EP0910431A1

EP0910431A1 - Static control wrist strap

Info

Publication number: EP0910431A1
Application number: EP97927679A
Authority: EP
Inventors: Brian M. Cox
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1996-06-28
Filing date: 1997-05-21
Publication date: 1999-04-28
Also published as: WO1998000196A1; TW500285U; JP2000513262A; MY128755A

Abstract

A static control wristband (10) comprises a strap (12) having a first polymeric, insulative layer and a second, conductive layer (14) integrally molded with the first layer, contact means (22) for interconnection of the second layer with a conductive tether, and means for adjusting an effective length of the strap to accommodate the size of a user's limb. A third, conductive layer (24) may be included to provide a constant monitoring device in the ground path. The contact means may comprise a conventional eyelet passing through a portion of the conductive layer with a snap for connection to the tether. The insulative layer may be overmolded onto the conductive layer (14) or, where the conductive layer is also polymeric, co-molded, co-extruded in sheet or tubular form, or insert molded with the insulative layer. The tether comprises a grounding cord that may be constructed of a conductive layer surrounding a flexible strengthening member, where the strengthening member may be stranded copper wire, foil tensile wrapped around an insulative polyamide fiber thread, or a metal coated fiber. The conductive layer of the tether may be constructed of a flexible elastomer loaded with a conductive filler, and a flexible polymer insulative layer may surround the conductive layer. The tether may be formed into a coil to remove excess length during use.

Description

STATIC CONTROL WRIST STRAP

Background of the Invention 1. Field of the Invention

The present invention generally relates to devices for preventing electrostatic discharge, and more particularly to a static control wristband and conductive (grounding) tether wherein the wristband and tether are primarily polymeric and are formed by molding processes to reduce the number of parts in the device.

2. Description of the Prior Art

Electrostatic discharge, as well as the mere presence of a static electric field, can be extremely detrimental to sensitive electronic (solid-state) parts. Modern semiconductors and integrated circuits may be degraded or destroyed by such static buildup. One common tool used to control static discharge and buildup is a conductive grounding tether which is designed to drain away excess electrostatic charge. A general discussion of such devices may be found in U.S. Patent Nos. 4,677,521, 5,018,044 and 5,184,274.

The wristbands and tethers disclosed in the above-mentioned patents have many parts which add to the cost of the devices. While those devices have features which make them advantageous for particular uses, they are relatively expensive when considered for the most basic task of safe and effective grounding. Less expensive and simpler bands have been designed, such as those shown in U.S. Patent Nos. 3,857,397 and 4,698,724, but those designs suffer from certain inherent limitations. For example, both of these designs are unitary wristband/tethers, meaning that if the tether portion of the device breaks, the wristband becomes useless, or vice- versa, and also meaning that if the user desires to walk away from his or her workstation he or she must completely take off the wristband, or must allow the tether portion to dangle about, in contrast with the designs that allow the tether to be detachably connected to the wristband. Both of these designs are also ill-suited for use in a dual conductor system (such as that shown in the '044 patent). Finally, the means for adjusting the effective size of the wristband must be applied separately to the band, such as the hook-and-loop fastening strips in the '397 patent, or the adhesive layer in the '724 patent. This separate application step increases processing cost, but it is critical that the device achieve a proper exact adjustment since they must have good contact with the skin to be sufficiently conductive, and yet not be so tight as to constrict circulation or otherwise be uncomfortable. It would, therefore, be desirable and advantageous to devise a durable low-cost, low part count wristband which overcomes the foregoing limitations.

