Quick Check of ESD Bags for Shielding Efficiency

José D. Sancho NASA – GSFC Bldg. 32 Rm. E120D, Greenbelt MD USA

tel.: 301-614-6083, fax: 301-614-5599, e-mail: jsancho@pop300.gsfc.nasa.gov

Abstract - This paper proposes an economical ESD bag test which would prevent defective ESD bags from being used to store ESDS hardware. This method measures voltage rather, than energy attenuation to classify the bags in a go/no-go test. There is a chance that some marginally good bags will be rejected but the cost of discarding those bags would be offset by the cost using the ESDA compliant equipment.

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I. Introduction The recent legal settlement between Honeywell and the Department of Justice awarding $2.3M to the US Government alerted the NASA Workmanship Standards Technical Committee to the fact that some of the ESD bags used to store “flight” boards may not be compliant with the NASA ESD requirements standard (ESD S20.20-1999). The ESD bags were made from a material which the company had certified as compliant with MIL-PRF-81705D but were never tested. During the investigation that followed at NASA Goddard Space Flight Center (GSFC), it was found that none of the ESD bag manufacturers would warranty their bags for more than a year. NASA GSFC had assemblies in bags for 10 years or longer! There was a need to quickly screen the ESD bags in use to verify that they were still protecting the hardware. GSFC recommends that all the Electrostatic Discharge Sensitive (ESDS) hardware be protected by ESD bags meeting Type III requirements and requires it for Flight hardware. Hardware to be used for ground support is normally protected by using Type I bags, per MIL-PRF-81705D, and then it is stored in protected cabinets. To reduce the task of testing all the bags in use, it was decided to check only the most critical electrical parameter to GSFC: the bags shielding effect. Only suspected (old) bags were checked using this parameter.

II. Rationale The test was designed to make use of readily available parts and to provide rapid results. It was intended to simulate the conditions encountered by the ESDS parts and assemblies during transportation within the regular lab environment. This scenario is the likeliest

cause of ESD-related damage of ESDS hardware while inside an ESD bag. The test setup does not check if the bag is compliant to ANSI/ESD S541, but rather checks for voltage attenuation comparable with that of the standard. The procedure uses a triboelectrically charged piece of polymer which provides several times the energy required in ESD STM11.31 under normal conditions. The main goal for the test is to validate the protection given to the parts while they are inside the bags.

III. Measurement Techniques This paper proposes a go/no-go test method to check for degradation of the ESD dissipative properties of the material used to fabricate the ESD protective bags. It is not intended to replace the ESD bag and material evaluation method described in ANSI/ESD STM 11.31. However, it will provide a rough indication of the ESD shielding protection provided by the bags being tested. The test takes a practical approach by using a triboelectrically charged sheet of polymer as the charging source and a block of aluminum as a discharge electrode. A sensing probe is positioned between the discharging block and a ground plane for calibration. During the actual test the probe is inserted in the ESD bag under test and both are positioned between the discharge block and the ground plane. The sensitivity of the system was calibrated by choosing the value of a 10 MΩ series resistor and a variable shunt resistor (500 KΩ) inserted between the capacitor sensor and the comparator. The maximum value of the shunt was experimentally chosen to give a good balance between range and ease of setting.

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IV. Equipment This technique takes advantage of an inexpensive, self-contained indicator consisting of a battery operated comparator circuit with a LED output (Figure 1). In fact the unit used was the “Mystery Widget 3” given as a “Promo” at the 2004 ESDA Symposium by Credence Technologies. The Sensor Probe consisted of a capacitor formed by two single sided PC boards, facing each other and separated by Nylon washers. The sensor is connected to the detector by a short piece of coaxial cable and isolated by a high resistance voltage divider (Figure 2).

The calibration of the setup was originally done using a 3M Model 718 Static Sensor and a Fluke 79 multimeter to check the capacitance of the sensor probe. A more accurate value of the attenuation of the bag shielding and the actual charging of the setup can be done using a Hi-pot tester and a high voltage capacitor. The use of a fast digital scope will allow

viewing the waveforms and calculating the actual energies involved.

