.2pF, 4.4pF and 11.5pF. resistance changed more than 1%. (Note: Use the MR
resistance failure for reference only, magnetic High Speed Power Supply To Oscilloscope Metal plate Oscilloscope + GND 10 MΩ resistor Power supply + 10M CT-6 CT6 Short Wire HGA Short wire Ground Metal Plate
performance is more susceptible to ESD damage).
Figure 2. Schematic representation of the DCDM manual test jig
for GMR head. Figure 1c. DCDM manual test jig for disk capacitors.
The bottom plate of the capacitor lies on a grounded gold-plated metal. The grounded metal plate is gold- III. Experimental Results and plated for good contact between the bottom plate of Analysis disk capacitor and metal plate. A voltage source
provides voltage to the top plate of the disk capacitor Figure 3 shows a simplified electrical equivalent through a 10MΩ resistor. The voltage source is circuit for DCDM manual test jig. The voltage source continuously charging the disk capacitor. A short is not shown in the circuit. Cdisk is the disk capacitor, wire of about half inch long is connected to a L1 is the inductance of the grounded wire, Cp is the grounded metal plate through a CT-6 current probe parasitic capacitance of the circuit, Rspark is the spark (See Figure 1c). The disk capacitor surface and the resistance, which is created during the grounded wire grounded wire must be cleaned. The discharge of the contacts the charged disk capacitor. disk capacitor is done manually contacting one end of
discharge current equation is given by: L1 −α
I (t) V = e t sin(
(3) L 2 1 R where α = R/2L, ω = 2πf = − ( ) , Rspark LC 2L Cp Cdisk I(t) is the discharge current and V is the voltage on the capacitor. The equation (3) shows that the voltage on SW the capacitor is doubled, the discharge current is also doubled. Figure 5b shows that the pulse width decreases as the voltage on the capacitor increases.
The hypothesis for the decreasing pulse width at higher voltage are the bandwidth limitation of a CT-6 Figure 3. Electrical equivalent circuit for DCDM manual testing current probe and the lower spark resistance as a for disk capacitor. result of smaller change of time constant. Figure 5c shows the discharge current saturated at high Figure 4 shows an example of discharge current capacitance. This is due to the smaller the waveform for a disk capacitor 4.4pF at charging capacitance, the higher the discharge path impedance, voltage of 4V. The first peak current reaches 54mA which limits the current. The magnitude of and the pulse width is about 0.47ns. There is a second impedance Z is given by: peak of opposite polarity or undershoot. Its peak
current level is about –21mA. Waveforms obtained in the same manner for the same capacitance and the 2 Z = R + ( ω ) 2
L , same charging voltage is mostly reproducible, i.e. peak current and pulse width values are within +/-
10%. Waveforms were captured for a range of where ω = 2πf, R is the equivalent resistance of the capacitor values and charging voltages. inductor L
1, the L is the inductance of the L1. Pspice simulation will be used to verify this behavior. Manual DCDM 4.4pF @4V Manual Charging/Discharging on Disk Capacitors 6.0E-02 160 11.5pF 5.0E-02 140 11.5pF y = 17.786x - 5.5478 4.0E-02 120 4.4pF y = 12.151x - 3.4174 3.0E-02 100 4.4pF 2.2pF y = 5.1043x + 0.0087 2.0E-02 1.0E-02 0.5pF y = 2.1426x + 0.3652 80 0.0E+00 Peak Current (A) 60 -1.0E-02 -1.E-09 0.E+00 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 2.2pF 40 -2.0E-02 measured Ip (mA) -3.0E-02 20 0.5pF Time (ns) Time (sec.) 0 0 2 4 6 8 10 Figure 4. Discharge current waveform of a 4.4pF disk capacitor at DCDM Voltage (V) 4V.
Figure 5a. Charging voltage versus discharge current for four Figure 5a shows the manual test discharge current for capacitors. four different capacitances and voltages. The
1.2 1.0 L1 0.8 11.5pF 0.6 4.4pF Rspark 0.4 2.2pF V CMR-to-grounded suspension 22pF 0.2 0.5pF Peak Width at Half Amplitude (ns) 0.0 SW 0 2 4 6 8 10 DCDM Voltage (V) Figure 5b. Pulse width versus charging voltage for four capacitances.
