Abstract:
An electronic apparatus that includes: a circuit board; a switch attached to the circuit board; an electronic part mounted on the circuit board; a wiring pattern extending between the switch and the electronic part; and a protrusion protruding from a surface of the wiring pattern, the protrusion being disposed adjacent to the switch on the circuit board and overlapping the wiring pattern.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims priority from Japanese Patent Application No. 2005-330768, filed on Nov. 15, 2005, the entire subject matter of which is incorporated herein by reference. 
   TECHNICAL FIELD 
   Aspects of the present invention relate to an electronic apparatus equipped with a circuit board having electronic parts mounted thereon. 
   BACKGROUND 
   Various electronic apparatuses such as a printer, a copying machine, a keyboard and an in-vehicle machine are equipped with a circuit board having electronic parts mounted thereon. With the increasing demand for lightness, thinness, shortness and smallness of the electronic apparatus, the wiring pattern of the circuit board is highly dense and thinned. Thus, static electricity discharged from a user may be discharged to signal lines of the circuit board. If the static electricity is discharged to the signal lines, a high voltage may be instantaneously applied to thereby break the electronic parts mounted on the circuit board. 
   JP-A-5-41568 discloses a configuration in which one set of discharge stylus patterns form air gaps between an earth pattern and input/output terminals. A mounting pattern, on which a resistor chip is mounted, is formed between input/output terminals and electronic parts. The distance of the air gaps formed by the discharge stylus patterns is set smaller than that of the mounting pattern of the resistor chip. Thus, the static electricity is discharged to the air gaps and released to the earth pattern and is prevented from being discharged to the mounting pattern of the resistor chip. 
   JP-A-10-270812 discloses a configuration, in which an insulated silk is printed on a pair of wiring patterns and on a solder resist layer arranged between the pair of wiring patterns. By this configuration, the dielectric strength is improved without widening the pattern gap between the pair of wiring patterns. Thus, the discharge to the wiring patterns can be prevented and the size of the circuit board can be reduced. 
   SUMMARY 
   JP-A-5-41568 is premised that the static electricity is discharged to a location between the input/output terminals and the mounting pattern of the resistor chip. Preventing the discharge to the input/output terminals themselves is not considered, and thus it is insufficient for protecting the electronic parts against the discharge. 
   According to the technique disclosed in JP-A-10-270812, the wiring patterns and the solder resist layer are covered with the insulated silk to thereby make it hard to discharge the static electricity to the wiring patterns or the solder resist layer. However, this technique does not positively discharge the static electricity to places other than the wiring patterns or the solder resist layer. Thus, the discharge to the wiring patterns or the solder resist layer cannot be completely prevented. 
   Aspects of the present invention provide an electronic apparatus, which can prevent the discharge to wiring patterns on a circuit board to thereby prevent electronic parts from being damaged. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a perspective view showing a multifunction apparatus according to a first aspect of the invention; 
       FIG. 2  is a sectional view of the multifunction apparatus shown in  FIG. 1 ; 
       FIG. 3  is a top plan view showing a portion of a circuit board used in the multifunction apparatus shown in  FIG. 1 ; 
       FIG. 4  is a sectional view showing an input switch of an operation unit provided on the multifunction apparatus shown in  FIG. 1 ; 
       FIG. 5  is a top plan view of  FIG. 4 ; 
       FIG. 6  is a diagram showing a resistor chip disposed in the circuit board shown in  FIG. 3 ; 
       FIG. 7  is a sectional view taken along line X 1 -X 1  of  FIG. 6 ; 
       FIG. 8  is a diagram showing an arrangement modification of the resistor chip shown in  FIG. 6 ; 
       FIG. 9  is a top plan view of a solder portion of a modification of protrusions; 
       FIG. 10  is a sectional view taken along line X 2 -X 2  of  FIG. 9 ; 
       FIG. 11  shows a modification of the operation unit; 
       FIG. 12  is a sectional view showing a conductive member attached to a circuit board to be used in an electronic apparatus according to a second aspect of the invention; 
       FIG. 13  is an enlarged view showing the periphery of through holes of an electronic apparatus according to a third aspect of the invention; and 
       FIG. 14  is a sectional view taken along line X 3 -X 3  of  FIG. 13 . 
