Patent Publication Number: US-2009224213-A1

Title: Variable impedance composition

Description:
BACKGROUND OF THE INVENTION 
     (A) Field of the Invention 
     The present invention relates to a variable impedance material, and more particularly, to a variable impedance material comprising a conductive powder, semi-conductive power, and an insulation adhesive. 
     (B) Description of the Related Art 
     Integrated circuits are externally fed with supply potentials and input signals to be processed and to have processed output signals received from them. In particular, the input signal terminals are very sensitive, since the conductor tracks that feed the potentials and signals lead directly to a gate terminal of an input switching stage. While the integrated circuit is being manually handled, or during the automated processing to solder the integrated circuit on a circuit board, there is risk that the sensitive input stage or output stage may be destroyed by electrostatic discharge. For instance, the human body may be electrostatically charged and then discharged via the terminals leading to the outside of the semiconductor component containing the integrated circuit. 
     Tools of automatic component-mounting machines or test equipment may also be electrostatically charged and discharged via the semiconductor component. As technology advances and the scale of pattern lines on the semiconductor body bearing integrated circuits becomes smaller, there is a need for protection against such electrostatic discharges. Integrated circuit devices are often provided with some protection against electrostatic discharge (ESD) with high input currents, such as electrical resistors connected in their input paths, thereby limiting the input current. 
     U.S. Pat. No. 6,642,297 discloses a composition for providing protection against electrical overstress (EOS) comprising an insulating binder, doped semiconductive particles, and semiconductive particles. The composite materials exhibit a high electrical resistance to normal operating voltage values, but in response to an EOS transient the materials switch to a low electrical resistance and limit the EOS transient voltage to a low level for the duration of the EOS transient. 
     U.S. Pat. No. 6,013,358 discloses a transient voltage protection device wherein a gap between a ground conductor and another conductor is formed using a diamond-dicing saw. Substrate material selection includes specific ceramic materials having a density of less than 3.8 gm/cm.sup.3 designed to optimize performance and manufacturability. An overlay layer can be provided to minimize burring of the conductors during formation of the gap. 
     U.S. Pat. No. 5,068,634 discloses a material and device for electronic circuitry that provides protection from fast transient over-voltage pulses. The electroded device can additionally be tailored to provide electrostatic bleed. Conductive particles are uniformly dispersed in an insulating matrix or binder to provide a material having non-linear resistance characteristics. The non-linear resistance characteristics of the material are determined by the inter-particle spacing within the binder as well as by the electrical properties of the insulating binder. By tailoring the separation between the conductive particles, thereby controlling quantum-mechanical tunneling, the electrical properties of the non-linear material can be varied over a wide range. 
     U.S. Pat. No. 6,498,715 discloses a stack up type low capacitance over-voltage protective device comprising a substrate, a conductive low electrode layer formed on the substrate, a voltage sensitive material layer formed on the conductive lower electrode layer, and a conductive upper electrode layer formed on the voltage sensitive material layer. 
     U.S. Pat. No. 6,645,393 discloses a material for transient voltage suppressors composed of at least two kinds of evenly-mixed powders including a powder material with non-linear resistance interfaces and a conductive powder. The conductive powder is distributed in the powder with non-linear resistance interfaces to relatively reduce the total number of non-linear resistance interfaces between two electrodes and, as a result, decrease the breakdown voltage of the components. 
     SUMMARY OF THE INVENTION 
     One aspect of the present invention provides a variable impedance material comprising a conductive powder, semi-conductive power, and an insulation adhesive, which presents a high resistance at a low applied voltage and a low resistance at a high applied voltage. 
     A variable impedance composition according to this aspect of the present invention comprises a conductive powder in an amount from 10% to 30% of the weight of the variable impedance composition, a semi-conductive power in an amount from 30% to 90% of the weight of the variable impedance composition, and an insulation adhesive in an amount from 3% to 50% of the weight of the variable impedance composition. 
     According to one embodiment of the present invention, the variable impedance material presents a high resistance at a low applied voltage and a low resistance at a high applied voltage. As the variable impedance material is positioned in a gap between two conductors of an over-voltage protection device, the over-voltage protection device as a whole presents a high resistance to a low voltage applied across the gap and a low resistance to a high voltage applied across the gap. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which: 
         FIG. 1  to  FIG. 5  illustrate an over-voltage protection device according to one embodiment of the present invention; 
         FIG. 6  shows the relationship between the resistance and the applied voltage of the variable impedance material according to one embodiment of the present invention; 
         FIG. 7  shows the response of the over-voltage protection device as a transient voltage is applied according to one embodiment of the present invention; and 
         FIG. 8  illustrates an over-voltage protection device according to another embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  to  FIG. 5  illustrate an over-voltage protection device  10  according to one embodiment of the present invention. Referring to  FIG. 1 , an electrode structure  20  is formed on a substrate  12  made of insulation material such as plastic material, i.e., the substrate  12  is a plastic substrate having an upper surface  12 A and a bottom surface  12 B. The electrode structure  20  includes a first nonrectangular conductor  14  having a first protrusion  14 A positioned on the upper surface  12 A of the substrate  12 , a second nonrectangular conductor  16  having a second protrusion  16 A positioned on the upper surface  12 A of the substrate  12 , a first side-electrode  22  positioned on one side of the substrate  12  and connected to the first nonrectangular conductor  14 , and a second side-electrode  24  positioned on the other side of the substrate  12  and connected to the second nonrectangular conductor  16 . 