Figure 7. Simplified equivalent electrical circuit model for 140 8V grounded suspension GMR heads. 120 100
80 Figure 8 shows the discharge current of 51mA and the 4V 60 pulse width of about 2ns at 4V for the grounded Ip (mA) 40 suspension GMR head. 2V 20 1V Manual DCDM: Grounded suspension 0 Criteria: MRR > 1% 0 1 2 3 4 5 6 7 8 9 10 11 12 13 Capacitance (pF) 0.06
0.05 breakdown at 4V Figure 5c. Discharge current versus capacitance for four charging 0.04 voltages. 0.03 0.02 0.01 Figure 6 shows an electrical circuit model of the 0 DCDM manual testing for grounded suspension GMR Total discharge current (A) -1.E-09 0.E+0 0 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 6.E-09 7.E-09 8.E-09 9.E-09 -0.01 heads. The capacitance from the MR leads to -0.02 suspension is larger than other capacitance when the Tim e ( n s ) Time (sec.) suspension is grounded. The total capacitance of the Figure 8. Discharge current waveform at 4V for grounded HGA depends on the type of flexcircuit. In this suspension GMR head. particular flexcircuit HGAs, the total capacitance is about 22pF from the MR leads to suspension. Figure Figure 9 shows the electrical circuit model of the 7 shows a simplified equivalent electrical circuit of DCDM, and the floating suspension GMR heads Figure 6. Where 22pF is the total head capacitance, (floating suspension means the suspension is not L connected to ground). The head capacitance is 1 is the total inductance of the MR lead and inductance of discharge path. dominated by the capacitance between the suspension and the system ground. 5pF 5pF 10MΩ MR pad+ 2Ω 10MΩ V MR
Figure 9. Electrical circuit model of the DCDM and the floating suspension GMR heads.
When the GMR head suspension is not grounded, the
M a n u a l D C D M : F l o a t i n g S u s p e n s i o n capacitance from suspension to grounded metal plate 0.0 8 is approximately 5pF. Figure 10a shows the CMR-to- Ip = 65mA @ 9V breakdown 0.0 6 suspension in series with the Csuspension-to-ground. The PW50 = 0.8ns 0.0 4 total head capacitance can be calculated as follows: 0.0 2 C=(22pF)(5pF)/(22pF+5pF)=4pF 0 - 1 . 0 0 E - 0 9 0 . 0 0 E + 0 0 1 . 0 0 E - 0 9 2.0 0 E - 0 9 3 . 0 0 E - 0 9 4 . 0 0 E - 0 9 5 . 0 0 E - 0 9 6 .00E-09 7 . 0 0 E - 0 9 8 . 0 0 E - 0 9 9 . 0 0 E - 0 9 Total discharge current (A) - 0 . 0 2 L1 - 0 . 0 4 Tim e ( n s ) 3 . 0 5 V 4 V 4 . 5 V 5 V 5 . 5 V 6 V 6.5 V 7 V 7 . 5 V 8 V 8 . 5 V 9 V 2 . 5 4 V Figure 11. Discharge current waveform at breakdown voltage of Rspark 9V for a floating suspension GMR head. 22pF Figure 12 shows that the comparison of the CMR-to-suspension breakdown voltage for the grounded suspension and the floating suspension GMR heads. The grounded SW Csuspension-to-ground 5pF suspension heads were broken down at about 3V to 4V, and the floating suspension head was broken down at about 9V at the GMR resistance changing
more than 1%. The floating suspension exhibits Figure 10a. Simplified electrical circuit model for a floating higher breakdown voltage than the grounded suspension GMR head. suspension GMR head because the grounded Figure 10b shows a simplified head capacitance from suspension has a larger head capacitance than the MR-to-ground for floating suspension GMR head. floating suspension GMR head reference to grounded An equivalent head capacitance can be represented by metal plate. The grounded suspension HGA has a a single 4pF. total head capacitance of about 22pF, and the floating suspension HGA has a total head capacitance of about 4pF. The total head energy to damage to MR sensor L1 can be calculated and compared as follows:
For grounded suspension HGA: Rspark A total head capacitance C of 22pF with a charging V voltage (V) at breakdown of 4V, then the total energy CMR-to-ground 4pF can be calculated based on the equation: SW
E=1/2(C V 2 )=0.176nJ
