Patent Publication Number: US-6341413-B1

Title: Method of making electronic equipment

Description:
This application is a divisional of U.S. application Ser. No. 08/809,176, filed Mar. 18, 1997, now U.S. Pat. No. 6,025,562, which was the National Stage of International Application No. PCT/JP96/01997, filed Jul. 18, 1996. 
    
    
     TECHNICAL FIELD 
     The present invention relates to an electronic component such as a proximity sensor as well as a method of manufacturing the same, and in particular to a structure of an electronic component provided with a casing filled with resin as well as a method of manufacturing such an electronic component. 
     BACKGROUND ART 
     FIG. 35 is a cross section showing an example of a proximity sensor of a high-frequency oscillation type in the prior art. A conventional proximity sensor  101  includes a core  102  provided with an annular groove, in which a coil  104  held by a coil spool  103  is buried. Core  102  is held at a front surface of a coil casing  105  made of resin as shown in the figure, and these parts are accommodated in a base member, i.e., metal casing  106 . A reference number  107  indicates an electronic circuit which includes an oscillator circuit including coil  104  as well as a signal processor unit for detecting lowering of its oscillation amplitude, and is mounted on a printed board  108 . After coil  104  is connected to printed board  108 , primary filler resin  109  is supplied into a portion of coil casing  105  near core  102  for stabilizing performance. In order to improve environmental resistance, epoxy resin  110  is supplied into the casing of the proximity sensor. When supplying epoxy resin  110 , it is hot and, for example, an injector is used. A clamp portion  111  holds a code  112  to complete the proximity sensor. Alternatively, there has been a proximity sensor, in which a casing is not filled with resin in a sealed manner, and thermoplastic resin is used to form an integral structure. A display element  113  is mounted on printed board  108 , and light beams emitted therefrom are led externally through a transparent light conducting portion  114 . 
     For filling the casing of the proximity sensor with the resin, primary filler resin  109  of a low viscosity is injected into the coil casing  105 , and coil  104  held by core  102  is inserted into the resin. Then, metal casing  106  is fitted to the coil casing  105 , and epoxy resin  110  is injected again. In this conventional manner of filling the structure with resin, many steps are required for resin injection, and a long time is required for hardening. 
     The conventional proximity sensor requires a preliminary process such as stirring of resin to be injected. Even if the filler resin is injected into the casing with an injector or the like, it shrinks when hardened, so that additional resin must be injected to fill the space formed by shrinkage. Therefore, many steps are required, and a long hardening time of about 1 hour is required. Further, in the case where thermoplastic resin is used for forming an integral structure, a high injection pressure is required for molding, so that accommodated parts may be impaired. 
     In the case where resin of a high viscosity is used as the filler resin for forming an integral structure, spaces around the coil casing, coil spool, core and others are not completely filled with the primary filler resin, and air remains in the primary molding dies. The remaining air expands due to a high temperature of the dies and a high temperature of the filler resin during the primary molding, and the coil casing may be deformed. The air remaining around the coil may cause unstable characteristics of the coil. 
     The present invention has been developed for overcoming the above disadvantages through low-pressure molding with thermoplastic resin, and has the following objects. 
     An object of the invention is to enable easy and reliable filling of a casing of an electronic component with resin. Another object of the invention is to simplify a resin filling process. 
     DISCLOSURE OF THE INVENTION 
     The invention provides an electronic component having a casing accommodating an electronic part and filled with resin, and including a resin inlet arranged at one end portion of the casing, and a passage arranged at the other end portion of the casing for communicating an interior and an exterior of the casing with each other. Since the passage for air is arranged remote from the resin inlet, air in the casing can be discharged through the passage to lower the pressure in the casing during supply of the resin. Therefore, resin can be easily and reliably supplied to portions, for example, around the electronic part in the casing, which cannot generally be filled with resin without difficulty. Further, the resin can be hardened in a short time, so that processing steps can be reduced in number, and a manufacturing time can be significantly reduced. 
     The passage arranged at the other end side of the casing may be arranged at a cover or lid covering an opening formed at the other end side of the casing. The passage formed at the other end side of the casing may be arranged between a cover over an opening formed at the other end portion of the casing and the casing. The resin inlet may be arranged at a clamp portion attached to an opening formed at the one end portion of the casing for holding a code. In a structure wherein a coil casing is arranged in the casing, the air passage may be formed of an air discharge groove arranged between a cylindrical casing and a coil casing. In a structure wherein a printed board and a shield film covering the printed board are arranged in a cylindrical casing, an opening may be formed at the shield film. The air passage may be formed by forming a linear projection at the coil casing. 