Summary of the Invention

The present invention provides a static control wristband generally comprising a strap having a first polymeric, insulative layer and a second, conductive layer integrally molded with the first layer, contact means for interconnection of the second layer with a conductive tether, and means for adjusting an effective length of the strap to accommodate the size of a user's limb. A dual conductor version of the wristband may further be constructed by providing a third, conductive layer also integrally molded with the first layer, and second contact means electrically isolated from the first contact means for interconnection of the third layer with a second conductive tether to allow a constant monitoring device in the ground path to signal the wearer by an alarm when the band or tether has failed to provide a continuous ground as taught in U.S. Patent No. 5.057,965. The adjusting means may comprise a clasp attached to an end of said strap, or the wrist strap may be self-adjusting by providing an elastic strap which is a closed loop with a series of corrugations in the strap. The contact means may comprise a conventional eyelet passing through a portion of the second layer, and a snap portion for connection to the tether, or the contact means may instead simply be provided by a hole formed in the strap for receiving a tether jack, the hole having an inner surface which is at least partially formed by the conductive layer.

The wristbands may be manufactured by overmolding the insulative layer onto the conductive layer or, where the conductive layer is polymeric, co- extruded, co-molded or insert molded with the insulative layer. The layers may be co- extruded in a sheet form, or co-extruded in a tubular member to provide the closed loop wristband embodiments.

A novel grounding cord is also provided, wherein the tether of the cord is constructed of a flexible strengthening member, a conductive layer surrounding the strengthening member, where the strengthening member may be stranded copper wire, or foil tensile wrapped around an insulative polya ide fiber thread, or a metal coated fiber. The conductive layer is constructed of a flexible elastomer loaded with a conductive filler, and an insulative layer surrounding the conductive layer, the insulative layer also being constructed of a flexible polymer. The conductive layer at a given end of the tether is formed for attachment to a particular connector style. A portion of the tether is preferably formed into a coil to remove excess length during use.

Brief Description of the Drawings The invention will best be understood by reference to the accompanying drawings, wherein:

Figure 1 is an exploded perspective view of one embodiment of the static control wristband of the present invention;

Figure 2 is a perspective view depicting a fabrication process for another embodiment of a wristband according to the present invention;

Figure 3 is a perspective view depicting another fabrication process similar to Figure 2 but for a dual conductor embodiment of a static control wristband;

Figure 4 is a perspective view depicting a fabrication process for a closed-loop embodiment of a wristband according to the present invention; Figure 5 is a perspective view depicting another fabrication process similar to Figure 4 but for a dual conductor embodiment of a closed-loop wristband; Figure 6 is a perspective view depicting the layered construction of a grounding cord made according to the present invention; and

Figure 7 is a perspective view of the grounding cord of Figure 6, fashioned into a tether having connectors at either end; Figure 8 is a perspective view of an alternative adjustment means for the static control wristband of the present invention; and

Figure 9 is a perspective view of a static control heel strap which may be made according to the present invention.

Description of the Preferred Embodiment

With reference now to the figures, and in particular with reference to Figure 1, there is depicted one embodiment 10 of the static control wristband of the present invention. Wristband 10 is generally comprised of a strap 12 formed of an electrically insulative, flexible, polymeric material, with an integrally molded, electrically conductive layer 14. As discussed further below, conductive layer 14 may be permanently attached to strap 12 by means of, e.g., overmolding, co-molding, insert molding or co-extrusion. Strap 12 is preferably constructed of Hytrel, a durable thermoplastic elastomer (polyether-ester block copolymer) available from E.I. duPont de Nemours. A pigment may be added for color-coding of the wristband. Conductive layer 14 may be a metallic material, but it is more preferably a polymeric material which has been loaded with a conductive filler to render it at least partially conductive (surface resistivity of less than 1 x 10⁶ Ω/sq), so as to be able to dissipate electrostatic buildup on the wearer of wristband 10. A suitable material is Hytrel loaded with a conductive carbon powder, between 15-40% (weight) depending on particle quality and size.

Conductive layer 14 is placed along strap 12 at a thickened portion 16 thereof An electrically conductive eyelet 18 passes through a hole formed in conductive layer 14 and a hole 20 formed in the thickened portion 16 of strap 12, and is affixed to a male snap part 22 which is used to interconnect wristband 10 to a grounding tether. Different size snaps (e.g., 4 mm, 5 mm, 7 mm or 10 mm) may be so attached to strap 12. A depression 23 in thickened portion 16 of strap 12 may be provided for receiving the snap. The eyelet and the snap may optionally be molded into the wrist strap. An optional layer 24 may be used as a conductive, hydrophilic barrier, placed over the carbon-loaded layer, for clean room applications. For such high-end applications, materials may be used which are less likely to sluff off (particles or fibers) or outgas.

