Patent Publication Number: US-2009226689-A1

Title: Pressure sensitive conductive  sheet and panel switch using the same

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a pressure sensitive conductive sheet and a panel switch using the sheet, which are used to operate various electronic apparatuses. 
     2. Background Art 
     In recent years, various electronic apparatuses including portable telephones and car navigation systems are becoming increasingly functional and diverse. In line with this, panel switches used to operate these apparatuses are expected to be diverse and to provide reliable operation. 
     One such conventional panel switch will be described with reference to  FIGS. 6 to 8  and  FIGS. 9A and 9B . Of these drawings, the sectional views are exaggerated in the thickness direction for clarity. 
       FIG. 6  is a sectional view of a conventional panel switch. As shown in  FIG. 6 , the panel switch includes pressure sensitive conductive sheet  4  having film-like base material  1  and resistive layer  2  formed on the bottom surface of base material  1 . Resistive layer  2  is made of a synthetic resin with carbon powder dispersed therein. Resistive layer  2  has different sized particles  3  dispersed therein which are made of a synthetic resin, glass, or the like, so that resistive layer  2  has a rough bottom surface. 
     The panel switch also includes board  5  on the bottom surface of pressure sensitive conductive sheet  4 , board  5  being provided on its top surface with fixed contacts  6 A and  6 B made of silver, carbon, or the like. Between pressure sensitive conductive sheet  4  and board  5 , there is provided spacer  7  which is made of an insulating resin and surrounds fixed contacts  6 A and  6 B. As a result, the bottom surface of pressure sensitive conductive sheet  4  is opposed to fixed contacts  6 A and  6 B with a predetermined spacing therebetween. 
     The panel switch thus structured is installed on the control surface of an electronic apparatus, with fixed contacts  6 A and  6 B connected to electronic circuits (not shown) of the apparatus via lead wires (not shown) or the like. 
       FIG. 7  is a sectional view showing a state in which the conventional panel switch is pressed. As shown in  FIG. 7 , when the user presses the top surface of pressure sensitive conductive sheet  4 , pressure sensitive conductive sheet  4  bends downward, so that the portion of the bottom surface of resistive layer  2  that has large particles  3 A and  3 B is brought into contact with fixed contacts  6 A and  6 B. As a result, fixed contacts  6 A and  6 B are electrically connected to each other via resistive layer  2 . 
     When the user applies a higher compressive force, the portion of the bottom surface of resistive layer  2  that has particles  3 C and  3 D smaller in size than particles  3 A and  3 B is also brought into contact with fixed contacts  6 A and  6 B. As a result, resistive layer  2  has a larger contact area with fixed contacts  6 A and  6 B, thereby changing the resistance between fixed contacts  6 A and  6 B. 
       FIG. 8  is a resistance characteristic diagram relative to the compressive force in the conventional panel switch. As shown in FIG.  8 , as the compressive force increases, it increases the contact area between fixed contacts  6 A and  6 B and the bottom surface of resistive layer  2 , which is rough because resistive layer  2  contains different sized particles  3 . In other words, a small compressive force produces a large resistance, and a large compressive force produces a small resistance. Thus, as shown in the curved line “A” of the resistance characteristic diagram of  FIG. 8 , the resistance characteristics gradually change according to the compressive force. 
     The electric connections or the resistance changed according to the compressive force are detected by an electronic circuit so as to perform various functions of the apparatus such as changing the speed of the cursor or the pointer on the display screen. A conventional technique related to the panel switch is disclosed in Japanese Patent Unexamined Publication No. 2008-311208. 
       FIGS. 9A and 9B  are enlarged sectional views showing a state in which the conventional panel switch has been repeatedly pressed. 
     In the case where particles  3  dispersed in resistive layer  2  are soft and elastically deformable, every time the user presses pressure sensitive conductive sheet  4 , the bottom surface of resistive layer  2  is pressed against fixed contacts  6 A and  6 B. As a result, particles  3  and resistive layer  2  around them are repeatedly elastically deformed. When pressing has been repeated hundreds of thousands or a million times, resistive layer  2 A around particles  3  is expanded and deformed as shown in  FIG. 9A . The expansion and deformation increases the distance between fixed contacts  6 A and  6 B, and hence, the same compressive force can produce a larger resistance shown in the curved line “B” than the original resistance shown in the curved line “A” of FIG.  8 . 
