Abstract:
A circuit board includes: an input terminal for inputting a signal from an external circuit; a first land and a second land; and an electrostatic-charge absorbing conductor for absorbing an electrostatic charge. The first land is electrically connected to the input terminal. The second land is separated from the first land by a predetermined distance therebetween. A distance between the electrostatic-charge absorbing conductor and the first land is shorter than a distance between the first land and the second land. In the circuit board, the electrostatic charge is effectively absorbed into the electrostatic-charge absorbing conductor through the first land so that the electrostatic charge is prevented from negatively affecting on electric parts on the board.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on Japanese Patent Applications No. 2004-311332 filed on Oct. 26, 2004, and No. 2005-259700 filed on Sep. 7, 2005, the disclosures of which are incorporated herein by reference. 
   FIELD OF THE INVENTION 
   The present invention relates to a circuit board and a semiconductor device having a circuit board. 
   BACKGROUND OF THE INVENTION 
   Conventionally, regarding ECU for an automotive vehicle, when a passenger or a driver operates a switch, electrostatic charge may penetrate into the ECU through a wiring harness, so that an IC in the ECU is damaged. Thus, to protect the ECU from the static charge, an electric part such as a resistor is formed on an input side of the ECU. The electric part for the protection of the ECU prevents not only the static charge but also external noise from penetrating into the ECU. 
   Further, recently, the number of electric parts in the ECU in the vehicle increases, and an accommodation space for the ECU becomes smaller. Thus, it is required to minimize the ECU and to increase the number of electric parts in the ECU. 
   Furthermore, to improve lifetime of a solder, an electrode is formed on a long side of the electric part in the ECU, instead of a short side of the part. In this case, a distance between electrodes becomes narrower, compared with a case where the electrode is formed on the short side of the part. 
   Thus, the electric part for the protection of static charge is minimized, and the distance between the electrodes of the parts becomes narrower so that a distance between lands on a circuit board becomes narrower, the lands corresponding to the electrodes. Therefore, conventionally, the electric part such as the resistor can prevent the static charge from penetrating into the IC of the ECU, i.e., the static charge penetrating into the IC is reduced. However, in the above device, since the distance between the lands is short, the static charge transmits between the lands through the protection part so that the static charge is not reduced in the protection part. Thus, the static charge directly penetrates into the IC so that the IC is damaged. 
   To protect the IC from the above direct transmission of the static charge, Japanese laid-Open Patent Publication No. S62-35480 discloses a circuit board. The circuit board includes the first wiring and the second wiring. The first wiring includes a resistor for limiting current, and formed on a signal input terminal side of a semiconductor circuit package. The second wiring works as a ground, and includes two parts, one of which protrudes and extends in parallel to the first wiring, the other one of which is connected to the semiconductor circuit package. The circuit board includes a protrusion facing the first and the second wirings so that a spark gap is provided. The spark gap is covered with a tape. 
   In general, a wiring is coated with an insulation film. Accordingly, in the device having the spark gap between the first and the second wirings, the static charge is not absorbed effectively, since the wiring is covered with the insulation film. Thus, the static charge may damage the IC. 
   SUMMARY OF THE INVENTION 
   In view of the above-described problem, it is an object of the present invention to provide a circuit board and an electric device having a circuit board. 
   A circuit board includes: an input terminal for inputting a signal from an external circuit; a first land and a second land; and an electrostatic-charge absorbing conductor for absorbing an electrostatic charge. The first land is electrically connected to the input terminal. The second land is separated from the first land by a predetermined distance therebetween. A distance between the electrostatic-charge absorbing conductor and the first land is shorter than a distance between the first land and the second land. 
   In the circuit board, the electrostatic charge penetrated from the input terminal is effectively absorbed into the electrostatic-charge absorbing conductor through the first land so that the electrostatic charge is prevented from negatively affecting on electric parts on the circuit board. 
