Container for protecting electronic components from static charges

Abstract: A container for packaging electronic components which are susceptible to damage from electrostatic discharge or fields comprises a container (10,40) made of paperboard or paper having an electrically conductive coating (30,43,45) of electrostatic shielding material on the exterior surface thereof, and a coating (31,46) of inorganic, non-carbonaceous electrostatic dissipative material on the interior surface thereof. The electrically conductive coating (30,43,44,45) preferably has an electrical surface resistivity of less than about 10⁵ ohms/square, and the inorganic, non-carbonaceous, electrostatic dissipative coating (31,46) has an electrical surface resistivity of greater than about 10⁵ ohms/square. The electrostatic shielding material is preferably a carbonaceous material, and the non-carbonaceous, electrostatic dissipative material preferably comprises particles of amorphous silica or silica-containing material coated with antimony-doped tin oxide.

Description: The present invention relates generally to containers for electronic components, devices and assemblies and more particularly, to containers for protecting such electronic components, devices and assemblies from electrostatic discharges and from the fields that emanate from electrostatic charges. A wide variety of conductive materials have been used as coatings or fillers for various packaging materials to protect electronic components packaged therein from electrostatic discharges. For example, carbonaceous materials such as particulate carbon black and graphite have been used as fillers in polymeric bags and films for packaging electronic components, and the same carbonaceous materials have been used to form coatings on paperboard boxes for the same purpose. Examples of such coated paperboard boxes are described in Ohlbach U. S. Patents Nos. 4,160,503; 4,211,324 and 4,293,070. It is generally preferred to coat both the interior and the exterior surfaces of such boxes with the conductive material. It has also been proposed to use metal powders in a similar manner. One of the problems encountered with carbonaceous coatings and metal powders is that the particulate carbonaceous or metallic material tends to slough off the packaging material, which can cause serious problems if the loose conductive particles happen to be deposited on sensitive electronic components. In an attempt to avoid such sloughing, coatings of such materials are sometimes sealed with a thin protective coating of another material. Another problem with conductive coatings formed from carbonaceous materials is that it is difficult to control the surface resistivity of such coatings. Carbonaceous materials tend to produce coatings which have extremely low surface resistivities, and attempts to increase the surface resistivity by using smaller amounts of the conductive material result in discontinuities which render the coating nonconductive, or produce coatings having non-uniform properties over the surface area of a given container. US-A-4 896 001 discloses a protective container for packaging electronic components. The container comprises paper having an electrically conductive coating of electrostatic shielding material on the exterior surface thereof, and a coating of inorganic, non-carbonaceous, electrostatic dissipative material on the interior surface thereof. EP-A-0 216 311 discloses a protective container of a similar kind comprising paper board having an electrically conductive coating of electrostatic shielding material on the exterior surface thereof and a coating of inorganic electrostatic dissipative material on the interior surface thereof. US-A-4 699 830 relates to laminated sheet material including conductive metal layers consisting of aluminium, nickel, cadmium, tin, chromium, lead, copper, zinc and combination or compounds thereof, as well as an anti-static layer including polyolefin-based materials treated with topical materials. It is a primary object of this invention to provide an improved protecting container with reduced contamination of the packaged items from sloughing of the coating material from the interior surface of the container. Another important object is to provide an improved protecting container in which the primary discharge path is on the exterior surface of the container, so that the discharge currents are spaced away from the packaged items by at least the thickness of the packaging material, and also take the path of least resistance to ground. The surface resistivities achieved ought to be relatively insensitive to humidity levels, even when the interior coating has a relatively high resistivity. Such improved containers ought to be biodegradable and manufacturable economically so that they are feasible for use in mass production operations. In accordance with the present invention, the foregoing objectives are realized by the feature combinations of claims 1, 9 and 11. The non-carbonaceous material comprising particulate silica coated with antimony-doped tin oxide used to form the protective coating on the interior surface of the container has a relatively high surface resistivity, this material is not likely to damage the packaged electronic components through discharges from charged components or through contamination of components, even if particles of the material become dislodged from the interior surface of the container. Furthermore, this type of electrostatic dissipative material can be made to adhere securely to the surface of the paper or paperboard. In addition by providing a much lower surface resistivity on the exterior surface of the container, by the use of carbonaceous materials in that coating, the primary discharge path for electrostatic charges is along the exterior surface of the container, rather than the interior surface, so that the discharge currents are spaced away from the packaged items by at least the thickness of the packaging material, and take the path of least resistance to ground. This further reduces the risk of any damage to the packaged electronic items by inadvertent conduction of the discharge current back into the items being protected, rather than going directly to ground. Although the cost of non-cubonaceous materials which are electrically conductive is generally much greater than the cost of carbonaceous