Editors Note: Figure numbering is continued from Part 1.

Previously, in Part 1 of this article, a discussion of ESD industry practices and Standard Test Methods for materials assessment was outlined describing the types of polymeric materials (Antistatic, Carbon Loaded and Inherently Conductive Polymers) used in handling and protecting ESD sensitive components. Here in Part 2, a comparison of the relative performance of the polymer types when subjected to a rigorous battery of ESD tests will be presented.

How do the polymer types compare?

This article summarizes a detailed investigation and comparison of three types of material – antistats, carbon loaded, and inherently conductive polymers (ICPs) in the following areas:

1. Surface Resistance

2. Non Contact Voltage Measurements

3. Faraday Cup Measurements

4. Static Decay

The test methods provide a step-by-step process for the evaluation of different material types inside and outside of the EPA (Electrostatic discharge Protected Area) for one-way transport, reuse and storage.

Surface Resistance per ANSI/ESD STM11.13-2004 and ANSI/ESD STM11.11-2001

Figures 7 and 8 show test methods for evaluating the resistance of a vacuum formed tray set on an ICP example.

Figure 7: Surface resistance measurements of ICP material.

Figure 8: Two point resistance measure-ment showing detail of material draw.

1. Topically Coated Antistatic Tray

A sample set of six vacuum formed trays were preconditioned at 10.3% RH, 72.5⁰F for 72 hours. Summary Table 3 illustrates the results of a topically treated antistatic polymer APET (Amorphous polyethylene terephthalate) tray. The constant voltage over the specimen was 100 volts. The vacuum formed tray results were insulative in part of the findings. Antistatic treatments are intended for one-way shipments, not for multiple uses. It is also important to note that passing surface resistance testing at 50% RH does not insure favorable results at 12% RH +/-3% RH after 48 -72 hours of preconditioning. If properly formed, an amine free, antistatic tray will function well for cleanroom compatibility for ISO Class 6 and above.

Table 3

2. Carbon Coated Tray

The carbon coated APET trays were measured in the conductive range (Table 3) per ANSI/ESD STM11.13-2004 at 10.3% RH, 72.50F after 72 hours of preconditioning. Constant voltage over the specimen was 2.7 volts. Depending upon the application desired, surface resistance can be targeted in both the conductive and static dissipative ranges. The carbon trays would be considered humidity independent. However, the drawback to this technology is poor rub resistance, lack of transparency, and unsuitability for cleanroom use.

3. ICP Planar and with Vacuum Formed Draw

The ANSI/ESD STM11.11-2001 results of the ICP planar sheets and with “draws” (Figures 7 and 8) were in the low static dissipative range at 12.3% RH, 73.10F after 72 hours of preconditioning (Table 3). The constant voltage over the specimen was 4.7 volts. At this low surface resistance, the ICP sheets have a secondary benefit of being not only humidity independent, but also transparent.

Table 3 illustrates vacuum formed surface resistance results per ANSI/ESD STM11.13-2004 at 12.3% RH, 73.10 F after 72 hours of preconditioning for the ICP sections that were elongated or drawn (Figure 8). Results above were close to the planar ICP surface resistance (Figure 7). The constant voltage over the specimen was 4.7 volts.

In short, the surface resistance findings for both the ICP and carbon coated trays were very stable and within the parameters of acceptance for the intended use. However, the ICPs do not slough particles and are optically transparent, thereby suitable for cleanroom use and bar coding. The antistatic trays did not produce favorable results in this evaluation.

Non Contact Voltage Measurement

1. Antistatic Tray and Tray with Cavities

Both antistatic trays generated less than +/-200 volts after being charged to +1000 volts and placed on a grounded surface for measurement (Table 4) at 13.3% RH, 73.10F after 72 hours of preconditioning.

Table 4

Figure 9: Non-contact voltage measure-ment of carbon coated sample. Note opacity of material.

2. Carbon Coated Base Tray

The carbon trays had very conductive surface resistance values that were low charging. However, the inside of one tray exhibited a hot spot of -159 volts (Table 4) at 11.1% RH, 72.10F after 72 hours of preconditioning.

3. PEDOT-Coated ICP Tray with Cavities

Table 4 illustrates that the ICP results that were well below +/-200 volts at 11.1% RH, 72.10F after 72 hours of preconditioning.

Overall, the products with lower surface resistance generated less voltage after being measured on a grounded plate with a non-contact voltmeter. With static control materials, charge generation does not necessarily correlate with surface resistance. A non-contact voltmeter is a useful tool in pinpointing hot spots that may be present after vacuum forming trays.

Faraday Cup Measurements

1. Antistatic Trays

All trays were low charge generating upon being dropped into a Faraday Cup after being charged to +1000 volts on a charge plate monitor (Table 5) at 12.1% RH, 72.50 F per Modified ESD Adv.11.2-1995 after 48 hours of preconditioning.