   Alternatively, the circuit board may further include a first electric part disposed on the first land. The first electric part is an electrostatic charge reducing element for reducing the electrostatic charge. Alternatively, the first land may include a first electric field concentration portion, at which an electric filed is concentrated, and the electrostatic-charge absorbing conductor is disposed near the first electric field concentration portion of the first land. Alternatively, the electrostatic-charge absorbing conductor may include a second electric field concentration portion, at which an electric filed is concentrated. The second electric field concentration portion of the electrostatic-charge absorbing conductor faces or is disposed near the first electric field concentration portion of the first land. 
   Alternatively, the electrostatic-charge absorbing conductor may be disposed between the first land and the second land, or disposed under the first electric part. 
   Alternatively, the circuit board may further include: a substrate having a first plane and a second plane; and an electrode. The first electric part is disposed on the first plane of the substrate. The electrode is disposed on the second plane of the substrate. The electrostatic-charge absorbing conductor is a via conductor in a via hole of the substrate. The via conductor is disposed on the first plane of the substrate, and electrically connected to the electrode on the second plane of the substrate. 
   Alternatively, the second land may be electrically connected to a ground through a capacitor. The circuit board may further include: a third land electrically connected to the first land and the input terminal; a second electric part disposed on the third land; and a fourth land separated from the third land by a predetermined distance, and connected to the ground. A distance between the third land and the fourth land is shorter than the distance between the first land and the second land. 
   Further, an electric device includes: a circuit board including an input terminal for inputting a signal from an external circuit; a first land disposed on the circuit board and electrically connected to the input terminal; a second land disposed on the circuit board and separated from the first land by a predetermined distance; a first electric part mounted between the first land and the second land; an input circuit element electrically connected to the second land and disposed on the circuit board; and an electrostatic-charge absorbing conductor disposed on the circuit board for absorbing an electrostatic charge. A distance between the electrostatic-charge absorbing conductor and the first land is shorter than a distance between the first land and the second land. 
   In the device, the electrostatic charge penetrated from the input terminal is effectively absorbed into the electrostatic-charge absorbing conductor through the first land so that the electrostatic charge is prevented from negatively affecting on electric parts on the circuit board. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
       FIG. 1  is a plan view showing an electric device according to a first embodiment of the present invention; 
       FIG. 2  is a cross sectional view showing the device taken along line II-II in  FIG. 1 ; 
       FIG. 3  is a plan view explaining a relationship between a via conductor and a land in the device according to the first embodiment; 
       FIG. 4A  is a graph showing a relationship between size of mounted parts and an electrostatic withstand voltage, and  FIG. 4B  is a table explaining the relationship between the size of the mounted parts and the electrostatic withstand voltage, according to the first embodiment; 
       FIG. 5  is a plan view explaining a relationship between a via conductor and a land in an electric device according to a second embodiment of the present invention; 
       FIG. 6  is a plan view explaining a relationship between a via conductor and a land in an electric device according to a third embodiment of the present invention; and 
       FIG. 7  is a plan view explaining a relationship between a via conductor and a land in an electric device according to a fourth embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   First Embodiment 
   An electric device  100  according to a first embodiment of the present invention is shown in  FIGS. 1 and 2 .  FIG. 3  shows a relationship between a via conductor and a land in the device  100 .  FIG. 4  shows a relationship between a distance between the lands and erector-static withstand voltage. The device  100  includes a circuit board  10 , which is a multi-layered circuit board composed of multiple sheets made of dielectric material. A conductive pattern is formed on the sheets. The circuit board  10  includes first to sixth wirings  20 - 25 , first to sixth lands  30 - 35 , an inner layer ground pattern  40 , and first to third via conductors V 1 -V 3 . On the circuit board  10 , a connector  50 , an input IC  60  for inputting an electric signal, first and second resistors R 1 , R 2 , and a capacitor C are mounted. Although the circuit board  10  is formed of a binary-layered circuit board, the circuit board  10  may be formed of other multi-layered circuit board such as a ternary-layered circuit board. 