     The present invention also provides a method of manufacturing an electronic component wherein an electronic part is accommodated in a casing filled with resin and having a resin inlet at its one end portion and a passage at the other end portion for communicating an interior of the casing with an exterior of the same, the method including the steps of lowering a pressure in the casing by externally discharging air in the casing through the passage, and filling the casing with resin by supplying the resin. In this manner, the pressure in the casing is reduced for supply of the resin. Therefore, it is possible to supply reliably and easily the resin to a portion which cannot be generally filled with the resin without difficulty. In this case, the resin is hardened in a short time, so that the processing steps can be small in number, and the manufacturing time can be significantly reduced. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a structure of a proximity sensor, before assembly, of a first embodiment of the invention. FIG. 2 is a cross section of the proximity sensor of the first embodiment. FIG. 3 a  is a side view of a coil casing of the proximity sensor of the first embodiment. FIG. 3 b  is a cross section of the coil casing of the proximity sensor of the first embodiment. FIG. 4 is a cross section of the coil casing of the first embodiment taken along line I—I. FIG. 5 is a cross section of the coil casing of the first embodiment taken along line II—II. FIG. 6 a  is an elevation of a clamp portion of the first embodiment. FIG. 6 b  is a side view of the clamp portion of the first embodiment. FIG. 7 a  is a bottom view of the clamp portion of the first embodiment. FIG. 7 b  is a rear view of the clamp portion of the first embodiment. FIG. 8 is a cross section of the clamp portion of the first embodiment. FIG. 9 a  shows a core and a coil to be accommodated in the coil casing of the first embodiment before assembly. FIG. 9 b  shows the core and the coil to be accommodated in the coil casing of the first embodiment before assembly. FIG. 10 shows a structure of a proximity sensor, before assembly, of a second embodiment of the invention. FIG. 11 is a cross section of the proximity sensor of the second embodiment. FIGS. 12 a,    12   b  and  12   c  show states of resin in a process of filling the proximity sensor with the resin in the second embodiment. FIG. 13 shows a state in which the structure is not completely filled with the resin in the second embodiment. FIG. 14 shows a structure of a proximity sensor, before assembly, of a third embodiment of the invention. FIG. 15 a  is a development of a shield film in the third embodiment. FIG. 15 b  is a cross section of the proximity sensor wrapped with the shield film of the third embodiment. FIGS. 16 a,    16   b  and  16   c  show states of resin in a process of filling the proximity sensor with the resin in the third embodiment. FIG. 17 is a cross section showing, on an enlarged scale, a tip end of a proximity sensor of a fourth embodiment. FIG. 18 a  shows a core and a coil to be accommodated in a coil casing of the fourth embodiment before assembly. FIG. 18 b  shows an assembly of the core and the coil to be accommodated in the coil casing in the fourth embodiment before assembly. FIG. 19 a  is an elevation of the core in the fourth embodiment. FIG. 19 b  is a cross section of the core in the fourth embodiment. FIG. 20 a  is an elevation of a coil spool in the fourth embodiment. FIG. 20 b  is a side view of the coil spool in the fourth embodiment. FIG. 20 c  is a cross section of the coil spool in the fourth embodiment. FIG. 21 shows a structure of a proximity sensor, before assembly, of a fifth embodiment. FIG. 22 is a cross section of the proximity sensor of the fifth embodiment. FIG. 23 shows a structure of a proximity sensor, before assembly, of a sixth embodiment. FIG. 24 is a cross section of the proximity sensor of the sixth embodiment. FIG. 25 shows a structure of a proximity sensor, before assembly, of a seventh embodiment. FIG. 26 is a cross section of the proximity sensor of the seventh embodiment. FIG. 27 shows a structure of a proximity sensor, before assembly, of an eighth embodiment. FIG. 28 is a cross section of the proximity sensor of the eighth embodiment. FIG. 29 shows a structure of a proximity sensor, before assembly, of a ninth embodiment. FIG. 30 is a cross section of the proximity sensor of the ninth embodiment. FIG. 31 shows a structure of a proximity sensor, before assembly, of a tenth embodiment. FIG. 32 is a cross section of the proximity sensor of the tenth embodiment. FIG. 33 shows a structure of a proximity sensor, before assembly, of an eleventh embodiment. FIG. 34 is a perspective view of a metal casing of the proximity sensor of the eleventh embodiment. FIG. 35 is a cross section of an example of a conventional proximity sensor. 
    
    
     PREFERRED EMBODIMENTS FOR IMPLEMENTING THE INVENTION 
     Embodiments of the invention will be described below with reference to the drawings. 
     FIG. 1 shows a structure of a proximity sensor, before assembly, of a first embodiment of the invention, and FIG. 2 is a cross section of the same. As shown in these figures, a proximity sensor  1  of this embodiment includes a core  2  provided with an annular groove. A coil  4  wound around a coil spool  3  is accommodated in the annular groove. To a rear surface of core  2  is connected an elongated printed board  5  as shown in the figure, and an oscillator circuitry on printed board  5  is covered with a shield film  6 . Core  2  is accommodated in a coil casing  7 . Coil casing  7  is a cylindrical member having a closed bottom and made of resin. Coil casing  7  is provided at a periphery of its axially inner portion with projections, which will be described later. Coil casing  7  is accommodated in a base member, i.e., a metal casing  8 . Metal casing  8  is a cylindrical casing made of metal and is provided at its outer periphery with a thread groove. A clamp portion  9  is attached to the rear surface of casing  8 . Clamp portion  9  is made of resin, and is provided for holding a code  10 . 