   

   DETAILED DESCRIPTION 
   Now, illustrative aspects of the invention will be described with reference to the accompanying drawings. 
   (First Aspect) 
     FIG. 1  is a perspective view showing the exterior of a multifunction apparatus  1 . 
   In this aspect, the multifunction apparatus  1  having multiple functions including a printer function, a copy function, a scanner function and a facsimile function is used as an electronic apparatus. The multifunction apparatus  1  is provided with an image forming unit  2  and an image reading unit  100 . An operation unit  121  is provided so as to switch the functions and to input information. 
     FIG. 2  is a sectional view of the multifunction apparatus  1  shown in  FIG. 1 . 
   In the image forming unit  2 , a paper sheet housed in a paper feeder cassette  6  is picked up by a paper feeder roller  11 . The paper sheet is conveyed through a separating roller  8  and a register roller  12  to an image forming portion  5 , by which an image is formed on the paper sheet. The paper sheet is then discharged to a paper discharge tray  46 . In the image forming portion  5 , the paper sheet is conveyed from the register roller  12  to an image forming position P 1  between a photosensitive roller  27  and a transfer roller  30 , at which an image is transferred to the paper sheet. After this, the image is fixed between a fixing roller  41  and a pressure roller  42 . The image forming unit  2  thus configured supports the image reading unit  100  rotatably on a first hinge pin J 1 . The image reading unit  100  covers the paper discharge tray  46 . 
   In the image reading unit  100 , a document cover  140  is rotatably held on an upper casing  101  through a second hinge pin J 2 . A document is placed on a platen glass  107  by opening the document cover  140 , so that the image of the document is read by an image sensor  115  provided in the upper casing  101 . Moreover, the image reading unit  100  is equipped with an auto document feeder (ADF)  145 , which takes documents one by one from a document feeder tray  147  (see  FIG. 1 ) and passes the taken document over the image sensor  115 , which is fixed, so that the image of the document can be read. 
   On the front side of the image reading unit  100  (or on the +X-axis side, as shown), there is disposed an operation unit  121 . This operation unit  121  comprises an operation cover  122  formed of a portion of the upper casing  101  and operation parts including input switches  125 . A circuit board  151  is fixed in parallel on the back side of the operation cover  122 . 
   As shown in  FIG. 1 , the operation unit  121  is equipped with various switches such as the input switches  125  for inputting the number and size of paper sheets, mode changing switches  128  for changing a copy mode, a facsimile mode and a print mode, and a start switch  129  for instructing the transmission of facsimile data and the start of a copy. Further, a display lamp  126  for displaying a mode being executed and a warning and a liquid crystal panel  127  are provided. The switches such as the input switches  125 , the mode changing switches  128  and the start switch  129  are attached to the circuit board  151 . A keytop  149 , which will be described below, is protruded from the operation cover  122  to the outside so that it can be operated. In order to prevent the discharge of static electricity, the multifunction apparatus  1  has its exterior made of resin, and the operation cover  122  of the operation unit  121  and the keytops  149  of the input switches  125 , the mode changing switches  128  and the start switch  129  are also made of resin. 
     FIG. 3  is a top plan view showing a portion of the circuit board  151 . On the circuit board  151 , a solder resist (or a covering material) is formed on the surface of an insulated substrate  152  so as to cover the entirety including wiring patterns  153  and earth patterns  154 . Further, a silk (or a protecting material) is formed to cover the wiring patterns  153  through the solder resist. For conveniences of the description, the solder resist and the silk are not shown in  FIG. 3 . 
   The circuit board  151  is a single-sided board, on which the wiring patterns  153  and the earth patterns  154  are formed on one side of the insulated substrate  152 . The wiring patterns  153  extend from input/output terminals  150  of switches  155  mounted on the insulated substrate  152  to an LSI (or an electronic part)  156 , thereby to form paths for electric signals. The earth patterns  154  are disposed around the wiring patterns  153  and the switches  155 . Resistor chips (or projecting portions)  157  straddle the wiring pattern between the earth patterns  154  disposed on both sides of the wiring pattern  153 . The resistor chips  157  are arranged at a constant interval on the wiring pattern  153  so as to prevent the discharge to the wiring pattern  153  reliably. 