     In contrast, in the case where particles  3  are hard and rigid, every time the user presses sensitive conductive sheet  4 , the bottom surface of resistive layer  2  is pressed against fixed contacts  6 A and  6 B by particles  3 . When pressing has been repeated, the bottom surface of resistive layer  2 B beneath particles  3  becomes almost flat as shown in  FIG. 9B . This increases the contact area between the bottom surface of resistive layer  2 B and fixed contacts  6 A and  6 B, possibly causing the same compressive force to produce a smaller resistance as shown in the curved line “C” than the original resistance shown in the curved line “A” of  FIG. 8 . 
     Thus, the conventional pressure sensitive conductive sheet and the panel switch using the sheet can cause variations in the resistance change according to the compressive force after pressing has been repeated hundreds of thousands or a million times. Therefore, it is necessary for an electronic circuit to detect the resistance in anticipation of such variations. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a pressure sensitive conductive sheet and a panel switch using the sheet which have small variations in resistance change after repeated pressing, thereby providing reliable operation. 
     The present invention provides a pressure sensitive conductive sheet including a film-like base material and a resistive layer formed on the bottom surface of the base material, the resistive layer having soft particles and hard particles dispersed therein and different in average particle size from each other. 
     With this structure, a combination of elastically deformable soft particles and rigid hard particles dispersed in the resistive layer allows the pressure sensitive conductive sheet to have small variations in resistance change after repeated pressing, thereby allowing the sheet to provide reliable operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional view of a panel switch according to a first embodiment of the present invention. 
         FIG. 2  is a sectional view showing a state in which the panel switch according to the first embodiment of the present invention is pressed. 
         FIG. 3  is a resistance characteristic diagram relative to the compressive force in the panel switch according to the first embodiment of the present invention. 
         FIG. 4  is a sectional view of another panel switch according to the first embodiment of the present invention. 
         FIG. 5A  is a partial plan view of fixed contacts in the panel switch according to the first embodiment of the present invention. 
         FIG. 5B  is a partial plan view of other fixed contacts in the panel switch according to the first embodiment of the present invention. 
         FIG. 5C  is a partial plan view of other fixed contacts in the panel switch according to the first embodiment of the present invention. 
         FIG. 6  is a sectional view of a conventional panel switch. 
         FIG. 7  is a sectional view showing a state in which the conventional panel switch is pressed. 
         FIG. 8  is a resistance characteristic diagram relative to the compressive force in the conventional panel switch. 
         FIG. 9A  is an enlarged sectional view showing a state in which the conventional panel switch has been repeatedly pressed. 
         FIG. 9B  is another enlarged sectional view showing a state in which the conventional panel switch has been repeatedly pressed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An embodiment of the present invention will be described as follows with reference to  FIGS. 1 to 5 . Of these drawings, the sectional views are exaggerated in the thickness direction for clarity. Like components are labeled with like reference numerals with respect to the panel switch described in the section of Background Art, and hence the detailed description thereof will be omitted. 
     First Embodiment 
       FIG. 1  is a sectional view of a panel switch according to a first embodiment of the present invention. As shown in  FIG. 1 , the panel switch includes pressure sensitive conductive sheet  16  including base material  11 , low resistive layer  12  on the bottom surface of base material  11 , and high resistive layer  13  on the bottom surface of low resistive layer  12 . Base material  11  is a flexible film with a thickness of 25 to 200 μm and made of polyethylene terephthalate, polycarbonate, polyimide, or the like. Low resistive layer  12  is made of a synthetic resin such as phenol with carbon powder dispersed therein, epoxy, phenoxy, or fluororubber, and has a sheet resistance of 50 Ω to 30 kΩ/square. Alternatively, low resistive layer  12  can be made of polyester or epoxy with silver, carbon, or the like dispersed therein, and have a sheet resistance of several ohms to several tens of ohms per square. Alternatively, the lower resistive layer than the low resistive layer  12  can be formed between base material  11  and the lower resistive layer  12 , the lower resistive layer being made of polyester or epoxy with silver, carbon, or the like dispersed therein, and have a sheet resistance of several ohms to several tens of ohms per square. 
     High resistive layer  13  is made of a synthetic resin with carbon powder dispersed therein, and has a sheet resistance of 50 kΩ to 5 MΩ/square and a thickness of 1 to 50 μm. High resistive layer  13  contains soft particles  14  with a large average particle size and hard particles  15  with a small average particle size, both of the average particle sizes being in the range of 1 to 100 μm. Soft particles  14  are made of urethane, acrylic, nylon, silicone, olefin, or the like and have a Shore A hardness of 30 to 90. Hard particles  15  are made of glass, alumina, zirconia, or the like and have a Vickers hardness of 500 to 1800. Soft particles  14  and hard particles  15  are dispersed in an amount of 10 to 80 wt %, so that high resistive layer  13  has a rough bottom surface. 