   The first wiring  20  electrically connects between an input terminal  51  and the third land  32 , the second wiring  21  electrically connects between the third land  32  and the first land  30 , the third wiring  22  electrically connects between the fourth land  33  and the third via conductor V 3 , the fourth wiring  23  electrically connects between the third via conductor V 3  and the sixth land  35 , the fifth wiring  24  electrically connects between the fifth land  34  and the second land  31 , the sixth wiring  25  electrically connects between the second land  31  and the input IC  60 . Each wiring  20 - 25  other than a connection portion for connecting to a terminal or a land is covered with an insulation film. 
   The first and the second resistors R 1 , R 2  and the capacitor C are mounted on the first to sixth lands  30 - 35 . Each land  30 - 35  is not covered with an insulation film so that the conductor of the land  30 - 35  is exposed outside. This is because the first and the second resistors R 1 , R 2  and the capacitor C are electrically bonded to the land  30 - 35  with a solder. Here, each wiring  20 - 25  is covered with the insulation film. 
   The inner layer ground pattern  40  is formed on a sheet made of dielectric material and composing the circuit board  10 . The inner layer ground pattern  40  is formed inside of the circuit board  10 . The inner layer ground pattern  40  is connected to the first to third via conductors V 1 -V 3  and a ground terminal  52  through via holes. The first to third via conductors V 1 -V 3  correspond to an electrostatic-charge absorbing conductor. 
   The connector  50  connects electrically between an external circuit and the device  100 . The connector  50  includes the input terminal  50  and the ground terminal  52 . The input IC  60  includes an AD converter for converting an analog signal to a digital signal so that the input IC  60  processes an input signal. 
   The first resistor R 1  and the capacitor C provide RC filter for protecting a noise from penetrating into the device  100 . The electrostatic charge penetrating from the input terminal  51  is reduced by the RC filter. The first resistor R 1  corresponds to an electrostatic-charge reducing device. The second resistor R 2  works in a case where the signal is not inputted into the input terminal  51 . Further, the second resistor R 2  is a pull-down resistor for fixing electric potential of a ground state so that the input IC  60  is connected to a ground potential. Although the device  100  includes the second resistor R 2  as the pull-down resistor, the device may not include the pull-down resistor. 
   A first distance between the first via conductor V 1  and the first land  30  is defined as L 1 , and a second distance between the first and the second lands  30 ,  31  is defined as L 2 , as shown in  FIG. 3 . The relationship between the first distance L 1  and the second distance L 2  is explained as follows. Here, a relationship between a distance between the second via conductor V 2  and the third land  32  and another distance between the third land  32  and the fourth land  33  is similar to the relationship between the first distance L 1  and the second distance L 2 . 
   The electrostatic charge inputted from the input terminal  51  transmits to the first land  30  through the first wiring  21 . The static charge is reduced by the first resistor R 1  mounted between the first land  30  and the second land  31 . However, when the dimensions of the first resistor R 1  are small, for example, equal to or smaller than 2.0 mm×1.25 mm, the second distance L 2  between the first and the second lands  30 ,  31  becomes short. In general, the electrostatic charge easily discharges as a distance between conductors becomes smaller. Thus, when the second distance L 2  becomes shorter, the static charge may easily discharge between the first and the second lands  30 ,  31 . In this case, the electrostatic charge transmits to the input IC  60  through the first and the second lands  30 ,  31  without reducing at the first resistor R 1 . Further, when the first resistor R 1  is small, the withstand energy of the first resistor R 1  is small. Accordingly, when the large static charge flows through the first resistor R 1 , the first resistor R 1  may be broken down. 
