Static protective laminated material

A static protective laminated material suitable for releasably supporting thereupon static-sensitive components having leads, such as integrated circuits and the like, and for protecting such components from electrostatic charges comprises a layer of antistatic material through which the leads of the component being supported may penetrate, the antistatic material being adapted to control the rate of discharge of any static charges through the component leads as the leads are inserted into the laminated material, and a layer of conductive material into which the component leads projecting through the antistatic layer may penetrate and be releasably supported thereupon, the conductive material being adapted to establish a conductive path between the component leads, thereby preventing the discharge of static charges through the component.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention generally relates to electrostatic discharge-protective 
materials, and more particularly to such materials for supporting 
static-sensitive electronic components having electrical leads. 
2. Description of the Prior-Art 
Electrostatic discharge (ESD) is widely recognized as possibly the most 
destructive phenomenon in the modern-day electronics industry. ESD is 
ubiquitous in the handling, packaging and shipping of electronic devices 
and components and leads to extensive damage by causing excessive internal 
heating or dielectric breakdown when such devices lie in the discharge 
path. Since static electricity cannot be eliminated altogether, a variety 
of approaches have been developed for restricting the destructive effects 
of ESD. 
For instance, static-sensitive components have been surrounded in various 
degrees by static-protective materials, depending on the properties of the 
material, so as to minimize charge generation and dissipate built-up 
charge by redistributing it on a surface, by providing a conductive path 
to ground, or by serving as a shield against external electrostatic 
fields. A popular approach in this regard has been the advantageous use of 
the "Faraday-cage" effect by means of a container (typically made of paper 
board) which is provided with a conductive coating (typically, a layer of 
carbon black) on its internal surface so that any electrostatic charge in 
the conductive layer is either bled off to ground or is forced to 
circulate harmlessly until the charge decays gradually to a negligible 
level. Such conductive containers, however, are incapable of providing ESD 
protection when the containers are open and are also susceptible to ESD 
due to triboelectric charge built up as a result of frictional action 
during insertion and removal of components; under these conditions, 
static-sensitive components remain subject to damage from static charges 
and fields. 
Containers and envelopes formed of different types of laminated materials 
have also been used to control ESD problems. The laminated materials used 
with such static shielding products are produced by using heat or adhesive 
to laminate a thin sheet of flexible antistatic material to a conductive 
layer or grid of carbon or metal, which is then laminated to a support 
layer, typically polyester or paper backing. The laminated material works 
by conducting the static charge through a small segment of the volume of 
the surface material to the conductive layer to ground. Since the surface 
is an antistatic material, while the carbon or metal layer is electrically 
conductive, the bulk of the charge current passes through the conductive 
layer to ground. U.S. Pat. No. 4,699,830 to White and U.S. Pat. No. 
4,738,882 to Rayford et al. are representative of such antistatic 
laminated sheet material which generally includes successive layers of 
antistatic material and conductive metal, with the laminated material 
being particularly adapted to forming protective containers or envelopes 
for electronic components. 
It should be noted that the term "antistatic material", as used herein, is 
intended to include materials traditionally defined within the electronics 
industry (particularly, in manufacturing environments) as "antistatic" 
(typically having a surface resistivity of 10.sup.9 to 10.sup.14 
ohms/square) as well as those defined as "static-dissipative" or "static 
shielding" (typically having a surface resistivity of 10.sup.5 to 10.sup.9 
ohms/square). 
Electronic components having leads are particularly susceptible to ESD 
problems because the conductive nature of the leads makes them the focal 
point for discharge of electrostatic charge, not only between the leads 
and surrounding static fields, but also from one lead to another. The 
prevalent practice in industry for protecting such leaded components is to 
supplement the use of static shielding containers for the same by shunting 
the components against static discharge. Shunting is accomplished by 
connecting all the component leads together through a common conductive 
path, thereby preventing a discharge through the component from one lead 
to another. A layer of conductive carbon foam (commonly known in the 
industry as "black" foam) is almost universally used for this purpose and 
leaded electronic components are supported on the foam by embedding the 
component leads into the foam. 
While the use of conductive foam as a shunting support layer reduces the 
possibility of inter-lead electrostatic discharge through a component, it 
has several inherent disadvantages. Since the conductive foam must 
necessarily be highly conductive in order to provide effective shunting, 
it is possible to have electrostatic discharge even as contact is 
approached and made between component leads and the foam, if either of 
them happens to be statically charged. Another problem prevalent with the 
use of conductive foam is that the foam gets broken up into small 
conductive particles as the component leads penetrate the foam during the 
embedding process or, more frequently, when embedded component leads are 
extracted from the foam to release a component supported thereupon. Such 
conductive particles frequently get lodged between the leads resulting in 
damaging short-circuits in the components themselves or in the circuit 
assemblies on which the components are subsequently installed. 
Accordingly, there exists a need for a static-protective material capable 
of avoiding the problems involved in protectively supporting 
static-sensitive electronic components having leads by using conductive 
shunt layers. 