     Pressure sensitive conductive sheet  16  having the above-described structure is formed as follows. First, low resistive layer  12  is screen printed on base material  11 . Then, high resistive layer  13  having soft particles  14  and hard particles  15  dispersed therein is screen printed on low resistive layer  12  using an SUS plate with a 100 to 300 mesh size. 
     The panel switch also includes board  5  formed under the bottom surface of pressure sensitive conductive sheet  16 . Board  5  can be a film made of polyethylene terephthalate, polycarbonate, or the like, or a plate made of paper phenol or glass-filled epoxy. Board  5  is provided thereon with fixed contacts  6 A and  6 B made of silver, carbon, copper foil, or the like with a spacing of 0.02 to 0.2 mm from each other under the bottom surface of pressure sensitive conductive sheet  16 . 
     Between pressure sensitive conductive sheet  16  and board  5 , there is provided spacer  7  made of an insulating resin such as polyester or epoxy in such a manner as to surround fixed contacts  6 A and  6 B. As a result, the bottom surface of high resistive layer  13  is opposite to fixed contacts  6 A and  6 B with a spacing of 10 to 100 μm therebetween. 
     The panel switch according to the first embodiment thus structured is installed on the control surface of an electronic apparatus, with fixed contacts  6 A and  6 B connected to electronic circuits (not shown) of the apparatus via lead wires (not shown) or the like. 
       FIG. 2  is a sectional view showing a state in which the panel switch according to the first embodiment is pressed. As shown in  FIG. 2 , when the user presses the top surface of pressure sensitive conductive sheet  16 , pressure sensitive conductive sheet  16  bends downward, so that the portion of the bottom surface of high resistive layer  13  that has soft particles  14 A and  14 B with a large average particle size dispersed therein is brought into contact with fixed contacts  6 A and  6 B. As a result, fixed contacts  6 A and  6 B are electrically connected to each other via high resistive layer  13  and low resistive layer  12 . 
     When the user applies a higher compressive force, the portion of the bottom surface of high resistive layer  13  that has hard particles  15 A and  15 B with a smaller average particle size than soft particles  14 A and  14 B is also brought into contact with fixed contacts  6 A and  6 B. This results in a change in the resistance between fixed contacts  6 A and  6 B. 
       FIG. 3  is a resistance characteristic diagram relative to the compressive force in the panel switch according to the first embodiment. As shown in  FIG. 3 , as the compressive force increases, it increases the contact area between fixed contacts  6 A,  6 B and the bottom surface of high resistive layer  13 , which is rough because high resistive layer  13  contains soft particles  14  and hard particles  15  different in average particle size. In other words, a compressive force produces a large resistance, and a large compressive force produces a small resistance. Thus, as shown in the curved line “A” of the resistance characteristic diagram of  FIG. 3 , the resistance characteristics gradually change according to the compressive force. 
     The electric connections or the resistance changed according to the compressive force are detected by an electronic circuit so as to perform various functions of the apparatus such as changing the speed of the cursor or the pointer on the display screen. 
     In the panel switch according to the first embodiment, pressing has been repeated hundreds of thousands or a million times, high resistive layer  13 , which is brought into or out of contact with fixed contacts  6 A and  6 B, is prevented from being expanded and deformed or from having a flat bottom surface. This is because high resistive layer  13  has elastically deformable soft particles  14  and rigid hard particles  15  dispersed therein, which are different in average particle size. This results in small variations in the resistance between fixed contacts  6 A and  6 B. 
     As described above, the amount of dispersion of soft particles  14  and hard particles  15  in high resistive layer  13  can be selected within the range of 10 to 80 wt %. When the amount is less than 40 wt %, however, high resistive layer  13  has too large a surface area, whereas when it is over 60 wt %, soft particles  14  and hard particles  15  are closely packed in high resistive layer  13 . Therefore, the amount of dispersion is preferably 40 to 60 wt % so that the particles  14  and  15  can be uniformly distributed across the surface of high resistive layer  13 . 
     As described above, soft particles  14  can have a larger average particle size than hard particles  15  in order to mitigate the impact on high resistive layer  13  when pressed. This reduces the variations in the resistance change after repeated pressing, thereby allowing the panel switch to provide reliable operation. 
     The average particle sizes of soft particles  14  and hard particles  15  can be selected within the range of 1 to 100 μm as described above. However, it is preferably 1 to 30 μm in order to make particles  14  and  15  uniformly dispersed in high resistive layer  13  having a thickness of 1 to 50 μm. It is further preferable to combine hard particles  15  with an average particle size of 5 to 15 μm and soft particles  14  with an average particle size of 10 to 25 μm. 
