Fowler Associates Labs

Part II: From MIL-P-81705 (AS)
to MIL-B-81705C

Over the past twenty years, MIL-B-81705 has helped shape static-control products and methods. Here's part two of the story of how this key military specification was itself shaped and grew.

(View Part I of this Article)

Charles R. Hynes
ESD Consultant, Contributing Editor, Minnitonka, MN

(Editor's Note: In the December/January issue, Chuck Hynes began his exposition of MIL-B-81705, a specification for the barrier materials from which sleeves, tubes, and Class-A or F, Style 1 or 2 Mil-B-117 bags are fabricated. The original version of this specification dates back to 1969 as MIL-P-81705 (AS). In Part 1, the author brought us up to the early 1970's; this month, his review continues from that point.)

We left MIL-B-81705 at Revision B, dated 15 August 1974. Since then, the specification has been amended three times. When Amendment Two was released, all changes made in Amendment One were repeated and some additional ones were added. In Paragraph 1.1 SCOPE, "opaque" was dropped and "associated airborne components" became "associated higher assemblies." This last change broadened the applicability of MIL-B-81705.

Amendments

The titles of Type-I and Type-II materials in Paragraph 1.2 were changed. Under Type I, the word "opaque" was dropped and "watervaporproof," and "greaseproof" were added, Type-I material was still designated electrostatic and electromagnetic protective. Type-II material changed from being simply "transparent" to "transparent, waterproof, electrostatic protective."

The color specification (Federal Standard 595) was dropped from Paragraph 2.1, and two new documents appeared: DOD-STD-1686 and DOD-HDBK-263. Refereeing to these two documents would have a major impact on MIL-B-81705. As we will see later, the material-classification system (Antistatic-Static Dissipative-Conductive) was born in those two documents.

In Paragraph 3.3.1, covering Type-I materials, the word "aluminum" was changed to "metal." That change opened the door to using nickel, stainless steel, copper, and possibly gold or silver.

Color Blind

In Paragraph 3.3.2, covering Type-II materials, "any tinted color" became acceptable, and words like "unsupported," "nonlaminated," "chemically neutral," and "homogenous antistatic resin mix" disappeared.

With the color identifier gone, it became mandatory in paragraph 3.6 to mark the materials. Type-I material was to be marked "EMI/STATIC SHIELD," and Type-II material was to be marked "ANTISTAT." The paragraph read, "markings (for Type II) shall be embossed," and "markings shall be printed" in groups that are one inch apart, on the length of the material, and every six inches across the width of the material.

Emboss or Paint?

The firms that began producing Type-II materials were faced with a production problem. If embossing was, indeed, a requirement, they would have to purchase costly embossing equipment and install it on every extrusion line that produced Type-II material.

Surface printing the material was a much less-expensive alternative; however, it had some problems, too. Waterproof inks that would adhere to the surface were available; however, the hygroscopic nature of the material and the migration of the antistatic additive caused the bond between the ink and the surface of the material to weaken. In less than a month, some of the printing could be obliterated in normal handling.

When changing from a color identifier to embossing or printing, the specification writer faces several problems. No one can think of everything, and no one can be an expert in every facet of life. The identification of Type-II material is a classic example of what can happen when good Government-Industry communications are lacking.

Antistat

Another critical aspect of Paragraph 3.6, Amendment Two, was including the word "antistat." This term took on new meaning when DOD-STD-1686 and DOD-HDBK-263 were released. In Paragraph 7.1.2.3 of HDBK-263; an antistatic protective material was defined as follows:

"Antistatic materials are those materials having surface resistivities equal to or grater than 10⁹ and equal to or less than 10¹⁴ W/sq. These materials include hygroscopic antistatic materials such as MIL-B-81705, Type II..."

Surface resistivity, as measured by ASTM D-257, was introduced as another test for Type II materials, and a range was defined; however, this test was not included in either Amendment Two or Three. Until the "C" revision of MIL-B-81705 is approved and released, surface-resistivity measurements are not part of qualification testing required for approval of an MIL-B-81705, Type-11 material, except by reference to a definition in HDBK-263. This is another example of the chain reactions that occur when documents are interrelated.

Another important point should be made. When MIL-B-81705 began, the title contained the words "Electrostatic-free." Those words still appear in the title of Amendment Three; however, the term "Electrostatic protective" was taken from the text of Amendment Two. With the advent of the term "anti-static," definitions of these terms became more important.