V. Models Two distinct theoretical models of the design were created to test the hypothesis that the setup was valid and the results consistent and comparable with the ANSI/ESD STM11.31 guidelines. The first model consisted of three capacitors in series to simulate a sensing probe positioned between the discharge block and ground (Figure 3). The values of the capacitors were chosen such that C1 || C3 ≈ C2 so that the voltage developed across the middle capacitor

is attenuated at least 30 times when measured across >10 MΩ resistor ladder and fed to the input of the comparator across a <500 KΩ variable resistor. To generate a peak voltage of ~1.5 V at the comparator input (the sensitivity of the Mystery Widget 3) we needed at least 45 V at the input discharge block. The second model includes the ESD bag in the circuit. It used the same scheme while modeling the ESD bag as a parallel resistor between the discharge block and ground (Figure 1). Depending on the bag type, the R1 value can vary from 103 – 1011 Ω. Under these conditions, the peak voltage at the discharge block must be at least 1500 V to provide the minimum dynamic range. This is easily achievable with a triboelectrically charged piece of polymer if the capacitance of the system is low. The choice of the capacitor values for the voltage divider and attenuator used, were based on the maximum value easily achievable. The voltage at discharge block is very hard to verify due to the I*R

0.2”

3.0”

2.2”

Edge Conn

5” coaxial cable

Figure 2 Capacitor Sensor Detail

3 V Battery

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+

R14 0 Ω

R5 1M Ω

R11 1M Ω

R6 100 KΩ

C1 0.1 µf

R12 475 Ω R7

249 Ω

R8 100 KΩ

R2 10 MΩ

R1 10 MΩ

R10 10 MΩ R4 10 MΩ

R3 10 MΩ R9 10 MΩ

8

4 7

2 1

3

5

6

TLV 3402 or equivalent

Courtesy and by permission of Credence Technologies

Figure 1 Credence Mystery Widget 3

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loss through R1. However, that loss is part of the designed protection offered by the bag; therefore it should be part of the measured attenuation. The values measured in the actual setup were:

C1 ≈ 57 pF, C2 ≈ 32 pF C3 ≈ 77 pF, R1 ≈ 103 – 1011 Ω

Figure 3 Models used for Test Setup Design

VI. Basic Assumptions and Verifications

1) The human eye would not respond to LED lighting of less than 0.1 second. This assumption proved to be wrong! Using a TTL pulse 1 µsec wide and a TTL LED indicator we were able to see the indicator light up. Since the input of the comparator indicator already has a 0.1 µF capacitor at the input, the model was modified to add a 10 MΩ resistor between the sensor probe and the indicator input. This modification will slow down the transfer of charge from the sensing probe and the indicator input capacitor.

2) The comparator have >10 MΩ input impedance and will respond instantly, when the voltage across the input leads reaches ±1.5 Volts. The response of the “Mystery Widget 3” solves the problem posed by the wrong assumption above. It allowed slowing down the transfer of charge from the capacitor sensor to the comparator indicator. The fast charging and slow discharging of the circuit allows for greater dynamic range for the test.

3) The limits for this test were based on the requirements of MIL-PRF-81705D Para. 4.9.6.4, it defines a maximum of 30 V inside the protected bag, for a pulse of 1 kV maximum on the outside of the bag. The limit set on ANSI/ESD 541 is given in terms of energy instead of voltage, but the converted value is essentially the same: 50 nJ for a

50 µJ input with 1 kV as maximum amplitude, which is ~31.6 V. The best response of the circuit was obtained when the input at the discharge block peaked at ~45 V (without a bag in the circuit). This value also provides a good safe voltage for the items usually stored inside the ESD bags at GSFC. This value is about 50% higher than the recommended test voltage calculated from the ANSI/ESD S541-2003 suggested limit. The minimum input voltage was then increased to 1500 V to obtain a similar attenuation value. This value can go as high as 15 kV depending on environmental conditions and materials used. Therefore, some good bags may be rejected if R1 is large and the ambient is too dry.

4) The polymer sheets used can be charged repeatedly to similar energy values if similar materials are used for triboelectric charging. The size of the polymer sheet used provided a limiting factor on the maximum amount of charge transferred to the discharge electrode under the environmental conditions proposed. The minimum amount of charge can be regulated by how close the sheet is positioned in reference to the discharge electrode before it is grounded.

5) There is enough energy transferred from the charged plastic to the discharge electrode to provide at least 40 dB dynamic range during the test of an ESD bag and can detect at least 30 dB during the test. Under ideal conditions the test provided ~42 dB dynamic range. Trying to increase the amount of charge (size) on the polymer sheet produced ESD events which obscured the results of the tests and the calibration was not repeatable. Setting the response limit to around 30 dB was fairly repeatable, that is, the indicator will light if the bag shielding falls below 30 dB.