     The ratio of soft particles  14  to hard particles  15  in high resistive layer  13  can be selected within the range of 1:9 to 9:1. It is preferable, however, that hard particles  15  are more dispersed when soft particles  14  have a large average particle size, and less dispersed when soft particles  14  have a small average particle size. 
     Alternatively, the present invention can be implemented without using low resistive layer  12  by directly forming high resistive layer  13  having soft particles  14  and hard particles  15  dispersed therein on the bottom surface of base material  11 . As described above, however, forming low resistive layer  12  and high resistive layer  13  in this order on the bottom surface of base material  11  makes the resistance change smooth and stable. More specifically, as shown in  FIG. 2 , when the compressive force is small enough that only the bottom surface of high resistive layer  13  beneath soft particles  14 A and  14 B having a large average particle size comes into contact with fixed contacts  6 A and  6 B, the resistance between fixed contacts  6 A and  6 B is the sum of the resistance of high resistive layer  13  between soft particles  14 A and  14 B, and the conductor resistance of low resistive layer  12 . 
     On the other hand, when the compressive force is high enough that the bottom surface of high resistive layer  13  beneath hard particles  15 A and  15 B having a small average particle size comes into contact with fixed contacts  6 A and  6 B, the sum of the conductor resistances between hard particles  15 A and  15 B is added in parallel to the sum of the resistance of high resistive layer  13  and the conductor resistance of low resistive layer  12 . As a result, the resistance between fixed contacts  6 A and  6 B is small. 
     Thus, as the contact area increases between the rough bottom surface of high resistive layer  13  and fixed contacts  6 A,  6 B with increasing compressive force, the sum of the resistance of high resistive layer  13  and the conductor resistance of low resistive layer  12  having different sheet resistances from each other between fixed contacts  6 A and  6 B continues to be added in parallel. As a result, as shown in the curved line “A” of the resistance characteristic diagram of  FIG. 3 , the resistance change can be smooth and stable. Alternatively, by setting the lower resistive layer than the low resistive layer  12  between base material  11  and the lower resistive layer  12 , the three-layered structure can be formed, and it makes the resistance change smoother and more stable. 
     In the above description, low resistive layer  12  has a sheet resistance of 50 Ω to 30 kΩ/square, and high resistive layer  13  has a sheet resistance of 50 kΩ to 5 MΩ/square. It is preferable, however, that low resistive layer  12  has a sheet resistance of 50 Ω to 10 kΩ/square, and high resistive layer  13  has a sheet resistance of 100 kΩ to 1 MΩ/square. 
       FIG. 4  is a sectional view of another panel switch according to the first embodiment. As shown in  FIG. 4 , low resistive layer  12  formed on the bottom surface of base material  11  is provided at the outer periphery of the center of its bottom surface with spacer  7 A. High resistive layer  13  having soft particles  14  and hard particles  15  dispersed therein is formed on the center of the bottom surface of low resistive layer  12  and on the bottom surface of spacer  7 A. Board  5  is provided with circular fixed contact  6 C in the center of its top surface, and substantially ring- or horseshoe-shaped fixed contact  6 D on the outer periphery of the top surface. 
     The portion of high resistive layer  13  that is formed on the bottom surface of spacer  7 A is placed on or adhesively connected to fixed contact  6 D. The center of the bottom surface of high resistive layer  13  faces fixed contact  6 C. The panel switch thus structured provides the same effect as the panel switch of  FIG. 1 . 
     The panel switch according to the present invention provides other various shaped fixed contacts.  FIGS. 5A to 5C  are partial plan views of fixed contacts used in the panel switch according to the first embodiment. In  FIG. 5A , circular fixed contact  6 C and annular fixed contact  6 D are concentrically arranged with respect to each other. In  FIG. 5B , fixed contacts  6 E and  6 F are semicircular. In  FIG. 5C , comb-shaped fixed contacts  6 H and  6 J are engaged with each other between two arc-shaped fixed contacts  6 G. 
     As described hereinbefore, according to the present embodiment, low resistive layer  12  and high resistive layer  13  are formed in this order on the bottom surface of film-like base material  11 , and soft particles  14  and hard particles  15  different in average particle size are dispersed in high resistive layer  13 . Fixed contacts  6 A and  6 B are arranged under the bottom surface of high resistive layer  13 . This structure provides pressure sensitive conductive sheet  16  and a panel switch using the sheet, which have small variations in the resistance change after repeated pressing, thereby providing reliable operation. 
     The pressure sensitive conductive sheet and the panel switch using the sheet according to the present invention are useful for the operation of various electronic apparatuses because of having small variations in the resistance change and providing reliable operation.