Patent Document

CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to, and the benefit of, Japan Patent Application No. 2007-104666, filed on 12 Apr. 2007, in the Japan Patent Office, the disclosure of which is incorporated herein by reference in its entirety. 
     FIELD 
     The present invention relates to electronic devices for surface mount, more particularly to electronic devices in which short-circuiting by solder is prevented when the devices are mounted on a circuit board or the like. 
     DESCRIPTION OF THE RELATED ART 
     Crystal devices are widely known for use as frequency-controlling elements such as crystal units, oscillators, and filters. Crystal devices are mounted on various types of circuit boards of electronic devices including, but not limited to, communication devices. In recent years, crystal devices for surface mount have been developed for mounting on circuit boards with other electronic devices, such as resistors and capacitors. In general, the electronic devices are mounted, using a surface-mount machine, on the circuit board to which solder paste has been applied. Then, the board on which the electronic devices have been placed is conveyed through a reflow furnace to achieve soldering of the electronic devices to the board. 
     In many instances the electronic devices must be closely arranged on the circuit board to satisfy current demands of high integration and miniaturization. Consequently, pads (corresponding to respective external terminals) for the electronic devices are situated closely together on the circuit board. As miniaturization of electronic devices for surface mount has progressed, the distances between external terminals on the electronic devices have narrowed, requiring corresponding reductions of distances between external terminals on the board. Consequently, when soldering the electronic devices on the circuit board, solder tends to overflow between external terminals and cause short-circuits. Even in situations in which short-circuits do not form between individual external terminals, solder overflow may become ball-shaped and thus adversely affect other regions of the mount board. 
       FIG. 7  shows a piezoelectric oscillator  200 , having a base board  210 , mounted on a circuit board PB. Specifically, the circuit board PB includes a pad  115 , and solder paste SOL has been applied to the pad  115 . When the piezoelectric oscillator  200  enters a reflow furnace after being placed in a state in which a predetermined amount of solder SOL has been applied, a solder ball is formed between the base board  210  and the circuit board PB. Thus, short-circuiting may be produced between the external terminals  215 . 
     If a somewhat small amount of solder paste is applied, the desired electrical connection between the external terminal  215  and the pad  115  may be insufficient. It is also difficult to detect whether or not connections between the electronic devices and the circuit board are satisfactory after performing solder reflow. Furthermore, if a connection fault should arise between an electronic device and the circuit board, the faulty connection state between the electronic device and wiring on the board may not be readily detected, which results in decreased product yield. 
     SUMMARY 
     To address the problems described above, an object of the present invention is to provide electronic devices for surface mount that prevent solder from overflowing between external terminals of the electronic device or between pads on a circuit board. 
     An electronic device for surface mount according to the first aspect comprises a base board made from an insulating material. An embodiment of the device includes at least one external terminal for surface mount on an outer surface. A groove is formed around the external terminal on a surface to be mounted on the printed circuit board. With this embodiment, even when solder is applied to a circuit board in a somewhat large amount during surface mounting, any over-flowed solder enters the groove. Hence, short-circuiting between external terminals is much reduced. 
     A base board on the electronic device for surface mount according to the second aspect comprises a resin board made of a thermoset resin. The groove is formed by thermal or mechanical processing. By making the base board on the electronic device as a thermoset resin board, the groove can be formed by thermal processing, e.g., laser processing. If mechanical processing is used, the groove can be formed by drilling or routing, for example. 
     A base board on the electronic device for surface mount according to the third aspect comprises a ceramic board. The groove is formed by embossing or stamping, for example. The external terminals can be printed using metallized ink. By making the base board on the electronic device of ceramic, the groove may be formed by embossing or stamping before performing calcination, followed by metallization to form the external terminals. 
     With an electronic device for surface mount according to the fourth aspect, the depth of the groove is from 0.1 mm to 80% of the thickness of the base board. By staying within this range, solder overflow is satisfactorily arrested. If the groove depth exceeds 80% of the thickness of the base board, the base board becomes too weak for adequate durability. 