materials, only a small amount of the non-cubonaceous material is needed to achieve the high surface resistivities provided on the interior surface of the container in accordance with this invention. Thus, the cost of the interior coating can still be approximately the same as the cost of coating the interior surface with carbonaceous material. In a preferred embodiment of particular importance, the coating on at least the exterior surface of said container contains graphite as electrostatic shielding material. With the help of specific bonding agents for coating the paper or cardboard, the graphite-flakes or -scales lead to superior shielding qualities of the coated surface compared to a coating containing particulate carbon black only. In combination with the high surface resistivity of the coating of inorganic non-carbonaceous material on the other surface, the low surface resistivity of the graphite coating and its excellent surface covering and uniformity properties result in a high-duty and high-safety packaging container. The surface properties of the coating can be enhanced even with a mixture of particulate carbon black and graphite. Preferred embodiments are contained in the dependent claims. Embodiments of the invention will be described with the help of the drawings. In the drawings is : FIG. 1 a perspective view of a corrugated fiberboard container; FIG. 2 a section taken generally along line 2-2 in FIG. 1; and FIG. 3 a perspective view of a multi-layer bag, with the various layers peeled apart at one end to show the layers more clearly. In FIG. 1, there is shown a paperboard box 10 which has both its exterior and interior surfaces covered with coatings of materials which protect electronic components packaged therein from electrostatic discharges and fields. Although the container may take a wide variety of different forms, the particular embodiment illustrated in FIGS. 1 and 2 is a conventional box configuration formed from a single sheet of paperboard. This box 10 includes a bottom panel 11, front and rear walls 12 and 13, a pair of end walls 14 and 15 and a lid 16. Each of the end walls 14 and 15 is formed from two panels of the paperboard blank, with the lower edge of the inside panel of each end wall forming a depending tab 14a or 15a which fits into a corresponding aperture in the bottom panel 11 so as to lock the end walls in place. The two panels of each end wall 14 and 15 form an interior cavity which receives end tabs from both the front wall 12 and the rear wall 13; and these end tabs are locked in place within the end walls after the depending tabs 14a and 15a on the inside panels of the end walls 14 and 15 have been inserted into the corresponding apertures in the bottom panel 11. The lid 16 of the box 10 includes a top panel 17 which is an extension of the rear wall 13. Depending from the top panel 17 are a pair of side flaps 18 and 19 which fit along the inside surfaces of the end walls 14 and 15, respectively, and a front flap 20 which extends along the outer surface of the front panel 12 when the box is closed. At opposite ends of the front flap 20 are a pair of tabs 21 and 22 which can be tucked into the cavities formed between the two panels of each end wall 14 and 15, thereby holding the lid 16 securely in the closed position. A coating 30 on the exterior surface of the box 10 is electrically conductive so as to provide a conductive path to ground for dissipating any electrostatic charges on the box whenever it comes into contact with a grounded surface. When the box is not in contact with a grounded surface, the conductive coating forms a Faraday cage. The effect of the Faraday cage is to enable an electrostatic charge on the surface of the box to circulate about and dissipate from the surface of the box into the surrounding atmosphere when the box is not in contact with a grounded surface. The conductive coating 30 on the exterior surface of the box 10 has a surface resistivity that is sufficiently low to provide electrostatic shielding. The particulate carbonaceous material in the coating may be either carbon black and/or graphite which is mixed with a liquid carrier so that it may be uniformly applied to the entire surface of the paperboard blank with conventional coating equipment. The liquid carriers may be the same as those used in carbon-containing printing inks for paperboard. The coating 30 is preferably applied to the paperboard blank before it is cut and folded into the shape of the box illustrated in FIG. 1. The concentration of particulate carbonaceous material in the liquid carrier will vary somewhat depending upon the particular carbonaceous material employed, the particular surface being coated, and the degree of surface resistivity desired in the coating, but in general the concentration of carbonaceous material will be within the range of from about 5 to about 20 percent by weight. The resulting deposit of carbonaceous material on the surface of the paperboard will typically be in the range of from about 1 to about 20 pounds per thousand square feet(∼0,4536 kg-∼9,072 kg/92,9 m 2 ). The surface resistivity of the carbonaceous coating is preferably in the range of from about 10 2 to about 10 4 ohms/square so that it forms an effective electrostatic shield and also quickly dissipates any charge that forms on, or comes into contact with, the coated surface. If desired, the conductive coating may be over-coated with a protective sealant, which is usually transparent. Instead of applying the carbonaceous material as a surface coating, the carbonaceous material may be impregnated into the surface sheet of the paperboard prior to the fabrication of the paperboard. As is well known in the art, paperboard usually has a corrugated core laminated to a pair of opposed surface sheets, and it is possible to impregnate one of the surface sheets with the carbonaceous material before it is laminated to the corrugated core. A coating 31 on the interior surface of the paperboard container is formed from an inorganic, non-carbonaceous material which enables the interior coating to have a much higher surface resistivity than the exterior coating. It will be noted that the coatings 30 and 31 on the inside and outside surfaces of the illustrative box are in contact with each other in several areas, thereby connecting the two coatings so that an electrostatic charge on the interior