Static shielding bag

A static shielding package is disclosed comprising an open end, a folded bottom and sealed side edges formed from a substantially rectangular transparent, flexible laminated segment of sheet material. The sheet material comprises an inner heat sealable carrier layer, an outer protective layer and a conductive layer sandwiched between said carrier layer and protective layer wherein said sheet material is folded over upon itself along a fold region to form the folded bottom of the package. The improvement comprises the addition of an auxiliary strip of static shielding material added to the package in the region of the folded bottom of the package to provide an alternative path for the conduction of electrical energy in the event of a disruption of the primary conductive layer of the package in the region of the fold area.

BACKGROUND OF INVENTION 
The present invention relates generally to packaging, and more particularly 
to packaging for electrically sensitive components which require shielding 
from electrical fields. Many electronic components such as metal oxide 
semiconductor devices are sensitive to electrostatic discharges when in 
their component form and can be damaged by electrostatic discharges prior 
to the components being installed in an electronic circuit. 
Generally two different methods for protecting such electronic components 
are presently in use. Either the leads of each component are shorted 
together via a metal or conductive plastic polymeric sheet member, or the 
entire component is enclosed in a protective envelope. By and large the 
most popular and least expensive method is the use of a static shielding 
bag or pouch which is preferably transparent or semi-transparent to allow 
visual inspection of the packaged electronic component. 
Currently there are several commercially available packaging products made 
from laminated sheets for electrically shielding electronic components. 
One of such products is an antistatic shield bag manufactured and 
distributed by Minnesota Mining and Manufacturing Company (3M Company). 
The 3M envelope or package is described in U.S. Pat. No. 4,154,344. Other 
U.S. Patents which show related products include U.S. Pat. Nos. 4,407,872; 
4,424,900; 4,496,406; and 4,699,830. In each case, the laminated sheets 
which comprise the static shielding bags include one or more conductive 
metallized layers buried within the laminated structure. While the 
laminated sheets described in the aforementioned patents provide a surface 
resistivity sufficient to dissipate static and other electrical charges 
that could otherwise damage an electronic component, packages made from 
these sheets do not always accomplish the intended result. They often fail 
to do so because of breakdowns in the conductive metallized layers which 
occur during the fabrication steps for converting the sheets into bags or 
pouches. 
It would be desirable, therefore, to provide a flexible, transparent 
laminated sheet which could be converted into a pouch or bag for packaging 
sensitive electronic components that would not lose its resistivity 
characteristics during the converting process, as well as repeated use in 
the field. It would also be desirable to create such a package that is 
transparent and capable of being sealed to itself. It would also be 
desirable to have a flexible sheet material which would provide a low 
surface resistivity for the outer structure of the bag or pouch and which 
would provide secondary static shielding protection in the event the 
resistivity of the outer structure is compromised and the static 
dissipation or static shielding properties are reduced, impaired or 
eliminated. 
SUMMARY OF INVENTION 
The invention herein is directed to an improved laminated sheet material 
and bag or pouch constructed therefrom which exhibits a low surface 
resistivity on its outer surface to provide static dissipation and static 
shielding. The improvement comprises the addition of an inner strip of 
static shielding material applied to the bag or pouch to provide a 
reinforcement against penetration of electrical fields in the most 
critical area of the bag or pouch formed from the laminated sheet 
material. 
Such bags are usually formed by unwinding the laminated material provided 
in roll form, and cutting the laminated material into blanks which are 
folded over upon themselves in a "U" shape. The "U" shaped blanks are then 
side seamed preferably by heat sealing or with adhesive to produce a bag 
or pouch with an open end and a folded bottom. When any laminated 
material, and particularly a laminated material including a metal layer is 
folded, the most critical area is in the region of the fold. In this 
region the laminate is subjected to great stress. The outside surface is 
subject to a strong stretching while the inside layer is compressed with a 
tendency toward wrinkling along the fold lines. When the laminate includes 
a metal layer, the metal layer has a tendency toward breaking when the 
bottom fold is made. The metal layer, conductive by nature, functions as a 
static shield (Faraday Cage Effect) unless its integrity and/or continuity 
is broken, in which case the bag or pouch so constructed is not only 
rendered useless, but counterproductive. In such a case, the broken metal 
layer of the laminate provides two separated metal ends at the fold which 
tend to function as a capacitor, i.e, they store electrical energy rather 
than dissipate the energy as is required for the static shield. 