Triboelectrification

The terminology became even more complex with the introduction of triboelectrification in HDBK-263. Here is a complete reprint of Paragraph 7.1.4 of HDBK-263, dated 2 May 1980:

"7.14 Triboelectric properties. Many antistatic materials such as MIL-B-81705 Type II, virgin cotton, unfinished wood, and some paper products provide good protection against generation of static electricity from triboelectric effects. As described in 4.2, the amount of charge created by triboelectric effects is dependent upon factors such as surface smoothness, lubricity, contact area, pressure, and speed of rubbing. Hygroscopic-type antistatic materials processing sweat layers have good lubricity and generally are poor generators of static.

"Many conductive and static-dissipative materials also provide protection from triboelectric generation. Some metals, however, will create significant charges from triboelectric generation as indicated in the triboelectric series. Aluminum, for example, when rubbed with a common plastic, can generate substantial electrostatic charges. Although a conductive material distributes a charge over its surface, the other substance with with it is rubbed, or from which it is separated, especially if it is insulative, can become highly charged."

If you have followed this series of articles, you can understand the difficulty we had with the definitions of "electrostatic protective" and "electromagnetic protective," which we encountered in MIL-P-116 and MIL-O-17555. Clearly defining these terms is an extremely difficult task.

Let's begin with "electrostatic free." This term can only apply to the material itself. It implies that the material will not hold a static charge. If we define a static charge as an excess or deficiency of electrons on or in a material, the definition is meaningless unless we use the term relative to ground or atmosphere.

Metallic Charging

Even a metal object can become electrostatically charged when isolated from the ground in a dry environment. While the charge may eventually dissipate, during the period of time that the object remains charged, it cannot be "electrostatic-free." This concept probably led to the use of the static-decay test in Method 4046 of Federal Standard 101. Thus, for a number of years, a material that could meet the two-second decay limit was considered electrostatic-free.

When the term changed to "electro-static protective," it could apply to more than the material itself. The term now implied that Type-II material had a relationship to whatever it contacted and would also provide barrier properties to external electrostatic charges. But that was already a function of the Type-I, electromagnetic-protective material. Coincidentally, the inclusion of a metal layer in the Type-I material, providing 25 decibels of attenuation for RF energy, automatically made Type-I material an effective Faraday cage, which also would act as an electrostatic barrier.

The interior surface of MIL-B-81705 Type-I material was a static generator. A paper presented at the First Annual EOS/ESD Symposium, by J. R. Huntsman and D. M. Yenni, entitled "The Deficiencies in Military Specification MIL-B-81705..." pointed out the shortcomings of MIL-B-81705 Type-I material.

On the Inside

Triboelectric tests showed that sensitive components could become charged if they contacted the "inside" surface of the material. How could this be? When tested by Method 4046 of Federal Standard 101, Type-I materials showed extremely fast decay times. Even buried conductive grid bags (then newly developed) and other metallized laminates that were transparent tested extremely well.

Investigators began to look at the interrelationship of triboelectrification, resistivity, static decay, electrostatic shielding, and electromagnetic shielding. The situation was confusing and became even more so when vendors with vested interests began using favorable test data to convince users that their products were effective.

If the surface resistivity of a specimen indicated that the material was insulative but a static-decay test showed that the specimen would drain a charge in 0.02 sec, the product was released for sale. Conversely, if a material had a relatively low resistivity number, some claimed that their material provided electrostatic shielding, even though static-decay test data did not reflect performance equal to that of a Type-I material.

The EIA forms PEPS

Because of the confusion, the Electronics Industry Association formed the Packaging of Electronic Parts and Supplies Committee and the EIA's PEPS Committee, as it became known, began to wrestle with the problems. It took over seven years to produce the new ANSI/EIA 541 Standard. During its development, the standard was referred to an Interim Standard No. 5 (IS-5).

Chronicling the history of IS-5 and 541 would make a separate book. The final contents of that document represent the combined thinking of 100 or more people who participated in it's drafting.

A list of PEPS committee members reads like the "Who's Who" of the EOS/ESD world; many of these individuals were also members of the EOS/ESD Association. In the beginning, representation came from users, vendors, and the military, with vendors dominating. Those vendor representatives, however, were extremely responsible. The voice of vested interests was tempered and, in some cases, rejected by those who wanted to end the confusion.