For floating suspension HGA:
A total equivalent head capacitance C of 4pF with a Figure 10b. Simplified electrical circuit model for a floating suspension GMR heads with a total equivalent capacitance of 4pF. charging voltage (V) at breakdown of 9V, then the
total energy can be calculated based on the equation: Figure 11 shows the discharge current waveforms at
different charging voltages from 3V to 9V. The MR resistance of this floating suspension head changed E=1/2(C V 2 )=0.162nJ from 42Ω to 54Ω at 9V breakdown. The peak current Comparing the two total energies, the floating amplitude at 9V is about 65mA, and the pulse width is suspension HGA requires higher voltage to reach the about 0.8ns. same energy level as for the grounded suspension
HGA that can damage the GMR sensor (assume same
MR resistance for both type). This is due to the
floating suspension HGA has lower head capacitance
than the grounded suspension HGA reference to
the discharge current amplitude and wider the pulse Manual DCDM: Grounded vs Floating suspension Criteria: MRR >1% width. Both the resistance and impedance of the 56 inductor L1 in the discharge path can affect the 54 Grounded suspension Floating suspension 52 discharge current amplitude and the pulse wide. Col 50 d M 48 RR 46 The total resistance, Rt, in the discharge path is 44 approximately related by the following expression: 42 40 0 1 2 3 4 5 6 7 8 9 10 Rt = Rseries + Rspark + | Z | (4) DCDM Voltage (V) Hd1 Hd2 Hd3 Hd4 The magnitude of impedance Z is given by: Figure 12. GMR resistance versus DCDM breakdown voltage for 2 grounded and floating suspension GMR heads.
Z = R + (
ω = Discharge current occurs when the grounded wire is where 2 f π , R is the equivalent resistance of the brought near a charged disk capacitor, the electric inductor wire and L is the equivalent inductance, f is field between them increases to an extreme value. the frequency. The Rspark and the impedance can be When the field exceeds the dielectric breakdown calculated from the Equation (4): strength of air, current begins to flow by an avalanche
process. Eventually a low resistance spark develops. Rspark + | Z | = Vc/Ip - Rseries The spark resistance can be affected by the contact conditions such as humidity, contact speed, grounded where Vc is the charging voltage on the capacitor, and wire/disk capacitor plate material, the size and the the Ip is the discharging current. shape of the grounded wire tip and the contamination between the grounded wire tip and the disk capacitor. 0.1 No Rseries 0.08 The following setup can be used for initial spark 11 Ohms 0.06 51 Ohms resistance study. 0.04 100 Ohms 0.02 Figure 13 shows the spark resistance test setup. A 0 low inductance surface mount resistor must be used. Discharge Current (A) -2.E-09 -1.E-09 0.E+00 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 6.E-09 7.E-09 8.E-09 -0.02 The purpose of using additional resistance in series -0.04 Time (sec.) with the discharge path is that the behavior of the pulse width and the amplitude of the discharge current Figure 14. Discharge current versus Rseries at charging voltage of 4V. can be observed. (See Figure 14) Figure 15 shows the Rspark and the impedance Z as a function of charging voltage. The manual DCDM test High Speed Power Supply jig was used to perform this experiment (See Figure Oscilloscope 13). The spark resistance plus the impedance Z without adding the series resistance is 44Ω at 4V. + GND Based on the experimental data, the Rspark plus the impedance Z increase as the charging voltage decreases. However, we expected higher spark 10M CT-6 resistance with the increasing of the charging voltage. Disk More experiments need to be conducted and analyzed Series resistor Capacitor in this area.
Figure 13. Schematic representation of the spark resistance test setup.