     Coil casing  7  will be described below more in detail. Coil casing  7  is a cylindrical member having a closed front side, and thus has a substantially rectangular section with one side open, as can be seen from a side view of FIG. 3 a  and a cross section of FIG. 3 b  as well as FIG. 4 showing a section taken along line I—I and FIG. 5 showing, on an enlarged scale, a section taken along line B—B. Coil casing  7  is provided at its outer periphery of an axially inner surface portion, which is located inside metal casing  8 , with first linear projections  21   a - 21   d  for preventing leakage of resin as shown in the figures. A series of linear projections  21   a - 21   d  for preventing leakage of resin extend substantially entirely over an outer periphery of coil casing  7 , and are interrupted at four positions. Axially inside projections  21   a - 21   d,  there are arranged second linear projections  22   a - 22   d  for preventing leakage of resin, each of which is shorter than the first projection and is axially opposed to a recess between circumferentially adjacent linear projections  21   a - 21   d.  The recesses between first and second linear projections  21   a - 21   d  and  22   a - 22   d  form grooves for discharging air. At positions axially corresponding to circumferentially middle positions of the recesses between linear projections  21   a - 21   d , there are arranged projections  23   a - 23   d  each having the same height and an arc-shaped section, respectively. At middle positions between linear projections  22   a - 22   d , there are arranged projections  24   a - 24   d  each having the same height and an arc-shaped section, respectively. These arc-shaped projections  22   a - 22   d  and  24   a - 24   d  are provided for holding coil casing  7  coaxially with metal casing  8  without inclining the coil casing  7 , and thereby providing uniform grooves for discharging air in a process of forcedly fitting coil casing  7  into metal casing  8 . An open end of coil casing  7  is provided at portions, which do not correspond to linear projections  22   a - 22   d , with axially inward recesses  25   a - 25   d , respectively, as shown in FIGS. 3 a  and  3   b.  When coil casing  7  is inserted into metal casing  8 , it stops when its innermost end is brought into contact with a boundary at the end portion of metal casing  8  having a slightly reduced thickness. Thereby, recesses  25   a - 25   d  are provided for ensuring a passage for air flow, when coil casing  7  is accommodated in metal casing  8 . A portion axially between linear projections  21   a - 21   d  and linear projections  22   a - 22   d  forms a groove  26  for accommodating or storing resin. Linear projections  21   a - 21   d  and  22   a - 22   d  have inclined surfaces at their axially inner sides, as can be seen from FIG. 5 showing, on an enlarged scale, a section taken along line II—II in FIG. 3 a,  and a thickness of the portion axially inside linear projections  22   a - 22   d  is larger than that of the portion axially outside the same. 
     As shown in FIG. 4, coil casing  7  is provided eight ribs  27   a - 27   h  located at positions, which axially correspond to the recesses between linear projections  21   a - 21   d  and intermediate portions of linear projections  21   a - 21   d,  respectively, and thus are circumferentially spaced by 45 degrees from each other. Ribs  27   a - 27   h  are formed at the inner periphery of the cylindrical portion of coil casing  7 , and extend axially inward and therefore parallel with the cylinder axis from the front side of the proximity sensor. These ribs  27   a - 27   h  are provided for holding core  2  coaxially with coil casing  7 . Further, coil casing  7  is provided at the inner surface of its front wall with linear projections  28   a - 28   d  arranged in a cross form. Ribs  27   a - 27   h  and linear projections  28   a - 28   d  are provided for ensuring an outlet of air flow in the process of injecting primary filler resin. 
     A specific structure of clamp portion  9  will be described below with reference to FIGS. 6 to  9 . FIG. 6 a  is an elevation, FIG. 6 b  is a side view, FIG. 7 a  is a bottom view, FIG. 7 b  is a rear view, and FIG. 8 is a cross section of clamp portion  9 . Clamp portion  9  has a cylindrical portion  31  located in metal casing  8 . Cylindrical portion  31  has the substantially same outer diameter as the inner diameter of metal casing  8  and is provided at its axially inner portion with two belt-like projections  32 . A flange  33  is arranged behind projections  32 . A light conducting portion  34 , which has a substantially half conical form and a substantially triangular section as shown in FIG. 8, is formed axially behind flange  33 . A code holding portion  35  is arranged behind light conducting portion  34  for elastically holding code  10 . Code holding portion  35  is provided with slit-like openings  36  which are equally spaced from each other. Code holding portion  35  has an inner diameter corresponding to the outer diameter of code  10  as shown in FIGS. 6 a  and  7   b.    