"Antistatic" Redefined

One of the most important statements to hit the ESD world is contained in paragraph two of the foreword to the document. It reads as follows:

" 'Antistatic' no longer refers to a resistivity range. In this standard, 'Antistatic' refers to a material's ability to resist triboelectric charge generation. A material's propensity depends upon the nature of the material itself and the material with which it is in contact also with the means of surface separation. The antistatic property is not a dependent function of material resistivity. Material resistivity is an intrinsic property used to define its degree of conductivity without regard to other materials. Resistivity measurements depend on the apparatus, sample size and shape, and voltage used. Not all materials are ohmic (linear) in their resistivity."

Suddenly, "antistatic" took on a whole new meaning. The classification system in HDBK-263 conflicted with this new definition. In addition, the term "anitstatic," when applied to MIL-B-81705, created problems. The development of ANSI/EIA 541 will be the topic of a future article, so for now, let's return to MIL-B-81705.

On December 1, 1982, a triservice meeting was held at the US Naval Air Development Center (Warminster, PA) to discuss updating of MIL-B-81705. A result of that was the definition of an effort to compare the following three test procedures for use in evaluating the electrostatic properties of materials:

The then-current static-decay test for evaluating the electrostatic properties of materials,

Surface and volume resistivity,

Triboelectric test method. (NASA developed the original "tribo" test).

Specimens from no less than six different MIL-SPEC Barrier materials were to become part of the test. Two of the six, of course, were MIL-B-81705 Type-I and Type-II materials.

On June 18, 1983, a contract was awarded to a private contractor for the services and equipment needed to run the tests. They completed the work and submitted a final report on March 28, 1984. The report, along with supporting data, is over 150 pages long, and, to my knowledge, has never been made public.

On Report

To paraphrase key sections of the report: All three tests can, to some degree, provide useful information about the antistatic properties of homogeneous MIL-B-81705, Type-II materials (for example, pink poly).

On the other hand, none of the test methods, as configured in 1983, could adequately evaluate the antistatic properties of homogeneous MIL-B-81705, Type-I material or any other laminated, nonhomogeneous material.

Laminates with buried, highly conductive layers behave differently from homogeneous materials.

When a nonconductive surface is backed by a more conductive surface, the nonconductive surface takes on the apparent decay characteristics of the most conductive layer when monitored by a field meter. The phenomenon of voltage suppression was apparently involved.

If a sample had an antistatic path as long as the specimen, a static decay test would reveal the existence of "dead spots" in the material but a surface resistivity test might indicate that the material was a good antistatic material.

The NASA triboelectric test was found to have some inherent problems; it was not reliable in all cases.

The dielectric constant of a material was found to have a major effect on the decy-time characteristic of that material. If a material's dielectric constant varies from sample to sample, capacitance and decay time will vary, even if the surface resistance is the same.

When attempting to correlate surface-resistivity test results with results from other methods, such as static decay, it is the measured resistance value and not the normalized resistivity value that must be used.

Nothing Works

The report stated that none of the test methods, as configured in the early 1980s could be relied upon to test both Type-I and Type-II MIL-B-81705 materials.

The contractor developed some new fixtures and procedures that eliminated the voltage-suppression effects of laminated Type- I material and provided evidence that with modifications to Method 4046 of Federal Standard 101, static-decay testing would do everything that the NASA tribo test would do. However, the NASA test could not do everything that the modified Method 4046 test could do.

The individuals responsible for revising MIL-B-81705 after receiving this report must have felt like tennis balls in a doubles match. The materials in question were used to package millions of dollars worth of high-reliability items. Changes made to test methods for MIL-B-81750 materials would create a tremendous chain reaction throughout the entire world of ESD protection.

New Proposals

Industry investigators suggested new tests for buried conductive layered laminates. NASA felt that its tribo/decay test was still a good one, and ASTM D-257 surface resistivity tests came under fire. Method 4046, as configured, was not the whole answer.

The EIA PEPS Committee has clearly changed the entire meaning of the word "antistat." Industries were using new, transparent, buried-conductive-layer materials. Vested interests pressed the ESD community to buy products that "met the requirements of MIL-B-81705."

Electrostatic protection is only one of the material properties listed in the specification's table of properties, shown in the December/January issue. We have yet to deal with EMI protection, corrosion, water-vapor properties, or triboelectric protection.

The next article in this series will deal with ANSI/EIA 541, including some of the issues raised in this article, and we will examine its impact on military specifications and standards. [EOS/ESD Technology Editor's correction: Part one of this article stated that "the Navy (NAVSEA)...(was) the custodian of (the specification)." The Navy is the custodian, but NAVAIR is the correct command. We regret the error.]