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First Published in EOS/ESD Technology Oct/Nov 1987 Tote Box Material: How Good is It? New Materials for multilayer tote boxes measure up well in lab comparison with tradition polyolefin plastic. by J.M Kolyer, W.E. Anderson
and D.E. Watson Until recently, all tote boxes for ESD control were made of polyolefin and either topically applied antistat, extruded-in antistat, or extruded-in graphitic (conductive) carbon. Each of these materials has its limitations. Topically applied antistat is only an expedient because the antistat wears off after some undetermined period of use. Also, a wall of plain polyolefin, even with an antistatic surface, provides little Faraday-cage shielding protection from external static fields or discharges (Table 1).
Extruded-in conductive carbon offers the advantage of permanence, but it has several problems. A carbon-loaded polyolefin tote-box wall is conductive enough to endanger people; a current of over 100 mA, which is usually fatal (DOD-HDBK-263), can be carried at 110 V (Table 2). If the conductive box itself is charged it is more dangerous than antistatic or nonconductive boxes (Table 3 and Ref 2).
A high-conductivity surface is also dangerous to devices (Ref 3), especially if the operator should be charged and touches a sensitive lead to the box (Table 4).
However, the wall of a carbon-loaded polyolefin box is not conductive enough to be a good Faraday cage; note that the volume resistivity of carbon-loaded polyoelfin is on the order of 102 W-cm versus 106 W-cm for aluminum foil. Refs 3 and 4 agree that highly ESD-sensitive components should always be protected by metal-foil bags, not carbon-loaded bags or bags with thin, see-through metallization. This conclusion is applicable to other containers such as boxes. Futhermore, carbon-loaded ployolefin imparts high triboelectric charges to nonconductors stroked on it (Table 5 and Ref 1) and sloughs conductive particles that could fall into open microelectronic devices and cause shorts.
Looking at New Materials The basic defect of conventional boxes is that they are insufficient Faraday cages. So, two multilayer designs with permanent ESD properties have been investigated: fiber-board-foil and vinyl-metal sheet. Multiple layers are necessary because a homogeneous wall is not capable of providing both a safe antistatic or nonconductive surface and Faraday-cage protection. Commercially available fiberboard-foil construction ("Cornshield" by Conductive Containers, Inc.) consists of aluminum foil sandwiched between layers of fiberboard. The fiberboard has a naturally antistatic surface but is covered with an antistatic coating to seal in sulfur-containing impurities that might tarnish silver-plated leads. The foil provides superior Faraday-cage protection. For example, a discharge from a key held by a person charged to 8000 V caused no damage to field effect transistors (MOSFETs), whereas an impractically heavy wall (0.140 in.) of carbon-loaded polyolefin allowed transistor damage (Table 1). Even the Tesla coil test was passed when the Corshield box was folded to give two layers of foil (Table 1). In previous work (Ref 5), only constructions with heavy foil or metal screen passed this test. Note that the Teslacoil test is the worst case in electrical stress and positioning of the device in the box. However, this test is not the worst case in statistical significance because only five parts are tested for a "pass" rating. Furthermore, our Tesla-coil test does not use worst-case acceptance criteria because subtle damage may not be seen by the curve tracer, and the MOSFETs used are sensitive to 100 to 200 V whereas some new devices may be affected by only 20 volts. If a cost-effective material can pass this test, we would use that material. Fiberboard-foil costs less than other materials. Also, it can be stored easily as flat sheets and then folded into boxes when needed. Its only major defect is its limited durability, but heavier fiberboard will probably prove sturdy enough for most applications. The vinyl-metal sheet design should satisfy the market niche requiring extreme durability. The metal, either steel or aluminum, provides high structural strengh and is coated on both sides with tough vinyl, e.g. 0.010 in thick, by either lamination or powder-coating. The resulting non-conductive surface is safe for people. Also, in a contrived charge-device model test (Table 6), the nonconductive surface was even safer for devices than an antistatic surface.
The relatively heavy metal wall, 0.0375-in. aluminum (20 gauge), is a virtually impregnable Faraday cage and suppresses the voltage of a static charge, no matter how much the surface may be stroked, so that the box never has an appreciable E field when the metal is grounded via bare metal feet on the bottom. In our test, the effectiveness of draining off charges onto an antistatic bench-top was better for a vinyl-metal sheet box than for a carbon-loaded polyefin box (Table 7). However, good contact with the bench surface, creating flatness of the bottom of the box, can be critical. The slightly flexible Corshield box benefited from being conformable and lying flat, whereas the rigid antistatic or carbon-loaded polyolefin boxes were slightly "dished" (concave) so that only edges or corners made contact. The nonconductive vinyl surface's only defect is that it can tribolelectrically charge conductors, whereas an antistatic or conductive surface cannot (Table 5). However, charging of nonconductive surfaces, e.g., conformally coated circuit-board modules, seems a more important issue, and nonconductive vinyl was less of an offender than carbon-loaded polyolefin (Table 5). Summary Table 8 summarizes our evaluations. Together, the two new multilayer boxes should satisfy all in-plant handling needs for an ESD-control program. These boxes are able to afford secure Faraday cage protection for even the most sensitive items when electrically continuous lids are being used. Cost advantage depends on many factors, most notably the number of units produced and the fabrication method. In general, a vinyl-aluminum sheet box would be competitive with an injection molded carbon-loaded polyolefin box. A vinyl-steel sheet box, though cheaper than aluminum, would be almost three times heavier in the same gauge. In contrast with the above choices, the fiberboard-foil box is inherently inexpensive (Table 8).
Last of all, fiberboard boxes (Corshield) are commercially available at this time; however, vinyl-metal sheet designs are still in the prototype stage. Another interesting multilayer design is one which utilizes an aluminum screen or foil sandwiched between layers of hard vulcanized fiber (Table 1). The vulcanized fiber material is naturally antistatic, even at low humidity (5 x 1011W/sq after seven weeks' storage at 72 deg. F and 12% RH). As in the case of the vinyl-metal sheet box, the commercial success of this design would depend upon the development of a practical fabrication method, but both appear to be excellent alternatives for future ESD control.
References 1. J.M. Kolyner and W.E. Anderson, "Permanence of the Antistatic Property of commercial Antistatic Bags and Tote Boxes," Reliability Analysis Center EOS/ESD Symposium Proceedings, EOS-5 (1983): 87-94. 2. J.M. Kolyer, W.E. Anderson and DE Watson, "Hazards of Static Charges and Fields at the Work Station," Reliability Analysis Center EOS/ESD Symposium Proceedings, EOS-6, Philadelphia, PA (1984): 7-19. 3. R.D. Enoch and R.N. Shaw, "An Experimental Validation of the Field-Induced ESD Model," Reliability Analysis Center EOS/ESD Symposium Proceedings, EOS-8, Las Vegas, NV (1986): 224-231. 4. G.C. Holmes, P.J. Huff and R.L. Johnson, "An Experimental Study of the ESD Screening Effectiveness of Antistatic Bags," Reliability Analysis Center EOS/ESD Symposium Proceedings, EOS-6, Philadelphia, PA (1984): 78-84. 5. J.M. Kolyner and W.E. Anderson, "Perforated Foil Bags: Partial Transparency and Excellent ESD Protection," Reliability Analysis Center EOS/ESD Symposium Proceedings, EOS-7, Minneapolis, MN (1985): 111-117.
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