U.S. Pat. No. 4,559,259 teaches a method of addressing the problem of 
stress cracking in the fold area of a laminated material involving the use 
of auxiliary crease lines in the region of the fold. However, in the 
present invention, the same problem is addressed by adding an auxiliary 
strip of the laminate material, or a similar material, to the blank used 
to fabricate the bag in the region of the fold. The auxiliary strip 
increases the mechanical characteristics of the laminate material in the 
fold area thus reducing the risk of internal collapse which could cause a 
crease producing disruption of the metal layer of the laminate. The strip 
also increases the interior radius of the bottom fold allowing the outer 
radius to maintain a higher profile to further reduce the possibility of 
disruption of the metal layer of the laminate. In the event that the 
auxiliary strip is prepared from the same material as the laminate 
structure, the metal layer of the strip may collapse and rupture during 
folding but the metal layer in the critical area of the outer material 
will remain intact. However, even if the conductive metal layer in the 
outer laminate is ruptured, the intact conductive metal layer in the 
auxiliary strip will provide an alternative path for the flow of 
electricity, thus maintaining the static shielding properties of the bag. 
The laminated material of the present invention preferably has an 
antistatic outer layer which provides a surface resistivity in the range 
of from about 10.sup.8 to about 10.sup.13 ohms per square. The antistatic 
outer layer can be selected from any transparent, polymeric, printable 
material such as polyethylene, cellophane, or polypropylene, but is 
preferably a polyester material of about 1 to 4 mils thickness. Adhered 
to, or deposited on the outer antistatic layer is a conductive metal layer 
having a surface resistivity of from about 10.sup.2 to 10.sup.3 but 
preferably less than about 10.sup.5 ohms per square. Bonded to the 
conductive metal layer by a suitable adhesive or the like is a carrier 
film layer which provides strength and body (puncture resistance) for the 
overall laminated sheet material. The carrier film layer is preferably a 
transparent, heat sealable polymeric material such as a polyolefin based 
material selected from the group consisting of co-extrusions of various 
polymers, but is preferably low density polyethylene having a thickness of 
from about 1 to 4 mils, preferably 2.6 mils. 
When the auxiliary strip material is prepared from the same laminating 
material used to form the bag or pouch of the present invention, the 
strips are attached to the bag blanks cut from the roll of laminated 
material by a suitable adhesive or the like, preferably a pressure 
sensitive adhesive. Of course other strip materials could be used for 
carrying out the present invention as long as the strip materials include 
a conductive element. Examples of such components include the materials 
used in the fabrication of susceptor elements for packages used in 
microwave ovens. Other examples inlcude semi-rigid or rigid profiles. In 
each case, the static shielding reinforcing strip would be adhered to the 
bag blank in the region where the bottom fold is to be made. 
The carrier film layer of the packaging laminate is selected from a 
material which can provide a bonding to itself through any suitable means. 
A particularly preferred carrier film layer is a heat sealable material 
which can bond to itself upon the application of heat and/or pressure. The 
laminated sheet material can thereby be formed into the packaging of the 
present invention in the form of bags or pouches. For example, a blank of 
the laminated sheet material including the static shielding reinforcing 
strip may be folded over upon itself in the region of the auxiliary strip 
and the carrier layer heat sealed to itself at the edges of the bag blank 
to form side seams. The top exposed edges would be left open so as to 
provide an opening for the introduction of the electronic components into 
the bag and such opening would be subsequently closed by heat sealing, 
zipper closure or folded-over.