6) Energy transfer losses do not seriously affect the final results of the test, nor does the polarity of the pulse. This assumption was verified during the tests above.

7) The detection range of the test is repeatable in a normal laboratory environment. By repeating the test many times and using a differential measurement on an oscilloscope under varying conditions, an average error of ±6 dB was observed.

8) EMI produced during the test is minimal and will only contribute to false reject results.

C1

C2

C3

C1

C2

C3

R1

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EMI type failures were not apparent until very high voltages were induced. Charging voltages in the plastic of >25 kV EMI became an issue during the charge transfer to the discharge electrode.

9) Arcing through the bag due to the high voltages used, will not have enough energy to permanently damage the bag and will only contribute to false rejections. Arcing through the dissipative layer was observed at higher discharge voltages. Subsequent measurements made at lower voltages duplicated measurements made before the arcing had occurred.

10)The test can distinguish between bad and good bags on a repeatable basis with low occurrences of false rejects. All the new bags tested passed the test. As older bags were tested the failure rate increased. These results were not directly measured for all the bags but were statistically significant and the decision was made for GSFC to avoid reusing bags on flight or critical equipment.

VII. Calibration Procedure The accuracy and repeatability of this setup are not as good as the values obtained with the test setup described in ANSI/ESD SMT11.31. However, it provides a good indication of degraded bag performance. The system was calibrated by charging the discharge electrode with ~45 V and discharging it to ground then varying the value of the shunt resistor

until the indicator no longer lighted. This low electrostatic voltage is difficult to achieve consistently, but by choosing a small piece of polymer about 0.5” by 2”, it was possible to adjust the voltage measured on the aluminum block used as the discharge electrode. This provides a fault indication when the bag has less than 30 dB of voltage attenuation if 1500 V is applied to the discharge electrode when the bag is tested. Much higher voltages tended to breakdown the inner layers of some bags but the normal ESD protection of the bag was not compromised by these discharges. Any combinations of materials which provide large triboelectric charges may be used. See the ESDA ADV11.2-1995 for guidance. Triobelectrically charge a small sheet of polymer with a piece of non-cotton cloth (a 3”x3” piece will provide enough energy to charge to the aluminum block to >1500 V) and place it on top of the discharge electrode which has been temporarily grounded. With the sheet in close proximity remove the ground, remove the polymer sheet and again ground the discharge electrode. If the bag is providing the proper electrostatic shielding the indicator will not light. Using the static meter verify that the charge in the polymer sheet is at least 9 kV. This test is valid when the room relative humidity hovers around 30% to 40% RH. Below 30% RH the polymer usually charges over 20 kV and marginally good bags will fail. Above 40% the charge in the plastic tends to drop during the test. See schematic of the test set up in Figure 4.

~1500V Triboelectric Charged Insulator

Discharge Electrode 3”X3”X1” Aluminum block

Credence Mystery Widget 3

~500 KΩ

~57 pf

~77 pf

32pf

Bag under test

Figure 4 Test Setup Schematic

10 MΩ

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Figures 5-7 show the actual setup and calculated and actual waveforms at the connector to the comparator with the “Mystery Widget 3” in place during calibration.

Recommendations As a result of these tests GSFC will recommend to its active projects to use only new ESD bags for Flight Hardware and also to transport highly sensitive items within ESD totes to provide double protection.

Acknowledgements Thanks to Mike Sampson, Jeanette Plante and Henning Leidecker from NASA-GSFC for the help and encouragement during the write-up, review and authorization request handling of the paper. Also to Frank Magnifico, Packaging & Lab Manager for NAVAIR for the information provided on the bag manufacturing process and tests and to Vladimir Kraz from Credence Technologies/3M for the review and suggestions on the draft of this paper and the use of the Mystery Widget 3.

Figure 6 Scope picture Figure 5 Actual Test Setup

1800

Measured at the divider output Min. Input Voltage for Indicator to

-1.00

0.00

1.00

2.00

3.00

4.00

Volt

0 200 400 600 800 1000 1200 1400 1600

Microsecond

V (t) = Vo*(e -t/RC - e -Rt/2L

)

Figure 7 Capacitor discharge as seen at the divider output

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