   Next, the structure of the switches is specifically described.  FIG. 4  is a sectional view showing the input switch  125  of the operation unit  121 .  FIG. 5  is a top plan view of  FIG. 4 . 
   The input switch  125  has its keytop  149  held vertically movably so that it is turned ON/OFF according to the position of the keytop  149 . The switch  155  is equipped with four input/output terminals  150   a ,  150   b ,  150   c  and  150   d  and is fixed to the circuit board  151  by means of solder. The input/output terminals  150   a  and  150   d  of the switch  155  are connected with wiring lines  153   a  and  153   b  of the wiring pattern  153 . The solder resist is not provided on the input/output terminals  150   a  to  150   d.    
   The keytop  149 . made of resin is so arranged over the switch  155  as to cover the switch  155 . The keytop  149  is inserted into an aperture  122   a  formed in the operation cover  122  and is always urged upward by a link member  123  arranged between the keytop  149  and the circuit board  151 . Between the resin keytop  149  and the resin operation cover  122 , there is defined a gap S for enabling the keytop  149  to move smoothly. At least a part of the keytop  149  is above the operation cover  122  through the aperture  122   a.    
   When the keytop  149  of the input switch  125  is pushed against the urging force by the link member  123 , the switch  155  is pressed by the keytop  149  to electrically connect the input/output terminals  150   a  and  150   d  thereby to feed the electric current from the wiring line  153   a  to the wiring line  153   b . When the keytop  149  is released from the pressure, the switch  155  disconnects the input/output terminals  150   a  and  150   b  to stop the current flow from the wiring line  153   a  to the wiring line  153   b.    
   When the air is dry (for example, in winter), static electricity is easily stored in the human body. In case the user charged with the static electricity brings a fingertip F to the input switch  125 , for example, for the copying operation, the static electricity V is not discharged to the resin operation cover  122  and the resin keytop  149  but may be discharged through the gap S formed around the keytop  149 . No problem arises if the static electricity discharged through the gap S releases from the earth patterns  154  to the ground, but the high voltage may be instantaneously applied to damage the LSI  156  or the electronic part, if the static electricity is discharged to the wiring lines  153   a  and  153   b  of the wiring patterns  153 . The static electricity V is likely to strike a protrusion by way of the easiest path in the same manner as thunder. Therefore, the circuit board  151  is equipped with the resistor chips  157  at least at positions on the shortest imaginary lines connecting the gap S with the circuit board  151 . As a result, the circuit board  151  arranges the resistor chips  157  at positions higher than the surface of the wiring pattern  153 , and gives the role as a lightning conductor to the resistor chips  157 . 
   The next description is made on the resister chip  157 .  FIG. 6  is a diagram showing the resister chip  157 , which is Disposed in the circuit board  151  shown in  FIG. 3 . For Conveniences of the description, the resist and the silk Are not shown in  FIG. 6 . 
   On the two sides of the wiring pattern  153  formed on the insulated substrate  152 , as shown in  FIG. 6 , the earth patterns  154  are formed through a spacing width Z. The resistor chip  157  has a rectangular shape and includes an insulated resin portion  157   a  and metallic electrodes  157   b  and  157   b  attached to two sides of the insulated resin portion  157   a . The resin portion  157   a  is made wider than the spacing width W. The electrodes  157   b  and  157   b  attached to the two sides can contact with the earth patterns  154  and  154 . The transverse width W 1  of the resistor chip  157  is wider than the spacing width Z. The resistor chip  157  is formed in such a rectangular shape having the transverse width W 1 , a longitudinal width W 2  and a thickness W 3  (se  FIG. 7 ). The resistor chip  157  is easily placed and mounted on the circuit board  151 . In this aspect, the spacing width Z is 0.35 to 0.50 mm, and the resistor chip  157  has the transverse width W 1  of 16 mm, the longitudinal width W 2  (see  FIG. 6 ) of 0.8 mm and the thickness W 3  of 0.8 mm. 
     FIG. 7  is a sectional view taken along line X 1 -X 1  of  FIG. 6 . 
   In order to stabilize the insulation of the circuit board  151 , the solder resist  158  is formed to cover the wiring pattern  153  and the earth patterns  154  over the insulated substrate  152 , and the silk  159  is formed along the wiring pattern  153  on the solder resist  158 , so that the wiring pattern  153  is protected against the discharge by covering it with the solder resist  158  and the silk  159 . In this aspect, the solder resist  158  is made of two-pack epoxy resin, and the silk  159  is made of one-pack epoxy resin. In the solder resist  158 , holes are formed on the earth patterns  154 . 