Several features are molded into the shape of strap 12. In order to allow adjustment of the wristband, retention pins or studs may be formed at one end, such as a pair of small studs 26 and a larger stud 28, which engage corresponding holes 30 and 32, respectively, formed in the other end. Since this provides for adjustability at discrete lengths, further tolerance may be provided by building in relief features such as stretchable links 34. A first rail 36 is provided near studs 26 and 28 for retaining the other end of strap 12 in place over the studs, forming an insertion aperture, and a second rail 38 is provided for retaining excess length of the strap. The other end 40 may be tapered to ease insertion under rails 36 and 38. Other relief features, such as ribs or grooves 42 may be formed along the strap to improve its flexibility and conformance about the user's wrist.

There are several advantages to the foregoing design. In addition to providing all of the basic characteristics of a grounding wrist strap, wristband 10 is so comfortably adjustable that one size fits all, with positive skin contact. It has a low manufacturing cost, particularly when as few as four parts are used (strap 12, conductive layer 14, eyelet 18 and snap 22). Pigment may also be changed in the manufacturing process to easily select appropriate colors. As discussed below, wristband 10 is otherwise relatively easy to manufacture through overmolding or co- extrusion processes which allow for simple fabrication of the integrated strap and conductive layer, and quick assembly of the addition components.

Figure 2 is a perspective view depicting a fabrication process for another embodiment 44 of a wristband according to the present invention. Wristband 44 is constructed by overmolding a sheet of the conductive (carbon-loaded) material onto a larger sheet of the insulative polymeric material, forming an integral sheet 46 from which the individual wristbands 44 may be formed. In the first step of processing sheet 46, lines may be cut in the sheet to define the boundaries of the strap portion 48 of wristband 44, and a hole punched through the thickened portion of the strap. In the next assembly step, the eyelet 50 and snap 52 are attached. In the final step, a clasp 54 is attached to one end of strap 48. Grooves or indentations 56 may be formed on that end of strap 48 to mate with the attachment features of clasp 54. That end of strap 48 may have a lead section 58 which is tapered to allow passage through clasp 54, with the lead section 58 cut off after attachment of the clasp.

A similar process and design is shown in Figure 3. The only difference is that the insulative layer is overmolded onto two pieces of the conductive layer, with the two pieces of the conductive layer being separated slightly so that, in the finished wristband 60, there are two separate conductive sections 62 and 64 formed along the inside surface of the strap. Two snaps 66 and 68 are attached to respective eyelets on the strap such that they are in contact with the two conductive sections 62 and 64, respectively. Dual conductor wristband 60 may then be used in conjunction with dual conductor monitoring systems as are known in the art.

Figure 4 depicts a fabrication process for another embodiment 70 of a closed-loop wristband according to the present invention. As with the processes of Figures 2 and 3, wristband 70 is fashioned by cutting a strap away from a larger piece of material having the conductive layer already integrally formed with the insulative layer, but in Figure 4 the wristband is formed from a tubular member 72 rather than from a sheet, allowing wristband 70 to be a closed loop, i.e., no clasp or other elements are necessary to attach any ends of the wrist strap. Tubular member 72 may be formed by co-extrusion of the conductive layer 74 with the insulative polymeric material. During the extrusion process, a hole 76 may be formed in the strap, in a thickened portion thereof, for receiving a plug on the end of a grounding tether. Hole 76 has an inner surface which is at least partially formed by the conductive layer Formation of hole 76 in this manner further reduces cost of the device since it provides for interconnection to the tether without adding any parts (i.e., eyelet and snap). Wristband 70 may be formed of an elastic material with bends or corrugations 78 such that it is self-adjusting, i.e., it is capable of collapsing securely about wrists of various sizes to achieve satisfactory skin contact, but without providing any additional components for adjustment, such as a clasp. In lieu of corrugations 78, a series of small transverse cuts made be made in a section of the strap to provide for self- adjustment. Figure 5 is similar to Figure 4 but it illustrates another dual conductor embodiment 80 of a wristband according to the present invention, formed by providing separate conductive layers 82 and 84 as with the process of Figure 3. Figure 5 also illustrates that snaps 86 may still be used with this design, instead of forming the plug- receiving hole 76 of Figure 4.