     In particular, a first conductive member  22 ′ is sandwiched between the substrate  12  and the first side-electrode  22 , and a second conductive member  24 ′ is sandwiched between the substrate  12  and the second side-electrode  24 . The first conductive member  22 ′ and the second conductive member  24 ′ can be plating metal layers or conductive through holes. Preferably, at least one of first protrusion  14 A and the second protrusion  16 A is a tapering protrusion with a tapering width. The second protrusion  16 A faces the first protrusion  14 A to form an arcing path  18  from the first protrusion  14 A to the second protrusion  16 A. Preferably, the first nonrectangular conductor  14  and the second nonrectangular conductor  16  are trapezoid and positioned on the substrate  12  in a mirror-image manner. In particular, the shape of the first nonrectangular conductor  14  can be different from that of the second nonrectangular conductor  16 . The first protrusion  14 A includes a first flat edge  14 B and the second protrusion  16 A includes a second flat edge  16 B facing the first flat edge  14 B. 
     Referring to  FIG. 2 , a cross-sectional view of the electrode structure  20 . The widths of the first protrusion  14 A and the second protrusion  16 A at their upper portions is larger than the widths at their middle portions such that the first protrusion  14 A and the second protrusion  16 A have a non-uniform thickness. Consequently, the first protrusion  14 A and the second protrusion  16 A are closer at the upper portion than at the middle portion such that the arcing path  20  is formed between the upper portion of the first protrusion  14 A and the upper portion of the second protrusion  16 A. 
     Referring to  FIG. 3 , a variable impedance material  26  is formed between the first protrusion  14 A and the second protrusion  16 A. Preferably, the variable impedance material includes a conductive powder in an amount from 10% to 30% of the weight of the variable impedance material, a semi-conductive powder in an amount from 30% to 90% of the weight of the variable impedance material, and an insulation adhesive in an amount from 3% to 50% of the weight of the variable impedance material. 
     Preferably, the conductive powder includes at least one element selected from the group consisting of Al, Ag, Pd, Pt, Au, Ni, Cu, W, Cr, Fe, Zn, Ti, Nb, Mo, Ru, Pb, and Ir, the semi-conductive powder includes zinc oxide or silicon carbide, and the insulation adhesive includes epoxy or silicone. In addition, the variable impedance material  28  may further include an insulation powder in an amount from 10% to 60% of the weight of the variable impedance material, and the insulation powder includes metal oxide such as aluminum oxide or zirconium oxide. 
     Referring to  FIG. 4  and  FIG. 5 , an arc-protection layer  30  is formed to cover the variable impedance material  28 , and an insulation layer  32  is then formed to cover the arc-protection layer  30  so as to complete the over-voltage protection device  10 . Preferably, the arc-protection layer  30  include inorganic insulation material and organic insulation material, wherein the inorganic insulation material includes metal oxide and the organic insulation material includes epoxy or silicone. The insulation layer  32  includes inorganic insulation material and organic insulation material, wherein the inorganic insulation material includes metal oxide and the organic insulation material includes epoxy or silicone. 
       FIG. 6  shows the relationship between the resistance and the applied voltage of the variable impedance material  26  according to one embodiment of the present invention. Obviously, the variable impedance material  26  presents a high resistance at a low applied voltage and a low resistance at a high applied voltage. With the variable impedance material  26  positioned in the gap between the first nonrectangular conductor  14  and the second nonrectangular conductor  16 , the over-voltage protection device  10  as a whole presents a high resistance to a low voltage applied across the gap and a low resistance to a high voltage applied across the gap. 
       FIG. 7  shows the response of the over-voltage protection device  10  as a transient voltage is applied according to one embodiment of the present invention. The transient voltage of 1900 Volts is applied to the first nonrectangular conductor  14  and the second nonrectangular conductor  16 , and the over-voltage protection device  10  switches to a low electrical resistance and limits the transient voltage of 1900 Volts to about 518 Volts. In other words, an electrical device connected to the over-voltage protection device  10  in parallel will not suffer transient voltage of 1900 Volts, but experiences a limited voltage about 518 Volts. 
       FIG. 8  illustrates an over-voltage protection device  10 ′ according to another embodiment of the present invention. Compared to the over-voltage protection device  10  shown in  FIG. 5 . The over-voltage protection device  10 ′ in  FIG. 8  further comprises at least one alignment block  34  positioned on the bottom surface  12 B of the substrate  12 , and the alignment block  34  is configured to align to another alignment block on a circuit board (not shown in the drawing) when the over-voltage protection device  10 ′ is attaching to the circuit board. In addition, the alignment block  32  is not electrically connected to the conductive member of the over-voltage protection device  10 ′, and the number of the alignment block  32  can be optionally designed to be two or more. 
     Conventional over-voltage protection devices all have two rectangular conductors a gap between the two conductors of uniform width; therefore, the arcing path is unpredictable. In contrast, the present over-voltage protection device  10  comprises the first nonrectangular conductor  14  having the first protrusion  14 A and the second nonrectangular conductor  16  having the second protrusion  16 A facing the first protrusion  14 A such that the distance between the first nonrectangular conductor  14  and the second nonrectangular conductor  16  is non-uniform. In particular, the gap between the first nonrectangular conductor  14  and the second nonrectangular conductor  16  is smaller at the protrusion portion than at other portions such that is the arcing path  18  is designed to be at the protrusion portion and the variable impedance material  26  covers the protrusion portion according to the embodiment of the present invention. 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.