surface can be discharged via the conductive coating on the exterior surface. For the purposes of the present invention, it is preferred that the coating 31 have a surface resistivity within the range of from about 10 5 to about 10 12 ohms/square. Coatings with surface resistivities in this range are commonly referred to as "electrostatic dissipative" materials. One particularly suitable material for the interior coating is "ZELEC ECP," which is an electroconductive powder commercially available from E.I. duPont deNemours and Co., Deepwater, New Jersey. This electroconductive powder comprises fine crystallites of antimony-doped tin oxide densely layered onto the surfaces of particles of amorphous silica or silica-containing material. The crystallites are dispersed in such a fashion that they form a dense layer on the silica surface, as described in more detail in published European patent application No. 359,569, dated March 21, 1990, by E.I. duPont deNemours Co., Serial No. 89309354 filed September 14, 1989. By controlling the amount of electroconductive powder in the coating applied to the interior surface of the paperboard, the surface resistivity of the interior coating may be accurately controlled. A coating having a surface resistivity within the range of 10 5 to 10 12 ohms/square requires a relatively small amount of the electroconductive powder and, therefore, is relatively inexpensive. The silica particles covered with antimony-doped tin oxide can be applied in accurately controllable densities, which enables the surface resistivity of the resulting coating 31 to be accurately controlled. For most applications, a surface resistivity of 10 8 ohms/square will provide the required protection, and such a coating requires only 0.6 lbs/1000 ft 2 (∼0,27216 kg/92,9 m 2 ) of the particulate material. Thus, such a coating is practical even for large volume applications. This coating material is also virtually insensitive to humidity levels, i.e., its surface resistivity changes only negligibly with changes in the relative humidity of the ambient air, and can be readily mixed with different pigments to form attractive coatings of virtually any desired color. In addition, paper or paperboard containers coated with such material are biodegradable. Moreover, there is virtually no risk of arcing between such a coating and the packaged article, as can occur when a highly conductive surface on the interior of a container "draws" an arc from a charged surface. One of the most significant properties of the preferred coating containing the antimony-doped tin oxide is that it bonds firmly to a paper or paperboard substrate and does not slough off. Consequently, there is virtually no risk of contamination of packaged electronic components by the sloughing of conductive particles. The risk of sloughing is further reduced by the extremely small amount of conductive particulate material needed to provide the desired surface resistivity of 10 5 to 10 12 ohms/square. In FIG.3, a container is in the form of a bag 40 comprising multiple plies 41 and 42 with both the outer and inner surfaces of the outer ply 41, and the outer surface of the inner ply 42, coated with electrically conductive coatings 43, 44 and 45 of electrostatic shielding material, and the inner surface of the inner ply 42 coated with an inorganic, non-carbonaceous electrostatic dissipative material 46. The electrostatic shielding coatings 43, 44 and 45 may be the same as the coating 30 described above for the exterior of the box illustrated in FIGS. 1 and 2, and the electrostatic dissipative coating 46 may be made of the same material described above for the coating 31 on the box of FIGS. 1 and 2. It has been found that two or more envelopes of electrostatic shielding material around a sensitive electronic component provide more protection than a single envelope. For example, if a single envelope dissipates 90% of an electrostatic charge or field originating from a source outside the outer envelope, the second envelope will then dissipate approximately 90% of the remaining 10%, so that the total dissipation by the two envelopes combined is approximately 99%. Although the illustrated bag has been shown with electrostatic shielding coatings on both of the opposed surfaces of the two plies of the bags, i.e., the inner surface of the outer ply and the outer surface of the inner ply, significant protection may also be afforded by a bag which has only one of these two coatings of electrostatic shielding material. Also, for certain applications it may be desirable to coat one of these two surfaces with an insulating material, to provide further protection against the conduction of electrical currents through the two plies of the bag. It will also be appreciated that this embodiment not limited to the use of only two plies, but is equally applicable to any desired number of plies of paper, so as to provide any desired number of envelopes of electrostatic shielding material, insulating material or electrostatic dissipative material around the article being protected. In one particular embodiment that is preferred for high-volume, low-cost applications, a single-ply paper bag is made from paper having an electrically conductive coating of electrostatic shielding material on the outer surface, a coating of non-carbonaceous electrostatic dissipative material on the inner surface, and having the inner and outer surfaces bonded to each other to form at least one of the seams of the bag. The bonding of the inner and outer surfaces is effected with an adhesive that is readily penetrated by the particulate material in the respective coatings on the inner and outer surfaces, so that the two coatings are in electrically conductive contact with each other. This intimate and conductive connection between the inside and outside coatings ensures that any charges formed on the inside surface of the bag, due to rubbing engagement of that surface with the packaged contents, for example, are immediately transferred to the outside surface of the bag, and then dissipated. If desired, the conductive, preferably carbonaceous, coating on the outer surface of the bag may be coated with a sealant that permanently seals the conductive particles to the paper; this prevents sloughing of the conductive particles and ensures retention of the desired degree of conductivity of the outer coating.

