Patent Publication Number: US-7214873-B2

Title: Electrical transmission line and a substrate

Description:
1. FIELD OF THE INVENTION 
   The present invention relates to an electrical transmission line for transmitting microcurrents and in particular, to an electrical transmission line wherein the air-wired electrical wire and guard pattern are separated by an insulation stud, as well as a substrate on which the guard pattern is formed. 
   2. DISCUSSION OF THE BACKGROUND ART 
   Equipment for measuring microcurrents and a circuit with microcurrent output sensors, as well as other equipment for handling microcurrents often has an electrical transmission line that has been air-wired in order to prevent contamination by outside current or leakage current generated when the microcurrent is transmitted (refer to JP (Kokai) 8[1996]-335,754). A guard pattern of the same potential as the air-wired electrical transmission line is generally made around the transmission line in order to prevent direct-current leakage current from flowing around the transmission line and to prevent charge current from flowing to the floating capacitance formed around the transmission line. 
     FIG. 4  is a typical example of an electrical transmission line  50  with a guard pattern  30 . Guard pattern (conductive region)  30  is formed parallel to an electrical wire  20  that transmits microcurrents on a substrate  40 . A plurality of studs  10  are disposed in guard pattern  30  along electrical wire  20 . Electrical wire  20  and guard pattern  30  are separated by supporting the electrical wire  20  using insulation studs  10 . 
   An example of a typical insulation stud  10  is shown in  FIG. 3 . Insulation stud  10  is a cylindrical insulator  12  made from Teflon (registered trademark) with a top electrode  11  and a bottom electrode  13  at either end. Electrical wire  20  is anchored by soldering it onto electrode  11 . Moreover, insulation stud  10  is anchored to guard pattern  30  by joining guard pattern  30  and electrode  13  by soldering. 
   However, the temperature around electrical transmission line  50  changes over time. Because of this, the surface area contacting the atmosphere and the heat capacity differ between top electrode  11  and bottom electrode  13  connected to guard pattern  30 ; therefore, the rate of change in temperature at the two electrodes is not the same. As a result, a temperature difference is produced between the two electrodes while the peripheral temperature changes. When this occurs, a thermally stimulated current is produced in accordance with the temperature difference of insulator  12  and this current flows into electrical wire  20 . In general, this thermally stimulated current is a very small microcurrent (usually on the order of several femtoamperes to several hundred femtoamperes), but the fact of thermally stimulated current cannot be disregarded when the current flowing to electrical wire  20  is a microcurrent on the same order as the thermally stimulated current or when the transmitted current must be measured at the same resolution as the thermally stimulated current. 
   There are methods whereby electrical transmission line  50  is closed in order to eliminate as much as possible the effects of peripheral temperature changes and thereby to control the thermally stimulated current. However, when the transmission line is closed, the effect of internal heat generation increases and there is an increase in the possibility of current leakage, and similar effects occurring due to the presence of humidity trapped inside the closed area. Therefore, it is preferred that the difference in the amount of temperature change between top electrode  11  and bottom electrode  13  be reduced without closing the electrical transmission line. Bottom electrode  13  and guard pattern  30  can be thermally separated in order to accomplish this, but when bottom electrode  13  and guard pattern  30  are completely electrically separated, bottom electrode  13  enters a state where it is said to be floating electrically and it becomes impossible to prevent direct-current leakage current from floating around the transmission line or to prevent the charge current from flowing to floating capacitance produced around the line because the line is not completely guarded. Therefore, it is preferred that bottom electrode  13  and guard pattern  30  be electrically connected while preventing heat conduction between the two. 
   SUMMARY OF THE INVENTION 
   The present invention solves the above-mentioned problem with an electrical transmission line comprising an electrical wire, a guard pattern disposed parallel to the electrical wire, and a plurality of insulation studs inserted between the electrical wire and the guard pattern, this electrical transmission line being characterized in that the guard pattern has a non-conductive region disposed around the part where the insulation studs are fastened as well as a wiring pattern for electrically connecting the conductive region to the outside of the non-conductive region and the studs. 