     Cylindrical portion  31  inserted into metal casing  8  is provided at its inner surface with guide grooves  37  and tapered portions  38  for guiding opposite edges of printed board  5  of the proximity sensor, as can be seen from the section of FIG.  8 . Tapered portion  38  is provided at its axially innermost portion with board holding grooves  39 . Board holding groove  39  has a width substantially equal to a thickness of printed board  5 , and is provided with a linear projection  40  as shown in the figure. Linear projection  40  is adapted to be deformed by printed board  5  inserted into to groove  39 , and thereby fixes printed board  5 . Printed board  5  which is held in board holding grooves  39  is opposed to the inner surface of light conducting portion  34 . A display element  41  such as an LED is mounted on the portion of printed board  5  opposed to the light conducting portion  34 . When printed board  5  provided with display element  41  is fitted into board holding grooves  39  through guide grooves  37 , the upper surface of display element  41  is substantially brought into contact with the inner surface of light conducting portion  34 . Even in the case where a gap is formed between them, light beams can be externally emitted from display element  41  by arranging a thick layer of transparent resin  42  on the upper surface of display element  41  as shown in FIG.  8 . More specifically, the light beams emitted from display element  41  are led to light conducting portion  34  through transparent resin  42  and then are externally emitted directly through light conducting portion  34  or after being reflected by a reflection surface  34   a.    
     In clamp portion  9   a,  as can be seen from the rear view of FIG. 7 a,  side portions of flange  33  have larger diameter, and is provided at its rear surface with an inward opening  43 . A rim  44  is formed at the edge of opening  43  as shown in FIG. 6 b.  Opening  43  is provided for internally injecting filler resin, as will be described later. 
     FIG. 9 a  shows a structure before assembly, and specifically shows core  2  to be accommodated in coil casing  7  and coil  4  wound around coil spool  3 . FIG. 9 b  shows the same structure viewed at a reverse angle. As can be seen from these figures, core  2  has the annular groove, and also has through-apertures  2   a  and  2   b  at its rear surface. Through through-apertures  2   a  and  2   b,  leads  4   a  and  4   b  at opposite ends of the coil  4  are extended from an end of the coil spool, and are connected to the oscillator circuit (not shown) on the printed board. 
     A process of manufacturing the proximity sensor of this embodiment will be described below. Coil  4  wound around coil spool  3  is put into the annular groove of core  2 . Leads  4   a  and  4   b  at the opposite ends of coil  4  thus arranged are connected to printed board  5 , and shield film  6  is wound around printed board  5 . Then, core  2  thus assembled is disposed in coil casing  7 , and the primary filling with coil unit sealing epoxy resin  51  is performed for stabilizing coil characteristics. Then, code  10  is inserted through metal casing  8  and clamp portion  9 , and a terminal of code  10  is connected to a predetermined portion of printed board  5 . Then, coil casing  7  is put into base member  8 . Transparent resin  42  already described is laid over the upper surface of display element  41  of printed board  5 , and clamp portion  9  is located inside metal casing  8 . Thereby, printed board  5  is inserted into guide grooves  37  and holding grooves  39 . In this manner, the proximity sensor is completed. Thereby, display element  41  is in contact with light conducting portion  34  of clamp portion  9  through transparent resin  42 . 
     The proximity sensor thus assembled is located in dies, and a vacuum condition is set. Thereby, air is externally extracted from the proximity sensor through the air discharge grooves formed of the spaces provided by linear projections  22   a - 22   d  and  21   a - 21   d  at coil casing  7 , and the pressure in the proximity sensor lowers. In this state, hot filler resin is injected with a low pressure, e.g., between 5 atm and 20 atm through opening  43  at clamp portion  9 . Thereby, the filler resin is first brought into contact with printed board  5 , and then entirely fills the casing. In this process, the temperature of the dies is appropriately selected, whereby the proximity sensor can be completed in a short time. In the above process, air flows between linear projections  21   a - 21   d  and  22   a - 22   d  as indicated by arrows C in FIG.  3 . The resin injected with a low pressure is brought into contact with linear projections  21   a - 21   d,  and stays in resin accumulation groove  26  shown in FIG.  5 . Therefore, the filler does not leak from coil casing  7 , and can be held within metal casing  8 . 
     A proximity sensor of a second embodiment of the invention will be described below. FIG. 10 shows a structure of a proximity sensor of the second embodiment before assembly, and FIG. 11 is a cross section of the same. As shown in these figures, a clamp portion  61  of this embodiment has a substantially conical form and is provided at its center with an opening as shown in FIG.  10 . An end of clamp portion  61  near metal casing  8  has a reduced diameter equal to the inner diameter of metal casing  8 . A code holding portion  62  of clamp portion  61  forming the rear end of the proximity sensor has a reduced inner diameter equal to the diameter of code  10 . Clamp portion  61  is provided at its side wall with an opening  63 . Structures other than the above are the same as those of the embodiment  1  already described. 
