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First Published in EOS/ESD Technology Dec/Jan 1990 From MIL-P-81705 (AS) Over the past twenty years, MIL-B-81705 has helped shape static-control products and methodologies. Here's how this key military specification has grown and changed with time. (View Part II of this article) Charles R. Hynes, ESD Consultant,
Contributing Editor, Minnetonda, MN MIL-B-81705 is a specification
for barrier materials (not bags) from which sleeves, tubes, and Class
-A or F, Style 1 or 2 MIL-B-117 bags are fabricated. (There is no such
thing as a Type-I or Type-II 81705 bag, despite frequent references
to the contrary.) The development of 81705 makes an interesting story. The original version of this specification was dated July 14, 1969, as MIL-P-81705 (AS). Unfortunately, my files do not contain a copy of that document. If someone still has a copy, I'd like to see it. As is the case with all Mil-Specs, the letter offset by hyphens relates to the first word in the title of the document. Thus, the "P" in the original version, and the "B" in the current document indicate a title change. The "(AS)" suffix indicates that the Navy (NAVSEA in this case) was the custodian of the document; it still is. The suffix further indicates that the specification was originally intended for Navy procurement. On February 2, 1972, the specification's
first revision become effective. The number changed to MIL-B-81705-A.
The title was: No reference was made to MIL-STD-1686 because it didn't exist is 1972. It appeared eight years later, when 81705 lost its (AS) suffix and became applicable to "all departments and agencies of the Department of Defense." The scope of the document covered "barrier materials for the packaging of missiles, explosive powder, and electrosensitive devices, micro circuits, semiconductors, thin-film resistors and associated airborne components." At this point, we referred to Paragraph 6.1, Intended
use. Under Paragraph 6.3, we find a footnote:
(*Because of the unavailability of the Type I Transport material, it has been deleted from the specification. There is a continuing need for an electrostatic-free transparent material and for this reason industry is encouraged to develop such a material). As of February 2, 1972, a transparent, electrostatic-proctective material had not yet been approved, although industry was at work on what was to become pink poly. From Table 1, we find that the only electrostatic property test that was required was a static-decay test per FedSTD 101, Method 4046 and a maximum 2-sec decay rate was established. A test for electromagnetic protection was not included. MIL-P-116, MIL-P-17555, and MIL-STD-794, while they existed, were not referenced. MIL-B-81705-A didn't last long. Revision B became effective two years later, August 15, 1974. The first amendment came on March 19, 1989. The second amendment came April 21, 1982, and the third came June 9, 1983. In May 1987, Revision C was approved for public release and became effective January 25, 1989. (When there is no effective date on a copy of a standard or specification, anything contained in that draft remains subject to change. Unless you see a release date on a copy of a specification in the upper-right-hand corner, don't let anyone tell you that the document has been approved for procurement purposes. Until a new revision is approved and dated, the most recent existing revision, with amendments, is operative.) With approval of the 81750's B revision in 1974, we find a change in the language of the scope: 1.1 Scope. This specification covers opaque and transparent heat-sealable, electrostatic-free, flexible, barrier materials for the packaging of missiles, explosive powered and electrosensitive devices, microcircuits, semiconductors, thin-film resistors, and associated airborne components, in addition to the Type I opaque material provides for the attenuation of electromagnetic radiation (see 6.1). 1.2 Classification. The barrier materials shall be of the following types as specified. When no type is specified Type I shall be furnished (see 6.2). Type I- Opaque. Type II- Transparent 1.2.1 Relationship of the materials specified in earlier issue of this specification is detailed in Section 6. In paragraph 1.2, we see "Type I- Opaque" and "Type II- Transparent" classifications. Pink poly was born! We are immediately referred to Paragraph 6.1. 6.1 Intended use. It is intended that these barrier materials be used to fabricate enclosures for electrostatic sensitive equipment and components. These barrier materials are especially formulated to prevent the buildup or retention of electrostatic potential, the objective being to maximize explosive safety, to protect miniature electronic parts, and to preclude ignition of stray flammable materials. The finished enclosure is to provide protection from corrosive environments. 6.1.1 Type I barrier materials. Type I barrier material is intended for use for the watervaporproof electrostatic and electromagnetic protection of microcircuits, certain semiconductor devices (such as microwave diodes and field-effect transistors, thin film resistors, and other miniature electronic parts requiring this protection) [sic]. 6.1.2 Type-II barrier material. Type-II barrier material is intended of ruse where transparency is required and contact with oil or grease is not contemplated. The transparent material is also intended for level C or commercial packaging use where a waterproof, electrostatic-free, barrier is needed. For level-A requirements use is limited to interior wraps or bags [sic]. For the first time, we see a distinction between electrostatic an electromagnetic protection. In paragraph 6.1.2, we can see MILP-116 begin to emerge, although it is still not referenced. Levels C and A refer to these levels as used in 116. Also not that Type II (pink poly), when used for level "A," is restricted to interior wraps on bags. Those who would have you believe that double wrapping is something new are incorrect; however, we must be more precise about when double-wrapping is required. Paragraph 6.1.1 of this document is one of the earliest references to distinguish between electrostatic and electromagnetic protection, and from this document and others, it becomes obvious that double-wrapping was mandated to provide a combination of water vapor, electrostatic, and electromagnetic-varrier protection. The definitions of Type-I and Type-II materials appear in Paragraph 3.3, Construction. 3.3.1 Type I. Type I barrier material shall consist of aluminum foil and other laminates constructed in any manner which will ensure compliance with the performance requirements of this specification and which will be suitable for the purpose intended. Butting of component materials or the finished product is not permitted except in the direction perpendicular to the rolling direction. The areas shall be plainly externally flagged to prevent use of that portion of the roll when a butt weld is made on the finished product or its components. 