   Specifically, as shown in  FIG. 4 , when the dimensions of the circuit board  10  are 4.5 mm×3.2 mm, the second distance L 2  is about 3.4 mm. In this case, the voltage of the electric discharge between the first land  30  and the second land  31  without reducing the electrostatic charge (i.e., the withstand voltage) is about 5.4 kV in case of positive polarity, or 6.4 kV in case of negative polarity. Accordingly, when the voltage is equal to or smaller than 5.4 kV in case of positive polarity and 6.4 kV in case of negative polarity, the electrostatic charge can be reduced by the first resistor R 1 . 
   When the dimensions of the circuit board  10  are 3.2 mm×1.6 mm, the second distance L 2  is about 2.2 mm. In this case, the voltage of the electric discharge between the first land  30  and the second land  31  without reducing the electrostatic charge is about 4.2 kV in case of positive polarity, or 5.0 kV in case of negative polarity. Accordingly, when the voltage is equal to or smaller than 4.2 kV in case of positive polarity and 5.0 kV in case of negative polarity, the electrostatic charge can be reduced by the first resistor R 1 . 
   When the dimensions of the circuit board  10  are 2.0 mm×1.25 mm, the second distance L 2  is about 11.0 mm. In this case, the voltage of the electric discharge between the first land  30  and the second land  31  without reducing the electrostatic charge is about 2.3 kV in case of positive polarity, or 2.5 kV in case of negative polarity. Accordingly, when the voltage is equal to or smaller than 2.3 kV in case of positive polarity and 2.5 kV in case of negative polarity, the electrostatic charge can be reduced by the first resistor R 1 . 
   When the dimensions of the circuit board  10  are 1.6 mm×0.8 mm, the second distance L 2  is about 0.8 mm. In this case, the voltage of the electric discharge between the first land  30  and the second land  31  without reducing the electrostatic charge is about 2.0 kV in case of positive polarity, or 2.0 kV in case of negative polarity. Accordingly, when the voltage is equal to or smaller than 2.0 kV in case of positive polarity and 2.0 kV in case of negative polarity, the electrostatic charge can be reduced by the first resistor R 1 . 
   As the dimensions of the circuit board  10  become smaller, the second distance L 2  becomes small. Thus, the voltage of the electric discharge between the first land  30  and the second land  31  without reducing the electrostatic charge becomes smaller. Therefore, to prevent the static charge from penetrating into the input IC  60  from the first land  30  without passing through the first resistor R 1 , and to protect the first resistor R 1  over the static charge from directly passing through the first resistor R 1 , the first via conductor V 1  as electrostatic-charge absorbing conductor is formed near the first land  30 . Here, the first distance L 1  between the first land  30  and the first via conductor V 1  is smaller than the second distance L 2  between the second land  31  and the first via conductor V 1 . 
   In the above case, since the first distance L 1  is smaller than the second distance L 2 , the electrostatic charge inputted from the input terminal  51  may easily discharge between the first land  30  and the first via conductor V 1 . The first via conductor V 1  is electrically connected to the inner layer ground pattern  40 . Accordingly, the electrostatic charge flows into the ground through the first via conductor V 1 . Thus, the static charge is safely removed. 
   Preferably, the first via conductor V 1  is disposed near a portion, at which electric field is concentrated. The portion at which the electric field is concentrated is, for example, a corner of the first land  30 . Since inequality electric field is formed at the corner of the first land  30  when the electrostatic charge is applied to the first land  30 , the electric field is concentrated at the corner. Thus, the discharge of the electrostatic charge may easily occur at the corner of the first land  30 . Accordingly, it is preferred that the first via conductor V 1  is disposed near the corner of the first land  30 . In this case, the discharge of the electrostatic charge is easily occurred between the first land  30  and the first via conductor V 1 , so that the electrostatic charge is much prevented from penetrating into the input IC  60 . 