   The resistor chip  157  is arranged such that the electrodes  157   b  and  157   b  are positioned to the holes formed in the solder resist  158  and arranged to cross the wiring pattern  153  orthogonally. The resistor chip  157  is fixed at its electrodes  157   b  and  157   b  to the earth patterns  154  by means of solder  160 . The resistor chip  157  and the solder  160  have little resistance so that the static electricity V easily flows from the electrodes  157   b  through the solder  160  to the earth patterns  154 . Generally speaking, the static electricity V to charge the human body is the to have a value of about 10 KV. The resistor chip  157  has a breakdown voltage of at least 10 KV so that the static electricity does not flow from the electrodes  157   b  and  157   b  to the resin portion  157   a  but flows from the electrodes  157   b  directly to the earth patterns  154 . In this aspect, the resistor chips having a breakdown voltage of 20 KV and a resistance of 0Ω are used. 
   Here, the resistor chips  157  are suitably selected in view of the size of the circuit board  151  and the allowable voltage of the wiring pattern  153 . Thus, the size and breakdown voltage are not necessarily limited to those of this aspect. Also, a resistor chip having several tens Ω or less may be used. 
   With the multifunction apparatus  1  thus constructed, for example, the mode changing switches  128  of the operation unit  121  are operated to select a copy mode. The document cover  140  is then opened to set a document on the platen glass  107 , and the input switch  125  is operated to set the number of copies. The image of the document is read by the image sensor  115 . The paper sheet picked up from the paper feeder cassette  6  is sent to form the read image on the paper sheet. The paper sheet subjected to the image formation is discharged to the paper discharge tray  46 . 
   When the static electricity V is discharged from the fingertip F of the user brought close to the input switch  125 , as shown in  FIG. 4 , the static electricity V is discharged through the gap S to the circuit board  151 . In this circuit board  151 , at this time, the resistor chips  157  protrude high from the surface of the wiring pattern  153 , and the earth patterns  154  and the wiring pattern  153  around the resistor chips  157  have a higher resistivity than that of the resistor chips  157  because they are covered with the solder resist  158  and the silk  159 . As a result, the static electricity V is discharged easily to the resistor chips  157  but not to the wiring pattern  153 . The static electricity V discharged to the resistor chips  157  is released through the solder  160  to the earth patterns  154 . As a result, no high voltage is applied through the wiring pattern  153  to the LSI  156  so that the LSI  156  is not damaged. According to the multifunction apparatus  1  of this aspect, therefore, the discharge to the wiring patterns  153  on the circuit board  151  can be prevented and the damage of the LSI  156  can be prevented. 
   In the multifunction apparatus  1  of this aspect, as shown in  FIG. 6  and  FIG. 7 , the electrodes  157   b  and  157   b  are fixed by the solder  160  on the earth patterns  154  which are positioned on the two sides of the wiring pattern  153 , so that the resistor chips  157  of a low electric resistance are mounted between the earth patterns  154 . The resistor chips  157  can be easily disposed at the arbitrary positions on the circuit board  151  in accordance with the circuit configuration of the wiring pattern  153  thereby to release the static electricity V through the resistor chips  157  to the earth patterns  154  having the low electric resistance. The static electricity V is easily discharged like a thunder to the protruding objects so that the resistor chips  157  play the role like the lightning conductor to promote the discharge to the earth patterns  154 . 
   In the multifunction apparatus  1  of this aspect, moreover, the gap S is formed around the keytop  149  of the input switch  125 , and the resistor chips  157  are disposed at such positions as to minimize the distance from the gap S to the circuit board  151  (see  FIG. 4  and  FIG. 5 ). As a result, the static electricity V, as discharged from that gap S to the circuit board  151 , can be efficiently discharged to the resistor chips  157  without being discharged to the wiring pattern  153 . 
   Moreover, at least the wiring pattern  153  is covered with the insulated solder resist  158  and the silk  159  thereby to raise the electric resistance so that the static electricity V can be more reliably prevented from being discharged to the wiring pattern  153 . 