     FIG. 12 a  shows various states during injection of filler resin  52  through opening  63  at clamp portion  61  in the second embodiment. As already described, the coil casing of proximity sensor  60  is filled with coil unit sealing resin represented by “X”, and the proximity sensor in this state is held in dies with a vacuum pressure. Resin represented by “.” is injected through opening  63  at clamp portion  61 , so that air is discharged through the gap at coil casing  7 . In accordance with this, filler resin  52  successively enters the proximity sensor, and it is filled with the resin as shown in FIGS. 12 a,    12   b  and  12   c.  Since code holding portion  62  of clamp portion  61  has the inner diameter equal to code  10 , resin does not leak through the gap around the code when filler resin  52  is supplied into the casing as described above. 
     A third embodiment of the invention will be described below. A proximity sensor of the third embodiment includes shield film  6  provided with an opening for allowing reliable injection of resin. If the proximity sensor had a small diameter and shield film  6  were not provided with an opening, air would not be completely discharged and would be left at a region between coil unit sealing resin and shield film  6  as shown in FIG.  13 . In view of this, the opening is formed at shield film  6 . Structures other than the above are the same as those of the second embodiment. 
     FIG. 14 shows a structure of a proximity sensor of a third embodiment before assembly. FIG. 15 a  is a development showing an example of a shield film  71  used in this proximity sensor. Shield film  71  is formed of a substantially rectangular flexible board, and a substantially comb-like pattern  72  is formed at one surface of film  71 . Shield film  71  is provided at its one end with a projection  73 , in which ends  74   a  and  74   b  of pattern  72  are formed. Two openings  75   a  and  75   b  are formed at a central portion of the conductor pattern of shield film  71 . A double-side adhesive portion  76  is formed at a right end of the rectangular board. Shield film  71  is curved to wrap printed board  5  as shown in FIG. 15 b  so that one end thereof covers and adheres to double-side adhesive portion  76 . In this wrapped state, a pair of openings  75   a  and  75   b  are located at positions corresponding to opposite ends of printed board  5 . This can reduce an influence on the electronic circuitry on printed board  5 . 
     FIGS. 16 a  and  16   b  show states during supply of resin into the proximity sensor thus constructed of the third embodiment. When resin is supplied through opening  63  into the proximity sensor maintained under vacuum, a portion near clamp portion  61  is first filled with resin which will successively expand therefrom to a portion filled with coil unit sealing resin  51  as shown in FIGS. 16 b  and  16   c.  Since through-apertures  75   a  and  75   b  are formed at shield film  71 , no air remains between shield film  71  and printed board  5 , and a gap is filled with resin in the above process. This improves an environmental resistance of the proximity sensor. Further, the filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. 
     Although the shield film has the two openings in the third embodiment described above, a similar effect can be achieved by at least one opening. 
     In the operation of filling the proximity sensor with the resin according to the embodiment described above, the resin is supplied into the proximity sensor maintained at a low pressure through the opening with a low pressure, so that the casing can be reliably filled with resin. In this case, the resin is hardened in a short time, so that the steps can be reduced in number, and the manufacturing time can be significantly reduced. Owing to provision of the opening at the shield film covering the printed board, the resin can be filled reliably. 
     A fourth embodiment of the invention will be described below. In the following description, a difference between the first and fourth embodiments will mainly be discussed. Except for portions which will be described below, the proximity sensor of the fourth embodiment is the same as that of the first embodiment. Accordingly, the same portions as those in the first embodiment will not be described below. 
     FIG. 17 shows, on an enlarged scale, a tip end of the proximity sensor of the fourth embodiment. Coil spool  3  is provided at its front surface with a plurality of through-apertures  3   c.  Through-apertures  3   c  function as will be described later. A plurality of through-apertures  2   d  are at core  2 . Through-apertures  2   d  function as will be described later. 
     Core  2  and coil spool  3  accommodated in coil casing  7  will be described below. FIG. 18 a  shows core  2  to be accommodated in coil casing  7  and coil  4  wound around coil spool  3  before assembly. FIG. 18 b  shows the same structure view at a reverse angle. FIG. 19 a  is an elevation of core  2 , and FIG. 19 b  is a cross section. As shown in these figures, core  2  has an annular groove  2   c,  and also has two substantially flat-D-shaped through-apertures  2   a  and  2   b  as well as a circular through-aperture  2   d,  which are circumferentially spaced by 120° from each other around the center of annular groove  2   c.  As can be seen from an elevation of FIG. 20 a,  a side view of FIG. 20 b  and a cross section of FIG. 20 c,  coil spool  3  has a spool portion around which the coil is wound, and also has terminals  3   a  and  3   b  for guiding and holding the opposite ends of coil. As shown in the elevation of FIG. 20 a,  a plurality of through-apertures  3   c  are formed at the spool portion. Coil  4  is wound around the spool portion, and has opposite ends which are held by terminals  3   a  and  3   b  and are externally led through through-apertures  2   a  and  2   b  at core  2  for connection to the oscillator circuit on printed board  5 . 