2. Carbon Based Trays

The Carbon based trays generated low charging results (Table 5) after 48 hours of preconditioning at12.1% RH 72.50F per modified ESD Adv. 11.2-1995.

Table 5

Figure 10: A carbon loaded material is placed in the Faraday cup for measurement of residual charge.

3. ICP Sheets with Draws

The ICP trays exhibited low charging after being charged up to +1000 volts and then allowed to free fall into a large Faraday Cup (Table 5) after 48 hours of preconditioning at 12.1% RH, 72.50F per modified ESD Adv. 11.2-1995. As illustrated in Table 5, the three material types exhibited residual charges that were less than 1.0 nano-Coulomb (nC). Table 5 depicts an average of six individual measurements along with the corresponding standard deviation results.

Figure 11: A sample of ICP material is tested in the Faraday cup. Note transparency of sample.

Static Decay

This test method is used to determine the rate of decay on a material to simulate performance after being grounded on an ESD workstation.

Figure 12: Instrumentation for measuring static decay

1. Antistatic Trays

The tray was small enough for placement into the fixturing of the Federal 101C Standard Method 4046 testing device. Six clear polymer tops were tested at  +/-1000 volts to +/-100 volts and produced readings for a static decay in less than 2.0 seconds. This test was designed for homogeneous materials of planar construction, but the evaluation was conducted so that results could be compared to the other polymer types at 12.3% RH, 73.80F after 48 hours of preconditioning. Note: Table 6 illustrates the average decay times as well as the standard deviation.

Table 6

2. Carbon Trays

The carbon tray results exhibited rapid decay (Table 6) after being preconditioned for 72 hours at 12.3% RH, 73.80F.

3. ICP Sheets with Draw

In this case, the PEDOT sheets with thermoformed trays would not fit into the fixturing of the decay testing device. Figure 13 illustrates an industry practice employed for evaluation of the ICP specimens. These samples were tested after 48 hours of preconditioning at 12.3% RH, 73.80F after being charged to +/-1000 volts to +/-100 volts. The decay time results for the ICP sheets with draws are under 2.0 seconds (Table 6). This test method is used to determine the rate of decay on a material to simulate performance after being grounded on an ESD workstation. The decay times were well under 2.0 seconds, in fact, less than 0.1 seconds. Both the ICP and carbon trays exhibited much quicker decay than the antistat material.

Figure 13: Charged plate Monitor fixture for static decay measurement.

Conclusions

The antistatic tray lids produced results in the static dissipative and insulative range. Surface resistance results should be less than 1.0 x 1011 ohms after preconditioning for 48 – 72 hours at 12% +/-3%RH, 730F +/-50F in accordance with ANSI/ESD S541-2003. Tribocharge generation, Faraday Cup and Static Decay results were within acceptable parameters. However, if a topically coated antistatic substrate is subjected to de-ionized water bathing or IPA rinsing, the electrostatic properties will be lost. Likewise, topically treated antistatic polymers are humidity dependent and their ESD properties are reduced or hindered in low relative humidities. One more consideration is to limit the use of antistats in cleanroom environments and in direct contact with devices where antistatic migration is considered unfavorable.

The carbon tray results generated conductive readings. The charge generation, Faraday Cup and Static Decay results were low. A drawback of carbon is unfavorable particle generation and lack of transparency. The use of carbon has been limited for use in Class 10,000 (ISO Class 7) cleanrooms and below due to particle generation or sloughing.

The new families of Inherently Conductive Polymers (ICPs) represented in this article have produced results in the static dissipative range with humidity independence. The tribocharge generation, Faraday Cup and static decay results were favorable. ICPs offer reusability, regular IPA rinsing or de-ionized water bathing, transparency that facilitates product identification, and bar code readability without opening a vacuum formed clamshell or tray. Although the ICP raw materials can be more expensive relative to topical antistatic agents or carbon fillers, their ability to function after repeated use in high and low RH reduces the need for reorders with overall cost savings. g

Note : Detailed results of the summary tables can be downloaded by going to www.esdrmv.com and selecting the Conformity reference tab.

About the Author

Bob Vermillion, CPP, is a Certified ESD & Product Safety Engineer-NARTE and holds a US Patent with several patents pending. One of his recent developments has been approved for a NASA Mars Mission. Bob is a member of the ESD Association Standards Committee and ESDA Packaging Working Group 11 coauthoring ANSI/ESD S541-2003. Bob conducts ESD Seminars in the USA and abroad as well as California State Polytechnic University, San Jose State and Clemson Universities. RMV Technology Group, LLC, a member of the American Council of Independent Laboratories, provides ESD materials testing, training, cleanroom and facility troubleshooting/auditing. Bob can be reached at 925-673-0225 or bob@esdrmv.com.