Figure 14 shows the discharge current waveforms
varying the different series resistances. The higher the series resistance in the discharge path is the lower
used to observe the current flows through the GMR sensor. Figure 17 shows the Pspice simulation of the 140 total discharge current (solid line), and the current 120 Series resistance 51 Ohms 100 flows through the GMR sensor (dashed line) for the (Ohm) Series resistance 11 Ohms 80 grounded suspension GMR head at breakdown + Z 60 Series resistance 100 Ohms voltage of 4V. The discharge current of 22mA flows 40 spark R 20 through the GMR sensor. The reason for the current 0 flows through the GMR sensor less than 50% of the 0 5 10 15 20 total current because the GMR sensor resistance limits DCDM Voltage (V) the current. The total discharge current using the Figure 15. Spark resistance versus DCDM voltage manual discharge (See Figure 8), and the Pspice simulation (See Figure 17) for a grounded suspension IV. Pspice Simulation GMR head are comparable. The Pspice simulated discharge current for the The DCDM manual discharge behavior can be floating suspension GMR head at the breakdown confirmed by Pspice simulation. voltage of 9V (See Figure 18) is comparable to the Figure 16 shows the Pspice simulation of discharge manual discharge (See Figure 11). The total current waveform from discharging a disk capacitor of discharge current is about 66mA; 22mA flows 4.4pF at 4V, inductance L1 of 10nH and spark through the GMR sensor, and the pulse width is about resistance of 24Ω . As can be seen, the DCDM 0.8ns. manual discharge waveform and the Pspice simulation
waveform are comparable. 0.08 Solid line: total discharge current = 66mA 0.06 Dashed line: discharge current through MR = 22mA PW50 = 0.8ns 0.06 Dashed line: simulated, Ip=54mA, PW50=0.47ns 0.04 0.05 Solid line: manual tested, Ip=54mA, PW50=0.47ns 0.04 0.02 0.03 0.02 0 Discharge Current (A) 0.0E+00 1.0E-09 2.0E-09 3.0E-09 4.0E-09 5.0E-09 6.0E-09 7.0E-09 8.0E-09 9.0E-09 0.01 -0.02 0 ischarge Current (A) -0.01 0.0E+00 1.0E-09 2.0E-09 3.0E-09 4.0E-09 5.0E-09 -0.04 D -0.02 time (sec.) -0.03 time (sec.) Figure 18. Pspice simulation of discharge current for a floating suspension GMR head. Dashed line is the current through GMR Figure 16. DCDM manual testing (solid line) and simulation sensor. (dashed line) comparison using 4.4pF at 4V. Figure 19 shows the Pspice simulation of discharge current per charging voltage (mA/V) relationship for 4 different disk capacitors. The simulation results agree 0.06 Itotal=52mA well with the DCDM manual discharge. 0.05 0.04 PW50=2ns 0.03 Pspice Simulated V vs I vs C IMR=22mA 180 0.02 160 0.01 y = 16.208x + 0.1125 11.5pF 140 0 y = 11.13x + 0.5917 4.4pF -1.E-09 0.E+00 1.E-09 2.E-09 3.E-09 4.E-09 5.E-09 6.E-09 7.E-09 8.E-09 9.E-09 ischarge Current (A) 120 -0.01 D y = 8.1211x - 0.275 2.2pF 100 -0.02 y = 2.7734x + 0.125 0.5pF 80 Ip (mA) time (sec.) 60 Figure 17. Pspice simulation of discharge current for the grounded 40 suspension GMR head at 4V. Dashed line is the current through 20 GMR sensor. 0 0 2 4 6 8 10 12 DCDM Voltage (V) The total current flows through the GMR heads can be measured but the current flows through the GMR
sensor can not be measured. Pspice simulation can be Figure 19. Pspice simulation of peak current versus DCDM voltage.