SUMMARY OF THE INVENTION 
It is a primary object of the present invention to provide an improved 
static-protective material for supporting and protecting electronic 
components having conductive leads from electrostatic discharge. 
A related object of this invention is to provide a material of the above 
type which is adapted to control the rate of discharge of any static 
charges built up on or about electronic components or generated as the 
components are affixed to or removed from the supporting material. 
Another object of this invention is to provide a static-protective material 
of this type which is capable of effectively shunting the leads of 
components supported thereupon, so as to avoid the discharge of 
electrostatic charges between component leads. 
An important object is to provide a static-protective material having the 
above properties which is capable of avoiding inter-lead shorts caused by 
mechanical breakdown of the conductive layer defined within the material. 
A further object is to provide static-protective material of the above type 
which is particularly adapted for use in ESD protection applications 
within clean-room environments. 
These and other objects are achieved, according to the present invention, 
by means of a static-protective laminated material suitable for releasably 
supporting thereupon static-sensitive electronic components having leads, 
and for protecting such components from electrostatic charges. The 
laminated material comprises a first layer of antistatic material through 
which the leads of the component being supported may penetrate, the 
antistatic material being adapted to control the rate of discharge of any 
static charges through the component leads as the leads are inserted into 
the laminated material, and a second layer of conductive material into 
which the component leads penetrating through the antistatic layer may 
penetrate and be releasably embedded therein, the conductive material 
having a substantially lower surface resistivity than that of the 
antistatic layer and adapted to establish a conductive path between the 
component leads, thereby preventing the discharge of static charges 
between the component leads. 
According to a preferred embodiment, the antistatic layer is formed of 
antistatic foam having a surface resistivity of at least 10.sup.6 
ohms/square and the conductive layer is formed of conductive foam having a 
surface resistivity of less than 10.sup.5 ohms/square. The layers of 
antistatic foam and conductive foam are preferably heat bonded together 
or, alternatively, bonded by means of a layer of adhesive material. 
According to an alternate embodiment adapted for use in clean-room 
environments, the layer of conductive foam is sandwiched between layers of 
antistatic foam which extend beyond the dimensions of the conductive foam 
layer on all sides and which are bonded together about the extended 
sections so as to effectively enclose the conductive foam from external 
exposure.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
While the invention is susceptible to various modifications and alternative 
forms, specific embodiments thereof have been shown by way of example in 
the drawings and will herein be described in detail. It should be 
understood, however, that it is not intended to limit the invention to the 
particular forms disclosed, but on the contrary, the intention is to cover 
all modifications, equivalents, and alternatives falling within the spirit 
and scope of the invention as defined by the appended claims. 
Referring now to the drawings, there are shown in FIGS. 1 and 2 side and 
cross-sectional views, respectively, of a static-protective laminated 
sheet material according to a preferred embodiment of this invention. The 
laminated material 10 is in the form of a sheet 10 comprising a top layer 
12 of antistatic material which is bonded to a bottom layer 14 of 
conductive material. Preferably, the top antistatic layer 12 and the 
bottom conductive layer 14 are heat bonded together. Alternatively, 
bonding may be achieved, as shown in the embodiments of FIGS. 1 and 2, by 
means of a layer 16 of adhesive material. Preferably, the adhesive 
material itself is conductive s that it effectively forms a part of the 
conductive layer 14. 
The antistatic layer 12 is formed of a material adapted to allow the 
electrical leads of an electronic component supported upon the laminated 
material 10 to penetrate through the layer 12. Similarly, the conductive 
layer 14 is also adapted to allow such component leads to penetrate into 
the conductive material and be releasably embedded therein. 
According to a preferred embodiment, the antistatic layer 12 comprises a 
layer of foam having antistatic properties. Polyolefin foams, either those 
which naturally exhibit characteristic antistatic properties or those 
surface treated with topical coatings or impregnated with migratory 
antistats to provide desired antistatic properties, may be used for this 
purpose. Such antistatic foams resist triboelectric charging and produce 
minimal static charges when separated from themselves or other materials. 
Polyethylene foam containing an antistatic additive, commonly referred to 
as "Pink Poly", is particularly suited for use as the antistatic layer 12. 
The primary requirement, of course, is that the selected foam material 
exhibit high surface resistivity. A preferred surface resistivity for the 
antistatic foam layer would range from about 10.sup.6 to about 10.sup.14 
ohms/square. 
Also according to the preferred embodiment, the conductive layer 14 is in 
the form of a foam having conductive properties. Preferably, a polyolefin 
or polyamide-based foam impregnated with conductive carbon black particles 
is used as the conductive layer 14. Such conductive foam is popularly 
known within the electronic component packaging industry as "black" foam 
and is used commonly as a single-layer material for supporting leaded 
components. Other types of conductive foams using conductive particles 
other than carbon black may also be used. In order to exhibit the desired 
conductivity, the conductive foam layer preferably has a surface 
resistivity of less than 10.sup.5 ohms/square. 
