Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – At least one aryl ring which is part of a fused or bridged...
Reexamination Certificate
1999-11-24
2004-04-13
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
At least one aryl ring which is part of a fused or bridged...
C524S495000, C252S511000
Reexamination Certificate
active
06720379
ABSTRACT:
TECHNICAL FIELD
This invention relates to electrostatic dissipative plastics and, more particularly, to electrostatic dissipative plastics that retain useful physical, chemical and electrical properties at elevated temperatures to at least 160° C. The invention has particular utility in the manufacture and use of electronic components and electronic devices though it is to be understood that it is not intended that the invention be so limited.
Definitions For purposes of this specification and the appended claims, the following terms are defined as follows:
a. Antistatic Agent. A material that can increase the conductivity of a plastic. It may be incorporated into a plastic or it may be applied as a surface coating or treatment.
b. Resistivity. Resistivity, the inverse of electrical conductivity, is a measure of the resistance to the flow of an electric current and is measured either as a surface or a volume phenomenon. Surface resistivity is expressed in ohms per square (&OHgr;/□) and is measured at the surface of a material, usually at room temperature. Information detailing methods for quantifying surface resistivity is given in ASTM D257 and EOS/ESD S11.11.
c. Conductor. A material that has a surface resistivity less than about 10
5
&OHgr;/□.
d. Insulator. A material that has a surface resistivity greater than about 10
13
&OHgr;/□.
e. Electrostatic Dissipative Material (ESD). A material that has a surface resistivity between that of a conductor and an insulator, usually defined as between about 10
6
and 10
12
&OHgr;/□.
BACKGROUND ART
Every major plastic resin is, in its natural state, an electric insulator and many have a significant tendency to accumulate static electric charges. The ability of plastics to generate static electric charges and accumulate them variously due to the relative movement of one piece to another, separation of surfaces and the turbulent flow of contaminates in the air is well recognized. As an example, it is the presence of static electrical charges on sheets of thermoplastic film, such as the familiar food wraps, that cause the sheets to stick to each other.
The increased complexity and sensitivity of microelectronic devices makes the control of static discharge of particular concern in the electronic industry. There are many instances in the manufacture and use of electronic components and electronic devices in which an excessive accumulation of static charges can range from being a general nuisance to being a disabling or destructive force. Even a low voltage discharge may cause damage to sensitive devices. The need to control static charge buildup by the controlled dissipation may require the total assembly environment to be constructed of electrostatic dissipative materials. It may also require that materials used in storing, handling and shipping electronic devices be made from electrostatic dissipative materials. Tote boxes, strapping tape and shipping containers are examples.
The prevention of the buildup of static electrical charges which accumulate on plastics can be controlled through the use of several kinds of antistatic agents (antistats) which variously are incorporated into the plastic as fillers or applied to the plastic as a surface coating. Most commonly, the electrostatic dissipative technologies include those that make use of hygroscopic agents, those that make use of conductive particulates, those that make use of conductive fibers, and those that make use of low molecular weight additives and conductive polymers. While the use of any of these techniques can be helpful in alleviating electrostatic build-up, they are all subject to inherent limitations.
Surface antistat agents are not always reliable and can be inconsistent in operation. Some of the surface antistats do not adhere to a surface with sufficient tenacity to avoid being wiped away over a period of use. Several surface antistats are essentially hygroscopic materials that cause a conductive film of water to form over a substrate. In this latter case, it is of course obvious that the resistivity of a given part will be dependent upon and change with the ambient humidity.
The other principal approach to the manufacture of ESD materials has been to fill a plastic with conductive materials like carbon black, metal fibers, carbon fibers, etc. This approach advantageously provides high conductivity, rapid static dissipation, reliability and permanence. Its disadvantages lie in the fact that the mechanical properties of the plastic materials may be adversely affected at the required loading levels. Some materials such as carbon black are of such black intensity that it is difficult to color the composite material. Also it should be noted, the reduction in strength attributable to the filler may cause portions of the surface to slough off. This is anathema to clean rooms and otherwise presents a surface that is never clean. Carbon blacks and particulate graphitic fillers are especially prone to sloughing.
Conductive fillers are also sometimes difficult to disperse uniformly throughout a plastic matrix. Lack of uniformity can create “hot spots” within a plastic matrix where arcing or damaging static discharge can occur. Some effective fillers like carbon fibers are relatively expensive while metal fibers cause excessive wear and abrasion to the flights of the screws in mixing extruders.
More recently, attention has been focused upon the use of synthetic organic materials as antistatic agents. These antistatic agents can range from low molecular weight compounds to comparatively high molecular weight compounds and polymeric materials.
Antistatic agents in the form of comparatively low molecular weight organic materials can be blended with the plastic by melt mixing. Typical examples would include quaternary ammonium salts, fatty acid esters and ethoxylated amines. Not uncommonly, however, low molecular weight electrostatic agents suffer from poor heat stability and, depending on the melting point of the base resin, they may not survive melt processing temperatures. Low molecular weight antistats, even when successfully incorporated into a molding powder can cause problems. Low molecular weight organic antistatic agents can migrate (bloom) to the surface of the molded article. Surface migration may impair the appearance and tactile properties of an article. Furthermore, most of these additives function by absorbing a layer of moisture on the surface of the article and so require a certain threshold of ambient humidity, below which they are ineffective.
It is known that some organic antistatic agents are thermally unstable or chemically incompatible with a polymer at processing temperatures and cause unacceptable degradation to or enter into unwanted side reactions with the host polymer. This is especially so with more complex host resins such as polyesters and polyamides.
Higher molecular weight organic and polymeric antistatic agents are available but here the miscibility of the antistatic agent with the base resins often becomes a problem. In addition to physical incompatibility (poor miscibility), thermal instability and chemical incompatibility may also cause problems.
There are even more difficult requirements imposed upon antistats in meeting special needs for a high temperature electrostatic dissipative material for use in applications such as wafer back-end testing in which chips are pressed against a fixture (nests, contactors and sockets) under elevated pressures and relatively high operating temperatures for plastics, such as 160° C. Electrostatic charges resulting from the movement of the equipment, the wafer itself or minute particles in the air surrounding the wafer can discharge suddenly and have significant and negative effects on the wafer.
As discussed generally above the requirements for an electrostatic dissipative plastics material fall in a specific range of resistivity. When the surface resistivity is less than about 10
6
&OHgr;/□, a composition has very little insulating ability and is generally considered a cond
Campbell Richard W.
Lau Chi Way
Cain Edward J.
Lee K. W.
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