   That is, it is possible to reduce the contact surface area between the bottom electrode and the conductive region (guard pattern) and to reduce the amount of heat conducted over a specific time by connecting the two by a linear pattern and not by a plane. When this is done, it is possible to control the effect of temperature changes of the conductive region on temperature changes of the bottom electrode; therefore, the temperature difference generated between the top electrode and the bottom electrode of the stud can be reduced, even in the case of changes in surrounding temperature. On the other hand, current does not flow between the bottom electrode and the conductive region (guard pattern); therefore, the contact surface area is reduced and the bottom electrode can be kept at the same potential as the guard pattern even if the resistance of the connection wiring increases. 
   The present invention provides an electrical transmission line with which the effect of thermally stimulated current is small, as well as a substrate which is used in this transmission line. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an enlarged view of the area near the stud of the working example of the present invention. 
       FIG. 2  is a working example of the current conduction line pertaining to the present invention. 
       FIG. 3  is an enlarged view of the region near the stud of a working example of the prior art. 
       FIG. 4  is a working example of an electrical transmission line pertaining to the prior art. 
       FIG. 5  is a diagram of another wiring pattern pertaining to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Preferred embodiments of the present invention will now be described in detail while referring to the drawings. 
     FIG. 2  shows an electrical transmission line  51  pertaining to the present invention, and  FIG. 1  shows an enlarged view near the part where an insulation stud  10  is fastened and a conductive region  31  is formed. Guard pattern (conductive region)  31  is made on a substrate  41  parallel to electrical wire  20  to which microcurrent is transmitted. Guard pattern  31  is set at the same potential as the electrical wire  20 . Guard pattern  31  of the present working example is simply a plane pattern made on printed substrate  41 , but a spatial enclosure can also be formed using a copper, aluminum, or other metal plate. That is, it is possible to control the leakage current from electrical wire  20  and the charge current to floating capacitance by covering with metal foil the region where air wiring is disposed in order to separate it from the outside space and by applying voltage of the same potential as that of electrical wire  20  to this metal structure. 
   A plurality of non-conductive regions  32  are disposed in guard pattern  31  parallel to electrical wire  10 . Non-conductive region  32  of the present working example is made by etching in a circle around the part of the conductive region  31  where insulation stud  10  is fastened. Of course, the method by which non-conductive region  32  is made is not limited to etching, and this non-conductive region can be made by forming holes, or another method. Electrical wire  20  and guard pattern  31  are separated by insulation stud  10  and are in an electrically non-conducting state. 
   Insulation stud  10  is a cylindrical insulator  12  with a top electrode  11  and a bottom electrode  13  at either end. Insulator  12  is made from Teflon (registered trademark). Electrodes  11  and  13  are made from a brass plated with a nickel foundation and a gold, but gold, nickel, or another metal with high electrical conductivity can also be used. Electrical wire  20  is anchored to top electrode  11  by soldering. 
   A wiring pattern  33  for electrically connecting bottom electrode  13  and conductive region  31  is disposed in non-conductive region  32 . Wiring pattern  33  is made by masking a region for this wiring pattern  33  in order to leave a conductive region when non-conductive region  32  is made by etching. As long as wiring pattern  33  is long, the amount of heat transmitted over a specific time between the bottom electrode  13  and conductive region  31  will decrease along this length. Therefore, it is preferred that wiring pattern  33  is longer than the distance between stud  10  and conductive region  31 . However, heat is also transmitted through glass epoxy substrate  41 ; therefore, even if the heat conductivity of wiring pattern  33  is less than the conductivity of substrate  41 , any increase in this effect is undesirable. By means of the present working example, a long pattern is made by making wiring pattern  33  go ¾ of the way around the stud, parallel to the part where stud  10  is fastened, but a spiral-shaped pattern can also be used in order to produce a long wiring pattern  33 . Moreover, the zigzag-shaped pattern in  FIG. 5  can be used in place of a pattern parallel to the outside periphery of the part where stud  10  is fastened. The amount of heat transmitted decreases as the line width of wiring pattern  33  becomes narrower. Wiring with a line width of 150 microns is used in the present working examples. 
   The technical concept of the present invention has been described in detail while referring to a specific working example, but it is clear that persons skilled in the art to which the present invention belongs can make various changes and modifications that do not stray from the gist or the scope of the claims.