3.3.2 Type II. Type II barrier material shall consist of a transparent, unsupported, nonlaminated plastic sheet both surfaces of which shall be chemically neutral and identical, extruded from a homogeneous antistatic resin mix. The material shall be unprinted and have no ink, talc, cornstarch, metallization or other foreign matter applied to the surface after extrusion. The basic definition of a Type I material did not change from Revision A. However, the definition of Type-II was completely new. Among key words introduced were "transparent," "unsupported," "nonlaminated," "chemically neutral," "homogeneous," "antistatic," and "resin mix." Each of these terms plays a vital role in controversies that developed later. In Paragraph 3.6 Identification of Material, we find that Type I material was to be printed with designated information using a water-resistant ink that approximated Color #27038 of Fed. Std. 595. What was that color? Black. In Paragraph 3.6.2, for Type II we find the following words: "Type II barrier materials shall be of uniform color, approximating the lusterless red (pink) which conforms to color number 31668 of Fed. Std. 595." As we will show later, identification of this material created numerous problems, some of which remain unresolved. The next section of MIL-B-81705-B is perhaps the most important part of ht document because it lists all of the physical properties and test methods used to quantify those properties. Table 1 of this article is a complete reprint of Table 1 as it originally appeared in MIL-B-81705-B. The next item a vendor hands you a piece of product literature that says, "meets requirements of 81705-B," don't be misled into the belief that the product meets all requirements of 81705-B. Likewise, anyone who published a statement that says "meets or exceeds static decay requirements of Fed. Std. 101, Method 4046" should become suspect. Method 4046 does not include any pass-fail criteria; it is simply a test method used to establish a value. Whether or not that value is acceptable is governed by the maximum 2.0-sec. decay rate given in Table 1 of 81705. I have never been able to determine why or how the 2.0 sec limit was established. The closest I ever came was during a roundtable discussion at NAVSEA headquarters in Washington, DC. When the question came up, someone said, "It takes more than two seconds for someone to pick up a package (or bag), open it, and touch what's inside the package." If someone can shed light on this, I'd like to see the item study or other justification for the time limit. For 15 years, we have accepted this value as gospel without any concrete rationale to support it. This is just one of the questions that a historical review of this document asks. The static-decay test in Method 4046, of Fed. Std. 101, has been the only approved test for the electrostatic-protective property of a barrier material since 1974. None of the three amendments added any new tests, nor did the maximum decay rate change. Since it was the only approved test, it was applied to nearly every conceivable static-control product, either on the market or developed subsequent to 1974. We suddenly saw product literature for totes, table mats, smocks, DIP tubes, foam, and even aerosol cans that met the "requirements of 81705." There was a second criterion and test method included in 81705-B, but it only applied to Type I material. The property to be tested was called "Electromagnetic interence attenuation frequency range 1 to 10 GHz." The requirement was "25 decibels (minimum) attenuation...limited to the 1 to 10 GHz frequency range." A series of tests was mandated at 500 MHz intervals between 1.0 and 10.0 GHz- the frequency range did not begin at 500 MHz as some misunderstood it.
As I see it, EMI is a three-legged monster. It can include field effects from a statically charged object. As long as the charge remains dormant, the primary field is considered a pure "E" field. This is the first leg of the monster. The second leg is the pure magnetic field, or "H" field found around a common magnet. The third leg is the RF energy associated with the entire radiant energy spectrum. The spectrum is well-defined by frequency and wavelength. Those who are familiar with the RF spectrum will recognize the 1 to 10 GHz range as beign associated with microwave and radar transmission. It was already known that the radiant energy created during a thunderstorm or an ESD event covered a wide frequency range. In the same roundtable discussion at NAVSEA where I raised the question about the 2-sec decay time, I asked, "Why is the RF frequency range limited to 1 to 10 GHz?" The answer, again, was quick and simple. The Navy had documented failures of components subjected to the radiant energy of its high powered radar units. Thus, if the sensitive component was wrapped in a Type II material, which provided electrostatic protection and this, in turn, was overwrapped with a Type I material which would provide electromagnetic protection, the problem of protecting static-sensitive devices from "field forces" was solved. It looked good on paper, and in theory, but technology refused to sit still. As noted in an earlier article, the definitions of electrostatic and electromagnetic have yet to be clearly defined. Lack of precise definition left a great deal of latitude for those who were to develop new static-protective barrier materials, and some raging battles ensued. One school of though said that use of a Type II material was sufficient to prevent damage to static-sensitive devices. If no charge accumulated on the material, no E-field would exist around the packaged device, and if a charged person or object touched the outside of the barrier layer, even when grounded, no spark would occur. The other school of though pointed out that Type II material was not conductive enough to form an effective Faraday Cage when a device surrounded by this material moved though an E-field strong enough for polarization to take place. As will we see in future articles, test methods came under scrutiny. Our entire understanding of ESD phenomena grew dramatically. Devices that had been packaged to level C of MIL-P-116 began to fail as sensitivity levels dropped. New forces of destruction were identified. And, above all, an explosion of static-safe products hit the market. In 1979, the EOS/ESD Assn. was formed, and from the very first symposium, responsible investigators began to report the results of their work. At about the same time, the Electronics Industry Assn. formed a committee to begin development of an ESD-protective packaging standard. These two organizations were to have a profound impact on 81705 and other ESD Mil-Specs. Charles Hynes concludes his exploration of MIL-B-81705 in the next issue of EOS/ESD Technology.
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From
Table 1, we are directed to Paragraph 4.8.6 for the test method, and
that paragraph says, "EMI ATTENUATION. This measuring technique
is intended for determining RF attenuation characteristics of the Type
I material."