Turning to Figures 6 and 7, those figures depict a novel grounding tether 90 which may be used in conjunction with the wristband of the present invention. Tether 90 shares some of the same design characteristics of the wristbands described above, viz., having primarily polymeric construction and including a conductive layer which is co-extruded which an insulative layer. Specifically, tether 90 has an outer layer 92 of an insulative polymer, again such as Hytrel, and an inner layer 94 of a conductive material, again preferably carbon-loaded Hytrel. With carbon loading of 32 %, a typical grounding cord (10' long) fashioned from tether 90 would have a resistance of about 2 MΩ. Conductive layer 94 is shown with a circular cross- section but those skilled in the art will appreciate that this shape is not mandatory. The layers may be overextruded or co-extruded using a tensile filament. Near the center of conductive layer 94 is a reinforcing member, preferably a strengthening fiber 96 made of polyester or Kevlar, an aromatic polyamide available from E.I. duPont de Nemours. The reinforcing member could also be, e.g., one or more solid metal wires, or tinsel foil wound over a strengthening strand of polyester or Nomex fibers (Nomex is a polyamide also available from E.I. duPont de Nemours).

Figure 7 depicts how tether 90 may be fashioned into a grounding cord having connectors at each end. In applications where a banana-style plug 98 is required or desirable, such a plug may be directly affixed to the exposed conductive layer at one end of tether 90 using a crimp tube formed on the banana plug. The banana plug may be a polymeric, molded part also. For those applications where interconnection with a snap such as snap 22 is required, the exposed conductive layer at one end of tether 90 may be swaged to flatten the end for heat stamping or welding of the polymer to the metal snap 100. The end may instead be connected to a resistor and the resistor connected to the metal snap. Alternatively, a female snap may be molded of conductive Hytrel and formed with prongs which grip the end of the tether. Overmolded insulative bodies and strain relief boots may additionally be used to strengthen the terminations.

Figure 7 also shows how, if the materials of the tether are elastomers, a portion 102 of tether 90 may be coiled over a mandrel which is heated and then rapidly cooled to set its memory in a coil shape, to minimize excess length of the tether in use. Insulative layer 92 is preferably a higher durometer material (in the range of 45-80 shore D) for durability (the durometer of conductive layer 94 is preferably in the range of 25-50 shore D). Fiber 96 preferably has a diameter of 0.003"-0.015", conductive layer 94 has an outer diameter of about 0. 070", and insulative layer 92 has an outer diameter of about 0.095".

Figure 8 depicts yet another alternative embodiment of a static control wristband 104 which have a different adjustment means. Specifically, a hinged clasp 106 is integrally molded into the strap, the clasp having a first slightly arcuate portion 108, and a second slightly arcuate portion 110 which, when the clasp is in the closed position, overlies portion 108. A living hinge 112 is formed between the two portions 108 and 110. Portion 108 has latch fingers 114 to releasably secure portion 110 in place. Other features such as bumps 116 and ridges 118 may be used to help align the two portions 108 and 110, i.e., there are corresponding holes or notches formed in the underside of portion 110. Figure 9 illustrates how the present invention may be applied to static control devices similar wristbands, particularly a heel strap 120. Heel strap 120 includes an electrically insulative, polymeric main strap portion 122 with an integrally molded conductive layer 124, similar to the above-described wristbands. Heel strap 120, however, is adapted to be worn such that conductive layer 124 is facing downward, i.e., it comes into contact with the ground or with, for example, a static control floor mat. Conductive layer 124 may be molded with a non-skid surface, such as a knurled surface. Strap 120 has front overlap sections 126 and 128 which wrap around the top, arch portion of a foot or shoe to secure strap 120 to the wearer. Another portion 130 is wrapped around the back side of the heel. Portion 130 is held in such a nested configuration by means of another eyelet or rivet 132. Eyelet 132 is in electrical contact with a resistor 134 whose other end is attached to another eyelet 136. Eyelet 136 in turn secures another conductive strip 138 to strap 120. Conductive strip 138 is tucked inside the wearer's shoe so as to be in physically contact with the wearer's foot.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiment, as well as alternative embodiments of the invention, will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present invention as defined in the appended claims.