     A process of manufacturing the proximity sensor of this embodiment will be described below. Coil  4  wound around coil spool  3  is located in the annular groove at core  2 . The leads at the opposite ends of coil  4  are connected to printed board  5 , and shield film  6  is wound around printed board  5 . Core  2  thus assembled is put into coil casing  7 , and primary filling with coil unit sealing epoxy resin  51  is performed for stabilizing the coil characteristics. Although coil unit sealing resin  51  for the primary filling has a high viscosity, no air remains around the coil owing to the following structure. Since coil spool  3  is provided with through-aperture  3   c,  and core  2  is also provided with through-apertures  2   a,    2   b  and  2   c.  In addition to this, coil casing  7  is provided at its inner surface with the projections already described. Therefore, the resin primarily supplied into coil casing  7  can fully fill the casing, and thus no air remains around the coil. Then, code  10  is inserted through metal casing  8  and clamp portion  9 , and the terminal of code  10  is connected to a predetermined portion of printed board  5 . Then, coil casing  7  is put into metal casing  8 . Clamp portion  9  is fitted into metal casing  8 . In this manner, the proximity sensor is assembled. 
     The proximity sensor thus assembled is disposed in dies and a vacuum state is attained. Thereby, air is externally extracted from the proximity sensor through the air discharge grooves formed of gaps between linear projections  22   a - 22   d  and  21   a - 21   d  at coil casing  7 , so that the pressure in the proximity sensor lowers. In this state, hot filler resin is injected through opening  9   a  at clamp portion  9  with a low pressure, e.g., between 5 atm and 20 atm. The resin thus supplied is brought into contact with printed board  5 , and further flows in the casing to fill entirely the casing. In this process, the temperature of dies is appropriately selected, whereby the proximity sensor can be completed in a short time. In the above process, air flows between linear projections  21   a - 21   d  and  22   a - 22   d  as indicated by arrows C in FIG.  3 . The resin injected with a low pressure is brought into contact with linear projections  21   a - 21   d,  and stays in resin accumulation groove  26  shown in FIG.  5 . Therefore, the filler does not leak from coil casing  7 , and can be held within metal casing  8 . 
     Although the fourth embodiment has been described in connection with the proximity sensor, the invention can be applied to various electronic components other than the proximity sensor such as a data carrier in an ID unit having such a structure that a coil is held in a cylindrical casing, a space around the coil is filled with primary filler resin, and an integral structure is subsequently formed in the casing. 
     According to the fourth embodiment described above, the space around the coil can be reliably filled with the primary filler resin. Therefore, no air remains, so that expansion of the coil casing can be prevented, and thus the coil characteristics can be stable. 
     The proximity sensor of a fifth embodiment of the invention will be described below. FIG. 21 shows a structure of a proximity sensor, before assembly, of the fifth embodiment. FIG. 22 is a cross section of the proximity sensor of the fifth embodiment. In the following description, only a difference between the first and fifth embodiments will mainly be discussed. 
     The proximity sensor of the fifth embodiment differs from that of the first embodiment in the following points. Coil casing  7  is not provided at its outer periphery with linear projections  21   a - 21   d.  Further, a ring code  45  is provided at an end of code  10  near the printed board. Ring code  45  is employed for filling a space between clamp portion  9  and code  10 . Metal casing  8  and coil casing  7  have sections of different forms. More specifically, metal casing  8  has an elliptic section, and coil casing  7  has a completely circular section. Since metal casing  8  and coil casing  7  have different sectional forms, gaps forming an air discharge passage is formed between the inner peripheral surface of metal casing  8  and the outer peripheral surface of coil casing  7 . Metal casing  8  is provided at its inner peripheral surface with a plurality of recesses (not shown), through which the air discharge passage is communicated with the interior of metal casing  8 . 
     Owing to the above structure, air is externally extracted through the air discharge passage in the process of injecting filler resin  52 . In contrast to the foregoing case, metal casing  8  may have a completely circular section, and coil casing  7  may have an elliptic section. Thus, any relationship may be present between the sectional forms of metal casing  8  and coil casing  7  provided that an air discharge passage, which allows passage of air but suppresses passage of resin, is formed between the inner peripheral surface of metal casing  8  and the outer peripheral surface of coil casing  7 . The gap or space forming the air discharge passage is preferably in a range from 0.1 mm to 0.01 mm. 
     According to this fifth embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done also in the first embodiment. 
     A proximity sensor of a sixth embodiment of the invention will be described below. FIG. 23 shows a structure of the proximity sensor before assembly of the sixth embodiment. FIG. 24 is a cross section of the proximity sensor of the sixth embodiment. In the following description, the proximity sensor of the sixth embodiment will be discussed only in connection with a difference from that of the first embodiment. 