   Now, modifications-of the resistor chips  157  are described with reference to  FIG. 8  to  FIG. 11 . In the circuit boards  151  of the modifications, the solder resist  158  and the silk  159  are formed but are not shown for conveniences of the description. 
     FIG. 8  is a diagram showing an arrangement modification of the resistor chips  157  shown in  FIG. 6 . 
   The resistor chips  157  of the aforementioned first aspect are disposed across the wiring pattern  153 , as shown in  FIG. 6  and  FIG. 7 . As shown in  FIG. 8 , the resistor chip  157  may also be arranged on one of the earth patterns  154  disposed on the two sides of the wiring pattern  153  and along the wiring pattern  153 . Although not shown, the resistor chips  157  may also be arranged along the wiring pattern  153  and on the two sides of the earth patterns  154 . 
     FIG. 9  shows a modification of the protrusions.  FIG. 10  is a sectional view taken along X 2 -X 2  of  FIG. 9 . Here,  FIG. 10  omits only the silk  159 . 
   In the foregoing first aspect, the protrusions are formed of the resistor chips  157 . As shown in  FIG. 9  and  FIG. 10 , the solder may be welded along the wiring pattern  153  on the earth patterns  154  positioned on the two sides of the wiring pattern  153 , thereby to form conical protrusions  161  protruding from the surface of the wiring pattern  153 . The reason for the conical shape is to facilitate the discharge of the static electricity V. The protrusions  161  are formed conical but may also be formed elliptical or semicircular. Here, the protrusions  161  of the solder can be easily formed by forming the through holes in the portions corresponding to the protrusions  161  of the solder resist  158  and then by heaping up the solder into those through holes. 
     FIG. 11  shows a modification of the operation unit  121 . 
   A keytop  149 A having a cylindrical body portion opened in one side and having a flange portion is used for the input switch  125 . This keytop  149 A has its cylindrical body portion inserted into the aperture  122   a  of the operation cover  122  so that the flange portion is engageable with a radially inner portion of the aperture  122   a . In this case, a crank-shaped gap S 1  is formed between the keytop  149 A and the operation cover  122 . The static electricity V is discharged through the gap S 1  to the circuit board  151 , but is discharged to the resistor chips  157  and released to the earth patterns  154 , if the resistor chips  157  are disposed at the positions to minimize the distance between the gap S 1  and the circuit board  151 . 
   (Second Aspect) 
   Next, a second aspect of the invention will be described.  FIG. 12  is a sectional view showing a conductive member  171 , which is attached to a circuit board  151  to be used in a multifunction apparatus  1  of the second aspect. 
   The multifunction apparatus  1  of this aspect is different from the first aspect, which has the resistor chips  157  mounted on the circuit board  151  of the single-sided board, in that the circuit board  151  is a double-sided board. The circuit board  151  has the wiring pattern  153  on the two sides of an insulated substrate  170 . The multifunction apparatus  1  of this aspect is further different from the first aspect in that the conductive member  171  is used as the protrusions. However, the remaining points are common. Therefore, the description is made by centering the conductive member  171 , which is different from the first aspect, but the portions common to those of the first aspect are omitted in description while being designated by the common reference numerals. 
   The conductive member  171  is constructed such that conductive wires  173  of a tip part  174  is penetrated through the circuit board  151  from the lower side to the upper side of the drawing thereby to fix the leading end portions of the conductive wires  173  by means of solder. In the conductive member  171 , the conductors  173  are so extended upward through the insulated substrate  170  and the earth patterns  154  as to contact with the earth patterns  154 . The solder is heaped up in conical shapes around the conductive wires  173  into the hole portions corresponding to the protrusions  172  of the solder resist  158 , thereby to constitute the protrusions  172 . Here, the conductive wires  173  may be jumper wires. 
   This conductive member  171  discharges the static electricity V, discharged from the gap S of the operation unit  121  to the circuit board  151 , to the protrusions  172 , and releases the static electricity V to the earth patterns  154  through the conductive wires  173 . 
   In the multifunction apparatus  1  of this aspect, therefore, the conductive member  171  can be easily disposed at an arbitrary position on the circuit board  151  in accordance with the circuit constitution of the wiring pattern  153  thereby to release the static electricity V from the conductive member  171  to the earth patterns  154  having a small electric resistance. The static electricity V is easily discharged like the thunder from the protrusion and because the conductive member  171  plays a role similar to that of the lightning conductor thereby to promote the discharge to the earth patterns  154 . 