     With an electronic device for surface mount according to the fifth aspect, the width of the groove is from 0.1 mm to 2.0 mm. By staying within this range, solder overflow is satisfactorily arrested. 
     The electronic devices can include crystal oscillators and crystal units. Crystal oscillators are categorized as large-sized among electronic devices. Consequently, a rather large amount of solder is applied to the circuit board. The present invention is especially advantageous for this application. 
     Electronic devices for surface mount according to the present invention advantageously prevent solder from overflowing between external terminals of the electronic device or between corresponding pads on a circuit board to which the electronic devices are mounted. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an exemplary surface-mount piezoelectric oscillator  100 . 
         FIGS. 2A and 2B  are elevational and plan views of a base board with external terminals. 
         FIGS. 3A and 3B  are elevational views of a piezoelectric oscillator being mounted on a circuit board. 
         FIGS. 4A ,  4 B, and  4 C are perspective, elevational, and plan views, respectively, of a crystal oscillator. 
         FIGS. 5A-5D  depict exemplary steps in a method for manufacturing a ceramic layer for use a bottom layer. 
         FIGS. 6A-6D  depict representative cross-sectional shapes of grooves. 
         FIG. 7  is a side view of a conventional piezoelectric oscillator mounted on a circuit board. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The invention is described in connection with representative embodiments, with reference to the drawings. 
     Construction of Piezoelectric Oscillator 
       FIG. 1  is a cross-sectional view of an embodiment of a surface-mounted, high-stability piezoelectric oscillator  100  of the temperature-controlled type (hereinafter referred to as a “piezoelectric oscillator”). The piezoelectric oscillator  100  comprises a base printed circuit board  10  (called a “base board”) and a sub printed circuit board  40 . The base board  10  is made of an insulating material. On the sub printed circuit board  40  are mounted a temperature-control circuit and/or electronic components  31  for an oscillation circuit. Also mounted to the sub printed board  40  is a crystal-vibrating  32  affixed using conductive adhesive  21 . On the under-surface of the base board  10 , external terminals  15  are arranged in multiple (e.g., four or six) places. The external terminals facilitate mounting of the piezoelectric oscillator  100  on the surface of a circuit board PB (refer to  FIG. 3 ). To visually observe a meniscus state of soldering after surface-mounting, the external terminals  15  can be electrically connected with the electronic component  31  or the crystal-unit  32  by plated wiring or by lead wires on the surface of the base board  10 . 
     Also mounted to the base board  10  are first ends of respective metal supports  50  made of brass or the like. The first ends are inserted in recesses  11  and affixed using conductive adhesive  21 . Opposing second ends of the metal support  50  are affixed to the sub printed circuit board  40  using conductive adhesive  21 . The entire assembly is covered with a metal case  48  so as to seal the two-tiered base board  10  and sub printed circuit board  40 . The piezoelectric oscillator  100  having such construction generally has a size from approximately 3 mm square to approximately 50 mm square. 
       FIGS. 2A-2B  depict the base board  10  and external terminal  15 .  FIG. 2A  is an enlarged view showing the metal support  50  affixed to the base board  10 , and also showing an external terminal  15 .  FIG. 2B  shows the under-surface of the base board  10 . 
     As shown in  FIG. 2A , the metal support  50  includes a flange  51  and a shaft  54 . The shaft  54  has a shank  52  extending from the flange  51 . The diameter of the shaft  54  is approximately 0.03 mm to approximately 1 mm, and the diameter of the flange  51  is approximately 0.04 mm to approximately 3 mm. The flange  51  may have a diameter of approximately twice the diameter of the shaft  54 . 
     The recess  11  is formed in the base board  10  such that the shank  52  may be inserted therein. The base board  10  is made of a glass-epoxy laminate or other insulating material. The thickness of the base board  10  is approximately 0.6 mm to approximately 3 mm, and the depth of the recess  11  is approximately 90% to approximately 30% of the thickness of the base board  10 . Alternatively, the base board  10  can be made of an insulating material other than glass-epoxy laminate, such as a thermoset resin for glass cloth or glass non-woven fabric base material, an epoxy-resin laminate, a composite laminate, a paper-base epoxy-resin laminate, or a paper-base phenolic resin laminate. Recess or groove processing may be easily applied to these various materials by laser processing, drilling, routing, or the like. 