   Alternatively, the first via conductor V 1  may be disposed on a portion facing the corner of the first land  30 , as shown in  FIG. 3 . The portion facing the corner of the first land  30  is the nearest position so that the corner of the first land  30  is nearest portion of the first land  30  when a distance between the first via conductor V 1  and the first land  30  is determined. The electric field concentration is easily occurred in a direction facing the corner of the first land  30 , so that the discharge of the electrostatic charge is easily occurred between the first land  30  and the first via conductor V 1 . Thus, the first via conductor V 1  as the electrostatic-charge absorbing conductor easily absorbs the electrostatic charge. 
   More preferably, the corner of the first land  30  is sharpened so that the inequality electric field is easily formed at the sharpened corner of the first land  30  when the electric field is applied to the first land  30 . Thus, the electric field is concentrated at the sharpened corner, so that the discharge of the electrostatic charge is easily occurred at the sharpened corner. 
   The electrostatic-charge absorbing conductor, i.e., the first via conductors V 1 , may be disposed on a portion between lands, for example, the portion between the first and the second lands  30 ,  31 , on which a circuit part such as the first resistor R 1  is mounted. In this case, the substrate area of the circuit board  10  can be effectively utilized. 
   Further, the electrostatic-charge absorbing conductor, i.e., the first via conductors V 1 , may be disposed under the circuit part such as the first resistor R 1 . In this case, the substrate area of the circuit board  10  can be effectively utilized. 
   Further, the device  100  includes one electrostatic-charge absorbing conductor, i.e., the first via conductors V 1 , the device  100  may include multiple electrostatic-charge absorbing conductors. In this case, when the electrostatic charge is inputted, the charge can discharge to multiple conductors, so that the charge is easily absorbed in the electrostatic-charge absorbing conductors. 
   The first distance L 1  between the first via conductor V 1  and the first land  30  is shortened as small as possible without providing a problem of arrangement of the circuit board  10  or the like. When first via conductor V 1  is approximated to the first land  30 , failure of the circuit board  10  such as short-circuiting between the first land  30  and the first via conductor V 1  through moisture in air may occur. Accordingly, it is preferred that the first distance L 1  between the first via conductor V 1  and the first land  30  is shortened as small as possible without raising the failure of the circuit board  10 . Specifically, when the dimensions of the circuit board  10  are 2.0 mm×1.25 mm, the first distance L 1  is set to be 0.5 mm so that the electrostatic charge is effectively absorbed in the first via conductor V 1 . This is because the second distance L 2  between the first land  30  and the second land  31  is about 1.0 mm, which is longer than the first distance L 1 . Further, in this case, the first land  30  is not electrically connected to the first via conductor V 1  through moisture or the like. When the first distance L 1  is equal to or larger than 0.25 mm, the first land  30  is not electrically connected to the first via conductor V 1  through moisture or the like. 
   Although the device  100  includes the inner layer ground pattern  40  connecting to the first and the second via conductors V 1 , V 2  as the electrostatic-charge absorbing conductor, the device  100  may include a power source pattern formed in an inner layer of the circuit board  10  or a conductive pattern connecting to a load, which is not deteriorated by the electrostatic charge substantially. 
   Second Embodiment 
     FIG. 5  shows a relationship between the first via conductor V 1  and the first land  30 , according to a second embodiment of the present invention. The shape of the first via conductor V 1  is different from that in  FIG. 3 . 
   When it is difficult to form the first via conductor V 1  at a portion facing the corner of the first land  30 , a sharp tip portion is formed on the first via conductor V 1  so that the electric field is concentrated at the sharp tip portion of the first via conductor V 1 . The first distance L 1  between the first land  30  and the first via conductor V 1  is set to be shorter than the second distance L 2  between the first and the second lands  30 ,  31 . Here, the shape of the first via conductor V 1  having the sharp tip portion is a tear drop shape. 
   In this case, when the electrostatic charge is applied to the first land  30 , the discharge of the static charge easily occurs between the first land  30  and the portion of the first via conductor V 1 , at which the electric field is easily concentrated. Accordingly, since the portion, at which the electric field is easily concentrated, is formed on the first via conductor V 1 , the electrostatic charge is prevented from penetrating into the input IC  60 , so that the device is protected from the electrostatic charge. 