   (Third Aspect) 
   Next, a third aspect of the invention will be described.  FIG. 13  is an enlarged diagram showing the periphery of through holes  181  formed in the multifunction apparatus  1 .  FIG. 14  is a sectional view taken along line X 3 -X 3  of  FIG. 13 .  FIG. 13  omits the solder resist  158  and the protecting material  159  formed on the wiring pattern  153 , for conveniences of the description. 
   The multifunction apparatus  1  of this aspect is different, in the provision of the through holes  181 , from the first aspect having the resistor chips  157 , but shares the remaining points. Therefore, the description is made by centering the through holes  181 , which is different from the first aspect, but the portions common to those of the first aspect are omitted in description while being designated by the common reference numerals. 
   In the circuit board  151 , as shown in  FIG. 13  and  FIG. 14 , the earth patterns  154  are formed on the two sides of the wiring pattern  153 , and the solder resist  158  is formed to cover the wiring pattern  153  and the earth patterns  154 . In the solder resist  158 , the through holes  181  are formed in a circular shape on the earth patterns  154  on the two sides of the wiring pattern  153 . The earth patterns  154  have a lower insulation than those of the solder resist  158  and the silk  159 . Therefore, the earth patterns  154  of the circular shape, which are exposed through the through holes  181 , are more easily subjected to discharge than the surrounding solder resist  158  and silk  159 . 
   In this circuit board  151 , the static electricity V is discharged through the through holes  181  to the earth patterns  154  thereby to prevent the discharge to the wiring pattern  153 , so that the high voltage is not instantaneously applied to damage the LSI  156 . According to the multifunction apparatus  1  of this aspect, therefore, it is possible to prevent the discharge to the wiring pattern  153  thereby to prevent the breakage of the LSI  156 , which is connected through the wiring pattern  153  with the input/output terminals  150  of the switch  155  to be operated by the user. 
   Here, if the through hole  181  is formed just below the gap S formed around the keytop  149  of the switch  155 , the static electricity V is easily discharged to the earth patterns  154  from the gap S through the through hole  181 . 
   Also in this aspect, the wiring pattern  153  is wrapped with the silk  159  on the solder resist  158  so that the static electricity V can be more reliably prevented from being discharged to the wiring pattern  153 . 
   Although the aspects of the invention have been described, the invention is not limited to the foregoing aspects but may be variously modified.
     (1) For example, the foregoing aspects have been described on the electronic apparatus to be used in the laser printer. However, the structure of the aspects can be applied to an electronic apparatus having the gap around the switch, such as an ink jet printer, a keyboard of a personal computer, or an operation unit of an in-vehicle electronic apparatus.   (2) In the foregoing aspects, the circuit board has been exemplified by the rigid circuit board, in which the wiring patterns are formed on the insulated substrate. However, the rigid flex wiring board or the flexible wiring board may be used as the circuit board.   (3) In the foregoing aspects, the resistor chips  157  are so arranged at a small pitch as to cover the wiring pattern  153  thereby to prevent the static electricity V from being discharged into the wiring pattern  153 . Using a lot of resistor chips  157  increases the cost. Therefore, the resistor chips  157  may be disposed at least at positions on the shortest imaginary lines connecting the gap S formed on the operation portion unit  121  with the circuit board  151 , thereby to reduce the costs.   (4) The foregoing aspects have been described on the case, in which the gap S is formed between the aperture  122   a  of the operation cover  122  and the keytop  149 . Even in case the gap S is formed between the adjoining keys such as the keys of the keyboard for the personal computer, the protrusions (e.g., the resistor chips  157  or the conductive members  171 ) may be formed on the circuit board  151  thereby to prevent the static electricity V from being discharged to the wiring pattern  153 . Also in this case, the resistor chips  157  may be fixed at positions on the shortest imaginary lines connecting the gap S between the keys with the circuit board  151 .   (5) In the foregoing aspects, the silk  159  is formed to cover only the wiring pattern  153 . However, the silk  159  may be disposed to cover the earth patterns  154  partially or wholly.