Figure 20 shows the Pspice simulation of discharge current pulse width as a function of DCDM voltage 0.035 0.03 10 Ohms for 6 different disk capacitors. The spark resistance, 0.025 0.02 inductance and capacitance are no changed. The 0.015 60 Ohms pulse width maintains constant throughout the 0.01 0.005 charging voltage ranging from 1V to 16V. However, 0 0.0E+00 2.0E-09 4.0E-09 6.0E-09 8.0E-09 1.0E-08 1.2E-08 1.4E-08 the simulation conditions and the experimental -0.005 -0.01 condition is slightly different. The experimental data Discharge Current thru MR (A) -0.015 -0.02 shows that the spark resistance changes as varying the Time (sec.) charging voltage. Figure 22. Pspice simulation of discharge current flows through the GMR sensor at the charging voltage of 4V with varying the 0.8 spark resistance Rsp of the DCDM from 10Ω to 60Ω . 0.7 11.5pF
0.6 Figure 23 shows that if the DCDM module varies the 0.5 4.4pF discharge path inductance from 5nH to 65nH, the 0.4 2.2pF energy (not shown) increases about 23% at 65nH. 0.3 PW50 (ns)
0.005 Figure 20. Pspice simulation of discharge current pulse width 0 versus charging voltage. -0.005 0.0E+0 2.0E-09 4.0E-09 6.0E-09 8.0E-09 1.0E-08 1.2E-08 1.4E-08 0
Discharge Current thru MR (A) -0.01 Figure 21 shows the discharge currents are saturated Time (sec.) as increasing the capacitance. The experimental data and simulation data are comparable. Figure 23. Pspice simulation of discharge current flows through the GMR sensor at the charging voltage of 4V with varying the 140 discharge path inductance from 5 to 65nH. 8V
120 100 V. Data Summary 80 60 4V 40 Table 1 shows the rise time of discharge currents for 2V 20 Peak Current (mA) four capacitors at charging voltage ranging from 1V, 1V 0 2V, 4V and 8V. The experimental (exp) and 0 1 2 3 4 5 6 7 8 9 10 11 12 13 simulation (sim) results are closed. Capacitance (pF)
Figure 21. Pspice simulation of discharge current versus 0.5pF 2.2pF 4.4pF 11.5pF capacitance for four charging voltages. tr (ns) exp sim exp sim exp sim exp sim
V Figure 22 shows that if the DCDM module varies the 1 0.25 0.177 0.21 0.203 0.39 0.215 0.53 0.32 spark resistance from 10Ω to 60Ω during ESD testing, 2 0.18 0.17 0.25 0.197 0.31 0.226 0.42 0.32 75% less energy (not shown) flows through the GMR 4 0.17 0.175 0.26 0.209 0.29 0.225 0.4 0.32 sensor at 60? . 8 0.14 0.175 0.19 0.192 0.34 0.228 0.44 0.32
Ave 0.19 0.17 0.23 0.20 0.33 0.22 0.45 0.32
Table 1. Discharge current risetime comparison for four
capacitances at charging voltages ranging from 1V, 2V, 4V and 8V.
Table 2 shows the sensitivity of discharge current per charging voltage (mA/V) for four capacitances. There REFERENCES is about 9% different between experimental and  Socket Device Model Testing, ESD Association simulation results for both 11.5pF and 4.4pF. technical report TR 08-00  The SDM Test Method: Past, Present, and Future. Michael Chaine, Ion Barth, Tilo Bordeck, Leo G. Henry, 0.5pF 2.2pF 4.4pF 11.5pF Mark A. Kelly, and Tom Meusen Compliance Engineering. exp sim exp sim exp sim exp sim Ce-mag.com 09-10/2001 mA/V 2.14 2.8 5.1 8.1 12.2 11.1 17.8 16.2  Charge Device Model. ESD Association standard test Table 2. Discharge current per charging voltage sensitivity method. ESD STM5.3.1-1999 comparison for four capacitances.  Standardized Direct Charged Device ESD Test for Magnetoresistive Recording Heads II. Lydia Baril, Tim Table 3 shows the pulse width for four capacitances at Cheung, Albert Wallash. Submitted to EOS/ESD charging voltage ranging from 1V, 2V, 4V and 8V. Symposium 2002 The experimental and simulation results are close, but  “Direct Charging” Charge Device Model Testing of the simulation data are slightly less than the Magnetoresistive Recording Heads. Tim Cheung, experimental data. Selection of component values for EOS/ESD Symposium 1997 simulation can affect the results.
VI. Conclusion We have learned from the manual testing, and Pspice simulations, the spark resistance; parasitic capacitance and inductance of the DCDM system can change the characteristic of a DCDM discharging current waveform. The Pspice simulation and experimental data are comparable. The data shows the grounded suspension GMR head is more sensitive than the floating suspension GMR head. The spark resistance, the DCDM parasitic inductance and capacitance can affect the results of ESD testing. More spark resistance experiments need to be conducted and analyzed. It is a big challenge for the test equipment vendors to design a DCDM tester with very small parasitic inductance and capacitance.