     The proximity sensor of the sixth embodiment differs from that of the first embodiment in the following points. Coil casing  7  is not provided at its periphery with linear projections  21   a - 21   d.  Ring code  45  is formed at an end of code  10  near printed board  5 . 
     Further, a plurality of through-apertures  71  are formed at the periphery of the end surface of coil casing  7 . The interior and exterior of coil casing  7  are communicated with each other by through-apertures  71 . The outer diameter of core  2  is smaller by a predetermined value than an inner diameter of metal casing  8 . This predetermined value is very small. Therefore, a gap is formed between the inner peripheral surface of metal casing  8  and the outer peripheral surface of coil casing  7 . This gap may be defined by an irregular surface which has many concavities and convexities and is formed by a powder of magnetic material coating over the outer peripheral surface of core  2 . The gap thus formed and through-apertures  71  form an air discharge passage. The air discharge passage communicates the exterior and interior of coil casing  7  and metal casing  8  with each other. 
     The structure is not filled with coil unit sealing resin  51 . Core  2  is fitted into casing  7 , whereby core  2  is fixed. An interior of the assembly of core  2 , coil casing  7  and metal casing  8  is entirely filled with filler resin  52 . Owing to this structure, air can be externally extracted through the air discharge passage in the process of filling the structure with filler resin  52 . 
     According to the above sixth embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. 
     A proximity sensor of a seventh embodiment of the invention will be described below. The proximity sensor of the seventh embodiment is of a so-called non-shield type, in which coil casing  7  is exposed outside metal casing  8 . FIG. 25 shows a structure of the proximity sensor before assembly of the seventh embodiment. FIG. 26 is a cross section of the proximity sensor of the seventh embodiment. In the following description, the proximity sensor of the seventh embodiment will be discussed mainly in connection with a difference from the first embodiment. 
     The proximity sensor of the seventh embodiment differs from the first embodiment in the following points. Coil casing  7  is provided at its outer periphery with linear projections  29   a - 29   d,  which are located at positions between positions of linear projections  21   a - 21   d  in the first embodiment and positions of linear projections  22   a - 22   d.  Further, only one end portion of coil casing  7 , at which linear projections  29   a - 29   d  and linear projections  22   a - 22   d  are formed, is fitted into metal casing  8 . Therefore, an air discharge passage is formed between the outer peripheral surface of coil casing  7  and the inner peripheral surface of metal casing  8  owing to provision of linear projections  29   a - 29   d  and  22   a - 22   d.  This air discharge passage forms a communication passage between the interior and exterior of metal casing  8 . Owing to the foregoing structure, air is externally extracted through the air discharge passage in the process of supplying filler resin  52 . 
     According to this seventh embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. 
     A proximity sensor of an eighth embodiment will be described below. The proximity sensor of the eighth embodiment is of a so-called non-shield type in which coil casing  7  is exposed outside metal casing  8 , and is not filled with coil unit sealing resin  51 . FIG. 27 shows a structure of the proximity sensor of the eighth embodiment before assembly. FIG. 28 is a cross section of the proximity sensor of the eighth embodiment. In the following description, the proximity sensor of the eighth embodiment will be described mainly in connection with a difference from the first embodiment. 
     The proximity sensor of the eighth embodiment differs from the first embodiment in the following points. Coil casing  7  is not provided at its outer periphery with linear projections  21   a - 21   d.  Ring code  45  is formed at an end of code  10  near printed board  5 . A plurality of air apertures  72  are formed at an outer periphery of an end of coil casing  7 . An air discharge passage is formed of air apertures  72  and a gap defined between the outer peripheral surface of core  2  and the inner peripheral surface of coil casing  7 . This air discharge passage forms a communication passage between the exterior and the interior of metal casing  8 . Owing to the foregoing structure, air is externally extracted through the air discharge passage in the process of supplying filler resin  52 . 
     According to the eighth embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. 
     A proximity sensor of a ninth embodiment will be described below. The proximity sensor of the ninth embodiment is of a so-called non-shield type in which the structure is filled with coil unit sealing resin  51 . FIG. 29 shows a structure of the proximity sensor before assembly of the ninth embodiment. FIG. 30 is a cross section of the proximity sensor of the ninth embodiment. In the following description, the proximity sensor of the ninth embodiment will be described mainly in connection with a difference from the eighth embodiment. 
     The proximity sensor of the ninth embodiment differs from the eighth embodiment in the following points. Coil casing  7  is not provided with air apertures  72 , but is provided at its outer peripheral portion near its opening with a plurality of air apertures  73 . Air apertures  73  are formed at such positions that are not covered with metal casing  8  when coil casing  7  is fitted into metal casing  8 . Therefore, the exterior and interior of coil casing  7  are communicated with each other through air apertures  73 . Air apertures  73  form an air discharge passage. Owing to the foregoing structure, air is externally extracted through the air discharge passage, i.e., air apertures  73  in the process of supplying filler resin  52 . 