Claims: A container (10,40) for packaging electronic components which are susceptible to damage from electrostatic discharge or fields, said container comprising paperboard or paper having an electrically conductive coating (30;43,45) of electrostatic shielding material on at least the exterior surface thereof, characterised in that a coating (31,46) of inorganic, non-carbonaceous electrostatic dissipative material is provided on the interior surface of the container, and that said coating comprises particles of amorphous silica or silica-containing material,coated with antimony-doped tin oxide. The container of claim 1, characterised in that said electrically conductive coating (30;43,45;44) on the exterior surface has an electrical surface resistivity less than about 10 5 ohms/square, and that said coating (31,46) on the interior surface has an electrical surface resistivity greater than about 10 5 ohms/square. The container of claims 1 and 2 characterised in that said electrostatic shielding material in said coating (30;43,45;44) on the exterior surface is a carbonaceous material, preferably particulate carbon black or graphite or a mixture of both. The container of claims 1 and 2, characterised in that said exterior and interior coatings (30;43,44,45;31,46) are connected so that an electrostatic charge on said interior surface can be discharged via said conductive coating on said exterior surface. The container of claims 1 to 4, characterised in that substantially the entire exterior and interior surfaces of said container (10,40) are covered by the respective coatings. The container of claims 1 and 2, characterised in that said interior coating (31,46) contains less than about 0,27216 kg/92,9 m 2 (0.6 lbs/1000 square feet) of particulate conductive material. The container of claims 1 and 2, characterised in that said exterior coating (30,43,44,45) has an electrical surface resistivity of about 10 2 to about 10 4 ohms/square, and that said interior coating (31,46) has an electrical surface resistivity of about 10 5 to about 10 12 ohms/square. The container of claim 1, characterised in that said container (10,40) is a biodegradable paperboard box or a biodegradable paper bag. A paper bag (40) for packaging electronic components of which are susceptible to damage from electrostatic discharge or fields, characterised in that said bag (40) comprises multiple plies (41,42) of paper, the exterior surface of the outermost ply (41) of paper being coated with an electrically conductive coating (43) of electrostatic shielding material, the interior surface of the innermost ply (42) of paper being coated with a coating (46) of an inorganic, non-carbonaceous electrostatic dissipative material, comprising particles of amorphous silica or silica-containing material coated with antimony-doped tin oxide, and at least one of the opposed interior surfaces of an adjacent pair of said plies (41,42) being coated with an electrically conductive coating (44, 45) of electrostatic shielding material. The paper bag of claim 9, characterised in that at least one of the opposed surface of adjacent plies (41,42) of paper is coated with an electrically conductive coating (44,45) of electrostatic shielding material. A paper bag for packaging electronic components which are susceptible to damage from electrostatic discharge or fields, characterised in that said bag comprises a single ply of paper that is coated on its exterior surface with an electrically conductive coating (30) of electrostatic shielding material, and on its interior surface with a coating (31) of an inorganic, non-carbonaceous electrostatic dissipative material, comprising particles of amorphous silica or silica-containing material coated with antimony-doped tin oxide, and the coated interior and exterior surfaces of said paper being bonded to each other to form a seam of the bag and to electrically connect said coatings.

Inventors: BECKER RAYMOND P, BECKER, RAYMOND P., BECKER, RAYMOND P., CC.CORSTAT CONDUCTIVE
Applicants: CONDUCTIVE CONTAINERS INC, KOLB WELLPAPPE HANS, CONDUCTIVE CONTAINERS INC., HANS KOLB WELLPAPPE
Application year: 1992
Main classification: H05K9
IPC codes: H05K13