Claims

CLAHvIS

1. An article for controlling electrostatic discharge, comprising a strap having a first polymeric, insulative layer and a second, conductive layer integrally molded with said first layer; contact means for interconnection of said second layer with a conductive tether, means for adjusting an effective length of said strap to accommodate the size of a user's limb

2 The article of Claim 1 further comprising a third, conductive layer also integrally molded with said first layer, and second contact means for interconnection of said third layer with a second conductive tether

3 The article of Claim 1 wherein said adjusting means comprises a clasp attached to an end of said strap 4 The article of Claim 1 wherein said strap is a closed loop and said first polymeric layer is formed of an elastic material, said adjusting means comprises a series of corrugations or cuts in said strap such that said strap is self-adjusting 5 The article of Claim 1 wherein said contact means comprises a hole formed in said strap for receiving a tether jack, said hole having an inner surface which is at least partially formed by said second layer

6. The article of Claim 1 wherein said contact means comprises a conductive eyelet passing through a portion of said second layer, said eyelet having a snap portion permanently attached to said eyelet for connection to said tether

7. A static control wristband comprising a strap having a first polymeric, insulative, elastic layer and a second polymeric, conductive layer integrally molded with said first layer, said first and second layers forming a closed loop and having a series of corrugations such that said strap is self-adjusting, and said strap further having a hole therein for receiving a tether jack, said hole having an inner surface which is at least partially formed by said second layer

8. A process of manufacturing a static control wristband, comprising the steps of: obtaining a first strip of polymeric, insulative material, obtaining a second strip of conductive material; permanently applying said first strip to said second strip, forming means in electrical contact with said second strip for interconnection of said second strip with a conductive tether, providing means on said first strip for adjusting an effective length of said first strip to accommodate the size of a user's wrist 9 The process of Claim 8 wherein said first strip is overmolded onto said second strip

10 The process of Claim 8 wherein said second strip is polymeric and co-extruded with said first strip.

11. The process of Claim 8 further comprising the steps of obtaining a third strip of conductive material, permanently applying said first strip to said third strip such that said second and third strips are electrically isolated, and forming second means in electrical contact with said third strip for interconnecting said third strip with another conductive tether 12 The process of Claim 8 wherein said adjusting means is provided by attaching a clasp to an end of said first strip

13 The process of Claim 8 wherein said interconnecting means is formed by forming a hole in the wristband for receiving a tether jack, said hole having an inner surface which is at least partially formed by said second strip 14 The process of Claim 8 wherein said interconnecting means is formed by passing a conductive eyelet through a portion of said second strip, said eyelet having a snap portion for connection to said tether

15 The process of Claim 10 wherein said first strip is polymeric and co- extruded in a sheet 16 The process of Claim 10 wherein said first strip is polymeric and co- extruded in a closed loop

17. The process of Claim 12 wherein said end of said first strip is formed with indentations to mate with attachment features on said clasp.

18. A grounding cord comprising: a tether having a flexible strengthening member, a conductive layer surrounding said strengthening member, said conductive layer being constructed of a flexible polymer loaded with a conductive filler, and an insulative layer surrounding said conductive layer, said insulative layer being constructed of a flexible polymer; a first electrical connector located at a first end of said tether and electrically connected to said conductive layer; and a second electrical connector located at a second end of said tether and electrically connected to said conductive layer. 19. The grounding cord of Claim 18 wherein said conductive layer at a given end of said tether is formed for attachment to a connector.

20. The grounding cord of Claim 18 wherein said strengthening member is electrically conductive.