     Description will now be given on a method of manufacturing metal casing  8  of the ninth embodiment. Core  2 , to which coil  4 , printed board  5  and shield film  6  are attached, is disposed in coil casing  7 , and the assembly is filled with coil unit sealing resin  51 . In this filling operation, an amount of coil unit sealing resin  51  is controlled to prevent closing of air apertures  73  at coil casing  7  by coil unit sealing resin  51 . Coil casing  7  is forcedly fitted into metal casing  8 . This operation is performed in such a manner that metal casing  8  does not close air apertures  73  at coil casing  7 . Air in coil casing  7  and metal casing  8  is externally extracted through air apertures  73 . Thereafter, filler resin  52  is supplied through resin inlet  43 . Air aperture  73  has a size which allows passage of air but prevents passage of resin. If an air aperture  73  of a size allowing passage of resin is employed for any reason, an apparatus for manufacturing the proximity sensor employs a countermeasure for preventing leakage of resin. 
     According to the ninth embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. 
     A proximity sensor of a tenth embodiment of the invention will be described below. The proximity sensor which will be described below as the tenth embodiment is an example of a structure in which a clamp portion holding a code connected to a printed board is integrally formed of filler resin. FIG. 31 shows a structure of the proximity sensor before assembly of the tenth embodiment. FIG. 32 is a cross section of the proximity sensor of the tenth embodiment. In the following description, the proximity sensor of the tenth embodiment will be described mainly in connection with a difference from the first embodiment. 
     The proximity sensor of the tenth embodiment differs from the first embodiment in the following points. A clamp portion  520  is formed of filler resin  52 . The configuration of clamp portion  520  formed of filler resin  52  is shown in FIG. 32. A structure before supply of filler resin  52  is shown in FIG.  31 . 
     Description will now be given on a method of manufacturing metal casing  8  of the tenth embodiment. Core  2 , to which coil  4 , printed board  5  and shield film  6  are attached, is disposed in coil casing  7 , and the assembly is filled with coil unit sealing resin  51 . Coil casing  7  is forcedly fitted into metal casing  8 , and then a whole structure including metal casing  8  is put in molding dies. Metal casing  8  in the dies is pinched by the dies. Metal casing  8  thus pinched is completely wrapped and covered with the molding dies. 
     Air in metal casing  8  is externally extracted with an air bend provided at the molding dies through a gap between coil casing  7  and metal casing  8  (or apertures formed at coil casing  7 ). Filler resin  52  is supplied into molding dies. Thereby, as shown in FIG. 32, the portion holding code  10  is integrally formed as clamp portion  520 . The configuration of clamp portion  520  thus formed is similar to that of clamp portion  9  in the first embodiment. In this case, filler resin  52  is transparent so that light beams emitted from display element  41  can be externally visible. 
     According to the tenth embodiment, the resin filling operation can be performed extremely easily and reliably in a short time, as can be done in the first embodiment. Owing to integral formation or molding of clamp portion  520  made of filler resin  52 , parts of the proximity sensor can be reduced in number. 
     A proximity sensor of an eleventh embodiment of the invention will be described below. The proximity sensor of the eleventh embodiment is another example of the structure in which clamp portion  520  is integrally formed of filler resin  52 , as is done in the tenth embodiment. FIG. 33 is a cross section of the proximity sensor of the eleventh embodiment. FIG. 34 is a perspective view of a sensor casing of the proximity sensor of the eleventh embodiment. In the following description, the proximity sensor of the eleventh embodiment will be described below mainly in connection with a difference from the tenth embodiment. 
     The proximity sensor of the eleventh embodiment differs from the tenth embodiment in the following points. A sensor casing  80 , which corresponds to metal casing  8  in the tenth embodiment, is made of resin. Sensor casing  80  has an axially extended portion at an end forming clamp portion  520 . This extended portion has a smaller diameter than the main body. In this embodiment, the extended portion of sensor casing  80  has a step-wise form and includes two portions of different diameters. More specifically, the extended portion forms a stepped portion at the end portion of sensor casing  80 , and the extended portion is formed to provide a first extension  811  of a smaller diameter than the main body of sensor casing  80  and a second extension  812  of a smaller diameter than first extension  811 . The extended portion has a semicircular section. The extended portion is covered with clamp portion  520  formed of filler resin  52 . 
     Since the extended portion provides the stepped portion at the end portion of sensor casing  80 , the stepped portion prevents disengaging of the portion formed of filler resin  52  from sensor casing  80 . 
     Industrial Applicability 
     As described above, the invention is suitable to various kinds of electronic components such as a proximity sensor, in which an electronic part such as a coil is held in a cylindrical casing and the casing is filled with resin.