     The diameter of the recess  11  desirably is smaller than the diameter of the flange  51 , and equal to or larger than the diameter of the shank  52 . The recess  11  can be formed in the base board  10  using a flat router in the edge. Copper plating  12  is applied around the recess  11 . The external terminal  15  and the copper plating  12  are electrically connected to each other. The flange  51  of the metal support  50  and the copper plating  12  are affixed using the conductive adhesive  21 . 
     The groove  13  extends at least part way around the external terminal  15 . In this regard, the groove  13   a  is formed only in the under-surface of the base board  10  destined to be surface mounted on the circuit board PB (refer to  FIG. 3 ). The groove  13   a  does not extend up the side surface in this embodiment. The groove  13  is configured to facilitate visual observation of a meniscus state of solder on the external terminal  15  from the side surface of the piezoelectric oscillator  100 . 
     The groove  13   b  is formed entirely in the under-surface of the circuit board PB because processing is easily applied to such end. The depth of the grooves  13  ( 13   a  and  13   b ) ranges from 0.1 mm to 80% of the thickness of the base board  10 . The width of the groove  13  is 0.1 mm to 2.0 mm. With these combinations of depth and width of the groove  13 , solder overflow is suppressed in the groove  13 , especially considering the size of the surface-mount piezoelectric oscillator  100 . (Solder overflow is still dependent on the amount of solder SOL applied to the circuit board PB, but this variable can be controlled.) In this embodiment, solder overflow is suppressed by flow of excess solder into the groove  13   a  or into the groove  13   b , or into both grooves. 
     Mounting Piezoelectric Oscillator on Circuit Board 
       FIGS. 3A-3B  show a piezoelectric oscillator  100  being mounted on the circuit board PB.  FIG. 3A  is a side view of the piezoelectric oscillator  100  before mounting, and  FIG. 3B  is a side view of the piezoelectric oscillator  100  after mounting. In  FIG. 3A  pads  115  are formed on a circuit board PB on which an electronic device or the like is mounted. The pads  115  form respective parts of a circuit. Solder SOL is applied to the pads  115  by application of a solder paste followed by passage through a reflow furnace of infrared type or hot-air type (not shown). 
     Solder is usually applied to the pads  115  at a predetermined thickness by application of solder paste SOL using a squeegee (not shown) that urges the paste through a perforated metal mask made from stainless steel (not shown). Then, the piezoelectric oscillator  100  is mounted to regions in which the solder SOL has been applied. The mounting of the piezoelectric oscillator  100  is usually performed by a numerically controlled (NC) surface-mounting machine. 
     As shown in  FIG. 3B , during mounting of the piezoelectric oscillator  100 , superfluous solder SOL may enter the groove  13 . This flow into the groove prevents formation of solder balls or the like even if a somewhat excessive amount of the solder paste is transferred to the pads  115 . A solder resist could be formed between the external terminals  15  to avoid generating short-circuits between the external terminals. However, with the depicted embodiment, the need for solder resist is eliminated because the grooves accommodate the excess solder. 
     The shape of the external terminal  15  can be similar to conventional shapes. The external terminals  15  on the under-surface of the base board  10  can extend up the side surfaces of the base board  10 . This configuration allows visual observations of a meniscus state of soldering. 