DETAILED DESCRIPTION 
Referring now more particularly to the Figures of drawing, a blank of 
laminated material 9 is illustrated in FIG. 1 that is useful for making a 
bag or pouch-type package well known in the prior art. FIG. 2 illustrates 
the construction of the laminate comprising an antistatic carrier layer 14 
which is heat sealable to itself and which forms the interior of the bag, 
an adhesive or heat sealable layer 13, a conductive metal layer 12 and an 
outer protective layer 11 which forms the outer surface of the bag. The 
bag 15 is constructed as shown in FIG. 3 wherein the blank of laminated 
material 9 is folded over upon itself along a fold area 18, and the side 
edges are side seamed by heat and pressure or adhesive at 16 and 17 to 
yield an open end 19. Unfortunately, the weakest point of the bag 
construction is the fold area 18 as shown in FIG. 4. When this fold is 
made it is possible to break the conductive metal layer 12 as shown at 20 
at the point of highest stress. When this occurs the electrical 
dissipative properties of the antistatic carrier layer 14 are compromised 
exposing any sensitive electrical components packaged in the bag to 
damage. 
In order to overcome this disadvantage of the prior art packaging, the 
present invention provides an auxiliary strip of material in the fold 
region of the bag which has the same or greater conductivity as the 
package laminate itself. The strip 23 is applied to the inner surface of 
the blank 21 of laminated material 22 as shown in FIG. 5. FIG. 6 
illustrates a typical cross sectional construction according to the 
present invention wherein both the strip 2 and laminate 22 comprise the 
same materials joined together by a layer of adhesive 24. For instance, in 
a preferred embodiment the laminate 22 comprises an antistatic carrier 
layer 14, adhesive layer 13, conductive metal layer 12 and outer 
protective layer 11, while the strip 23 comprises a carrier layer 25, 
adhesive layer 26, conductive layer 27 and an outer protective layer 28. 
However, it will be understood that the auxiliary strip material 23 may 
take other forms as long as it includes at least one conductive metal 
layer or as long as its profile assures the integrity of the primary 
conductive metal layer in the region of any fold. When the construction 
according to FIGS. 5 and 6 is formed into a bag as shown in FIG. 7, the 
fold 32 has a greater radius than a conventional prior art bag, and 
increased mechanical strength provided by the strip 23 to reduce stresses 
that might cause disruption of the conductive metal layer 12 in the region 
of the fold. Moreover, even if metal layer 12 is disrupted by some outside 
force in the region of the fold 32, the continuity of the electrical 
characteristics of the bag 31 are not compromised since the conductive 
layer 27 of the auxiliary strip 23 will remain intact providing a 
continuous path for the flow of electricity to dissipate any accumulated 
electrical charges. This is accomplished because of the characteristic of 
electrical energy to follow the path of least resistance. Thus, with the 
bag of the present invention, accumulated electrical energy would flow 
from the conductive metal layer 12 of the outer bag structure through the 
bag structure to the conductive metal layer 27 of the auxiliary strip 
member 23 (and vice versa) to avoid any disruption in the outer conductive 
metal layer 12. 
The thickness of the conductive metal layers 12 and 27 can vary depending 
upon the type of metal selected, since the controlling criteria for the 
amount of metal in the layer is the preferred surface resistivity to be 
provided. For the laminated sheet material herein, it is preferred that 
the metal layer provide a surface resistivity of less than about 10.sup.5 
ohms per square. At such a surface resistivity the buried metal layer 
provides the protection of a Faraday cage to the overall structure of the 
bag or pouch formed from the laminated material. The conductive metal 
layer is continuous and can be deposited on the protective substrate layer 
by any available means such as vacuum deposition or sputter metallization. 
Such products are well known and readily available on the market. 
Likewise, suitable adhesives are readily available for bonding vacuum 
deposited films to carrier substrates as required by the present 
invention. The auxiliary strips of the present invention may be readily 
bonded to the blanks of laminated material for making the bags using a 
similar adhesive with the same type apparatus used to patch window 
envelopes or the like. The result being separate blanks or segments of 
laminated material electrically reinforced in the region of the fold. 
Although there has been described hereinbefore the specific details of an 
improved static shielding bag according to the present invention, it will 
be appreciated that the invention is not limited thereto. Accordingly, any 
and all modifications, variations, equivalents or the like which may occur 
to those skilled in the art should be considered to be within the scope of 
the invention as defined in the appended claims.