As specifically shown in FIG. 2, an electronic component 20, such as an 
integrated circuit or "chip", is protectively supported upon the laminated 
material 10 by forcing the electrical leads 22 of the component 20 into 
the top antistatic layer 12 until the leads penetrate through the layer 12 
and the adhesive layer 14 into the conductive layer 14 so as to be 
embedded therein. The top layer of antistatic foam 12 serves as a high 
resistivity layer which causes any static charge to be conducted through a 
small segment of the volume of the surface material to the conductive 
layer. More importantly, the antistatic foam, by virtue of its high 
resistivity, functions as means for controlling the rate of discharge of 
electrostatic charges as the component leads are inserted into the 
laminated material. The presence of the antistatic layer avoids the sudden 
discharge of electrostatic charge that would occur if the component leads 
22 were to be brought into contact directly with the conductive layer 14. 
A major advantage of the dual-layered construction of FIGS. 1 and 2 is that 
any conductive particles loosened from the conductive layer 14 due to 
mechanical breakdown of the conductive material as a result of the 
insertion and removal of component leads 22 are prevented from being 
lodged inbetween the leads 22 the frictional action involved in extracting 
the leads from the antistatic layer 12 after they have been first 
extracted from the conductive layer 14 dislodges any such particles. The 
commonly used "black" foam is particularly susceptible to breaking into 
small conductive particles and the layer 12 effectively wipes such 
conductive particles off the leads. Thus, the possibility of damaging 
short-circuits in the component itself or in the circuit assembly on which 
the component is to be subsequently installed is avoided. 
Once the component leads 22 penetrate through the antistatic layer 12 and 
into the conductive layer 14, they are effectively shunted since the 
conductive material establishes a conductive path linking the various 
component leads 22. Thus, any discharge of static charges between the 
component leads and through the component is avoided. It should be noted 
that effective shunting of the component leads 22 can be accomplished by 
using only a small thickness T.sub.B of the conductive layer 14, as long 
as the total thickness T of the laminated material is less than the length 
of the component leads 22 and the component 20 is firmly pressed in a 
flush-fit manner onto the laminated material 10. Under these conditions, 
the components leads necessarily come into contact with and are shunted by 
the conductive layer 14. 
With the conventional use of a single layer of conductive foam for 
supporting electronic components, it is essential that the thickness of 
the foam be equal to or greater than the length of the component leads. In 
the laminated material of FIGS. 1 and 2, the thickness of the antistatic 
layer 12 is generally selected to be greater than the thickness of the 
conductive layer 14. This arrangement provides effective shunting and yet 
brings about a significant cost reduction since commercially available 
conductive foam is much more costly than antistatic foam of comparable 
thickness. The total thickness T of the laminated material, as represented 
by the sum of the thickness T.sub.A of the antistatic foam and the 
thickness T.sub.B of the conductive foam (the thickness of the adhesive 
layer being negligible), is selected to be equal to or, preferably, 
greater than the length of the component leads 22 so that the leads do not 
protrude through the conductive layer when the component 20 is firmly 
supported thereupon. 
The thickness of the laminated material may be varied depending on the 
length of the electrical leads of the electronic components which the 
material is designed to support. Standard thicknesses T, T.sub.A and 
T.sub.B of the laminated material, the antistatic foam layer 12, and the 
conductive foam layer 14, respectively, adapted for use with commonly used 
integrated circuits are listed below: 
______________________________________ 
(i) T = .250" - 
T.sub.A = .125"; T.sub.B = .125" 
(ii) T = .375" - 
T.sub.A = .250"; T.sub.B = .125" 
(iii) T = .500" - 
T.sub.A = .375"; T.sub.B = .125" or 
T.sub.A = .250"; T.sub.B = .250" 
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According to an alternative embodiment of this invention, the laminated 
material using layers of antistatic foam and conductive foam is formed as 
a triple-layered laminate in which a layer of conductive foam is 
sandwiched between two layers of antistatic foam. Such a sandwiched 
construction is particularly adapted for use in clean-room static control 
applications where it is crucial to avoid contamination of the "clean" 
atmosphere by discharge of even minute conductive particles from the 
conductive foam layer 14. Such a sandwiched laminated material is 
illustrated in cross-section in FIG. 3. 
As shown in FIG. 3, the laminate 30 has a layer of conductive foam 32 
sandwiched and bonded between layers 34 and 36 of antistatic foam which 
extend beyond the dimensions of the layer of conductive foam 32 on all 
sides. The extending segments of the conducting foam are then bonded 
together, using heat bonding or a suitable adhesive, so as to effectively 
surround the conductive foam 32 and prevent it from being externally 
exposed. The cross-sectional view of FIG. 3 shows the extension of the 
antistatic foam layers 34 and 36 beyond the width "W" of the conductive 
foam layer 32 and the bonding together of antistatic foam about 
transversely opposing ends 38 and 40 relative to the width of the 
conductive foam. It should be noted that a similar extension and bonding 
(not shown) of the antistatic foam around the conductive foam also applies 
about the longitudinal dimension of the laminated material 30.