     Construction of Crystal Oscillator 
     A crystal oscillator  150  is now described with reference to  FIGS. 4A-4C .  FIG. 4A  is an overall perspective view;  FIG. 4B  is a cross-sectional view; and  FIG. 4C  is a top view with the metal lid  61  removed. The crystal oscillator  150  is a surface-mount type, comprising an insulating ceramic package  60  and a metal lid  61  that covers the package. The metal lid  61  desirably is made of Kovar (iron (Fe)/nickel (Ni)/cobalt (Co) alloy). The ceramic package  60  comprises a bottom ceramic layer  60   a , a wall ceramic layer  60   b , and seat ceramic layer  60   c . These layers are punched from green sheets formed from a slurry containing ceramic powder including alumina as a main material, a binder, and the like. Instead of using ceramic powder containing alumina as the main ingredient to form the material of the ceramic package  60 , any of various other materials can be used such as glass ceramic, zero X-Y shrinkage glass ceramic substrate, aluminum nitride, mullite, or the like. As understood from  FIG. 4B , the package  60  constructed from the ceramic layers  60   a - 60   c  forms a cavity. The electronic component(s)  31  and/or tuning-fork type crystal-vibrating piece  33  is mounted in the cavity. 
     Copper plating  12 , electrically connected with the electronic component(s)  31 , is formed in a portion of the top surface of the seat ceramic layer  60   c . At least two external terminals  15 , formed in the lower surface of the ceramic package  60 , are mounted on the surfaces of the pads  115  of the circuit board PB. The copper plating  12  connects to the external terminals  15 . A metallized layer is provided on the upper surface of the wall ceramic layer  60   b . A sealing material  39 , made from a low-temperature-brazing filler metal, is formed on the metallized layer for bonding the metal lid  61 . The wall ceramic layer  60   b  and the metal lid  61  are welded together by the sealing material  39 . 
     The tuning-fork type crystal-vibrating piece  33  has, in its proximal portion, an adhesion region intended to be electrically connected using conductive adhesive  37 . Specifically, copper plating  12 , electrically connected with an external electrode, is formed on the seat ceramic layer  60   c , and the proximal end of the tuning-fork type crystal-vibrating piece  33  is bonded to the seat ceramic layer  60   c  using the conductive adhesive  37 . As affixed, the crystal-vibrating piece extends parallel to the bottom ceramic layer  60   a  and produces a predetermined vibration. 
     As disclosed in  FIGS. 4A-4C , a groove  13  is formed around the external terminals  15  of the crystal oscillator  150 . Consequently, when mounting the crystal oscillator  150  on the circuit board PB, any superfluous solder SOL flows into the groove  13 . Hence, even if an unintended larger amount of solder paste is applied to the pads  115  (e.g., using a squeegee), a solder ball or the like is not formed, and short-circuits are avoided. 
     Manufacture of Bottom Ceramic Layer 
       FIGS. 5A-5D  show a method for manufacturing the ceramic package  60 , specifically the bottom ceramic layer  60   a .  FIG. 5A  shows a first green sheet  60   a   1  made from alumina. The lattice-shaped broken lines  69  denote expected partition lines. In this example, a portion of the first green sheet enclosed by the parting lines  69  is a rectangle of 5 mm by 7 mm. To form the groove  13 , as shown in  FIG. 5A , rectangular through-holes  18  are formed in the first green sheet  60   a   1  along the parting lines  69  using a punching machine or the like. The thickness of the first green sheet  60   a   1  dictates the depth of the groove  13 . 
     Next, a second green sheet  60   a   2  sized identically to the first green sheet  60   a   1  is prepared. The second green sheet  60   a   2  is a flat plate lacking the through-holes. Then, the first green sheet  60   a   1  and second green sheet  60   a   2  are stacked. Thus, as shown in  FIG. 5B , the through-holes  18  become blind via-holes  19 . 
     Next, when the stacked sheet is cut along the parting lines  69  to form multiple units each destined to become a bottom ceramic layer  60   a  having the overall configuration as shown in  FIG. 5C . Then, when the wall ceramic layer  60   b  and seat ceramic layer  60   c  are stacked on and integrated with the bottom ceramic layer  60   a , a pre-calcination ceramic package  60  is formed. Although the wall ceramic layer  60   b  and seat ceramic layer  60   c  are not shown in  FIG. 5(D) , printing is performed at the blind via-holes  19  of the bottom ceramic layer  60   a  during application of vacuum suction. Thus, the external terminals  15  are formed by screen printing of a conductive paste including tungsten, molybdenum, or the like. The screen printing is not performed to the entire blind via-holes  19 . Rather, the conductive paste is applied only in the central portions of the blind via-holes  19  to form the grooves  13 . Although not specifically described, this screen-printing technique is also performed to the copper plating  12  of the wall ceramic layer  60   b  and to the seat ceramic layer  60   c.    