   Preferably, the position of the first via conductor V 1  is near the portion of the first land  30  such as the corner of the first land  30 , at which the electric field is concentrated. 
   Although the shape of the first via conductor V 1  is the tear drop shape, the shape of the first via conductor V 1  may be another shape as long as the electric field is concentrated at a part of the first via conductor V 1 . For example, the first via conductor V 1  may include a part having a right angle. 
   Third Embodiment 
     FIG. 6  shows a relationship between the first via conductor V 1  and the first land  30 , according to a third embodiment of the present invention. The shape of the first via conductor V 1  is different from that in  FIG. 3 . As shown in  FIG. 6 , the first via conductor V 1  includes a sharp tip portion as an electric field concentration portion. The first land  30  includes a corner as the electric field concentration portion. The first distance L 1  between the first land  30  and the first via conductor V 1  is shorter than the second distance L 2  between the first and the second lands  30 ,  31 . The electric field concentration portion of the first via conductor V 1  faces the electric field concentration portion of the first land  30 , i.e., the sharp tip portion of the first via conductor V 1  faces the corner of the first land  30 . 
   In this case, the electrostatic charge is effectively absorbed in the first via conductor V 1  as the electrostatic-charge absorbing conductor. 
   Fourth Embodiment 
     FIG. 7  shows a relationship between the first via conductor V 1  and a surface layer ground pattern  70 , according to a fourth embodiment of the present invention. 
   The surface layer ground pattern  70  includes an exposed portion  72  as the electrostatic-charge absorbing conductor. 
   When the surface layer ground pattern  70  is formed on a side of the circuit board  10 , on which the first land  30  is disposed, the surface layer ground pattern  70  works as an electrode pattern. A solder resist  71  or the like covers a conductor of the surface layer ground pattern  70 . A part of the solder resist  71  is removed so that the exposed portion  72  of the surface layer ground pattern  70  is formed. The first distance L 1  between the first land  30  and the exposed portion  72  of the surface layer ground pattern  70  is set to be shorter than the second distance L 2  between the first and the second lands  30 ,  31 . 
   Thus, by using the part of the surface layer ground pattern  70 , i.e., the exposed portion  72 , the electrostatic charge discharges between the first land  30  and the exposed portion  72  when the electrostatic charge is inputted from the input terminal  51 . Accordingly, even when the electrostatic charge is inputted from the input terminal  51 , the electrostatic charge is effectively reduced. Preferably, the exposed portion  72  is disposed near the portion, at which the electric field is concentrated, such as the corner of the first land  30 . Further, the exposed portion  72  is disposed on a portion facing the corner of the first land  30 , at which the electric field is concentrated. 
   Although the exposed portion  72  has a right angle shape, the exposed portion  72  may have another shape such as a sharpened angle shape. 
   Although the device  100  includes the surface layer ground pattern  70 , the device may have other pattern such as a power source pattern formed on the surface of the circuit board  10  and a conductor pattern connecting to a load, to which the electrostatic charge does not substantially affect. 
   (Modifications) 
   A distance between the third via conductor V 3  and the fourth land  33  may be shorter than the second distance L 2  between the first and the second lands  30 ,  31 . Here, the third land  32  is electrically connected to the input terminal  51 , and the fourth land is electrically connected to the third via conductor V 3 . In this case, when the electrostatic charge is inputted from the input terminal  51 , the electrostatic charge is absorbed into the first and the second via conductors V 1 , V 2 . Further, the electrostatic charge discharges between the third land  32  and the fourth land  33  so that the static charge is absorbed into the third via conductor V 3 . Accordingly, the device  100  is protected from the discharge of the electrostatic charge effectively. 
   Although the device  100  includes the resistor R 1 , R 2  for reducing the electrostatic charge, the device may have other elements such as a capacitor and a coil for reducing the electrostatic charge. 
   While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.