     The stacked structure formed as described above is calcinated for a predetermined time at approximately 1500° C. to form the ceramic package  60  having the grooves  13 . 
     In the foregoing description, screen printing is performed after cutting along the parting lines  69 . However, the ceramic package  60  may be produced by a process having a different other than that described above. For example, screen printing of the conductive paste may be performed to the large green sheet  60   a  before partition. Then the sheet is calcinated and cut along the parting lines  69 . 
     The foregoing description pertained to the package  60  being made of ceramic. Alternatively, the package can be made of a filled resin, with the same grooves  13  being formed around the external terminals  15 . Exemplary filled-resin materials are epoxy resin, bismaleimide-triazine (BT) resin, polyimide resin, glass epoxy resin, glass BT resin, and the like. With a resin package, the groove  13  may be formed by laser processing, drilling, routing, or the like. 
     In the foregoing description, the first green sheet  60   a   1  and the second green sheet  60   a   2  are stacked to form the bottom ceramic layer  60   a . Alternatively, a boss, die, or the like defining a shape complementary to the shape of the groove  13  may be urged against a single green sheet to form the grooves  13 . 
     Depth Profiles of Grooves 
     As explained above, the grooves  13  extend depthwise into the base board and can be formed by laser processing, drilling, routing, or the like to a base board made of a resin laminate. Alternatively, the grooves  13  can be formed by punching or similar method before calcining a ceramic base board. 
       FIGS. 6A-6D  show representative sectional profiles of the grooves  13 . In  FIGS. 1 to 5  described above, the sectional profile of the grooves  13  was rectangular. But, any of various other sectional profiles can alternatively be used.  FIG. 6A  depicts a triangular profile for the grooves  13 . Such a profile can be formed easily by drilling or routing. However, if the width and the depth of a triangular-profiled groove  13  are the same as a corresponding rectangular groove, the volume of the triangular groove is less than of the rectangular groove. Hence, the triangular groove can accept less overflowing solder SOL than a rectangular groove having the same depth and width. 
       FIG. 6B  shows a groove  13  having a sectional profile that is semi-circular. This profile is suitable if the grooves are formed by embossing. 
       FIG. 6C  shows a groove  13  that provides progressively larger cross-sectional area with increased depth. Although special routing or the like must be used to form such grooves, since the volume of the groove  13  increases with depth, the amount of overflowing solder SOL that can be accommodated in such a groove may be larger than with other types of grooves. 
       FIG. 6D  shows rectangular grooves  13  formed with shoulders (i.e., the grooves are separated from the external terminals  15  by a distance ΔL). 
     The grooves  13  described above are formed directly at the sides of the external terminals  15 . However, the grooves need not be formed directly to the sides. 
     The grooves  13  described above formed as a single groove around each respective external terminal  15 . Alternatively, multiple grooves (e.g., two) can be formed around the terminals. 
     The foregoing description has been in the context of mounting an electronic device, such as piezoelectric oscillator  100  or crystal oscillator  150 , to a circuit board PB. This is not intended to be limiting. The principles described herein can be applied to other types of electronic devices, such as a package having Chip on Board (COB) structure, and Pin Grid Array (PGA) structure, or a Ball Grid Array (BGA) package. These various electronic devices are often manufactured using resin packages. Since a resin package has rich mechanical processability, grooves may be formed economically and with high precision using mechanical techniques such as drilling or routing. 
     The description has been in the context of crystal oscillators. Alternatively, a crystal unit may be used and, in particular, a large-sized device is preferable among electronic devices. Before applying the solder SOL, a solder resist may be applied to the circuit board PB between places where the solder SOL is to be applied.

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