Patent Publication Number: US-2017354035-A1

Title: Flexible printed circuit board

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a Division of application Ser. No. 14/815,413 filed Jul. 31, 2015, which claims priority from Japanese Patent Application No. 2014-159097 filed on Aug. 4, 2014, and titled “ FLEXIBLE PRINTED CIRCUIT BOARD”, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present teaching relates to a flexible printed circuit board which can be electrically connected to a counterpart electrode reliably. 
     Description of the Related Art 
     For example, a strain gage measuring the strain or distortion of a structure is formed in one flexible printed circuit board (a first flexible printed circuit board) which is very thin. In order to extract or take out the output from the strain gage, another flexible printed circuit board (a second flexible printed circuit board) for transmitting a signal is joined to the first flexible printed circuit board with soldering so that the electrode of the first flexible printed circuit board is electrically connected to the counterpart electrode of the second flexible printed circuit board. 
     The technology for joining the electrode of the first flexible printed circuit board to the counterpart electrode of the second flexible printed circuit board with soldering is generally known and is disclosed, for example, in Japanese Patent Application Laid-open No. 2006-303354 and Japanese Utility Model Publication No. H5-29178. 
     SUMMARY 
     The lands of solder joints of the flexible printed circuit boards described in Japanese Patent Application Laid-open No. 2006-303354 and Japanese Utility Model Publication No. H5-29178 have no special structure for allowing air to escape outside at the time of thermocompression bonding. Thus, air could remain in the solder joints at the time of thermocompression bonding and cause the detachment or exfoliation of the solder joints. Specifically, when pre-solder (pre-tin) is melted so that the through holes are filled with the pre-solder, the air in the through holes is not allowed to escape. This forms a layer of air between copper foil and solder to cause the detachment or exfoliation of the solder joints. 
     An object of the present teaching is to provide a flexible printed circuit board having an electrode, which is connected or joined to a counterpart electrode via a solder joint with sufficient strength so as to provide satisfactory conductivity for a long time. 
     According to a first aspect related to the present teaching, there is provided a flexible printed circuit board including: a flexible insulation layer having a first surface and a second surface; a first land which is conductive and which is provided on the first surface of the flexible insulation layer; and a conductive member which is provided on the second surface of the flexible insulation layer, wherein a recess is formed on the first land. 
     According to a second aspect related to the present teaching, there is provided a flexible printed circuit board including: an insulation layer which has a first surface and a second surface, and through which an through hole connecting the first surface and the second surface are formed, a metallic first land provided around an opening, on the first surface of the insulation layer, defined by the through hole; a metallic second land provided around an opening, on the second surface of the insulation layer, defined by the through hole; a metallic connecting portion provided in the through hole to connect the first land and the second land, wherein a channel which connects the through hole and an outside of the first land are formed at a position lower than a top of the first land. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a flexible printed circuit board related to an embodiment of the present teaching as viewed from the side of one of lands provided in the flexible printed circuit board. 
         FIG. 2  is a perspective view depicting a counterpart electrode to be connected to the flexible printed circuit board depicted in  FIG. 1  with soldering. 
         FIG. 3  is an illustrative view of a welding process depicting the cross-section taken along the line III-III in  FIG. 1  and depicting a state immediately before the electrode of the flexible printed circuit board depicted in  FIG. 1  is joined or connected to the counterpart electrode depicted in  FIG. 2  with soldering. 
         FIG. 4  is an illustrative view of the welding process depicting a state, subsequent to the state depicted in  FIG. 3 , in which the flexible printed circuit board depicted in  FIG. 1  is completely pressed against the counterpart electrode with a pulse heater. 
         FIG. 5  is a plan view depicting an exemplary flexible printed circuit board, which has lands each corresponding to the land depicted in  FIG. 1 . 
         FIG. 6A  is a perspective view of the shape of the land related to the embodiment of the present teaching, and  FIG. 6B  is a perspective view of the shape of a land related to a modified embodiment of the present teaching. 
         FIG. 7  is a bottom view of a flexible printed circuit board related to a modified embodiment of the present teaching. 
         FIG. 8  is a cross-sectional view taken along the line VIII-VIII in  FIG. 7 . 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     In the following, an explanation will be made with reference to the drawings about a flexible printed circuit board  1  related to an embodiment of the present teaching.  FIG. 1  is a perspective view of the flexible printed circuit board  1  related to the embodiment of the present teaching as viewed from the side of one of lands (heating side land  130 ) of an electrode provided in the flexible printed circuit board  1 .  FIG. 2  is a perspective view depicting a counterpart electrode to be connected to the flexible printed circuit board  1  depicted in  FIG. 1  with soldering.  FIGS. 3 and 4  are illustrative views chronologically illustrating states that the flexible printed circuit board  1  related to this embodiment is crimped or pressure welded to a part of another flexible printed circuit board having the counterpart electrode through thermocompression bonding. 
     In the drawings, the sizes, thicknesses, and dimensions of components or parts related to the embodiment and modified embodiments are depicted exaggeratedly for easy understanding of the present teaching. 
     As depicted in  FIGS. 3 and 4 , the flexible printed circuit board  1  related to this embodiment includes a flexible insulation layer  11 , a heating side land  130 , and a conductor pattern  12 . The insulation layer  11  is made of polyimide (PI). The heating side land  130  (a first land) is formed on the upper surface (one surface) (a first surface) of the insulation layer  11 . The conductor pattern  12  is formed on the lower surface (the other surface) (a second surface) of the insulation layer  11 . The conductor pattern  12  is electrically connected with the heating side land  130  via a through hole  140 . The conductor pattern  12  is made of rolled copper foil and includes a welding side land  120 , a thin wire pattern for electrical connection  125  (wiring), and an insulating coating  129 . The welding side land  120  has a circular or annular shape. The thin wire pattern for electrical connection  125  extends from the welding side land  120  along the insulation layer  11 . The insulating coating  129  covers the entire flexible printed circuit board  1  except for the welding side land  120  and the part, of the thin wire pattern for electrical connection  125 , required for electrical connection with the counterpart electrode. 
     As depicted in  FIGS. 1 and 6A , the heating side land  130  has a circular plate shape or disk shape in plan view (hereinafter referred to as “disk”). The heating side land  130  is composed of two pieces of rolled copper foil and a groove  131  (slit). The two pieces of rolled copper foil are disposed to face each other with a small space intervening therebetween, and each piece has a substantially semicircular shape in plan view (hereinafter referred to as “semicircular plate”). The groove  131  is sandwiched by the two pieces of rolled copper foil. That is, the groove  131  is formed as one exemplary aspect of a recess of the heating side land  130  (The groove  131  is an example of recess formed in the land related to the present teaching). The heating side land  130  is formed on the upper surface of the insulation layer  11  to be concentric, in plan view, with the welding side land  120 . The heating side land  130  is electrically connected with the welding side land  120  via a rolled copper foil  141  covering the inner circumferential surface of the through hole  140 . That is, the heating side land  130  and the welding side land  120  are electrically and thermally connected with each other via the rolled copper foil  141  (a metallic connecting portion) in the through hole  140 . Note that, in this specification, the expression of “thermally connected” mean a state that objects are connected with a metallic material, such as copper, having good thermal conductivity. 
     Subsequently, an explanation will be made about a structure of a counterpart electrode  220  to which the flexible printed circuit board  1  is connected. In this embodiment, the counterpart electrode  220  depicted in  FIG. 2  is provided at an end of a flexible printed circuit board  20  having a strain gage (not depicted in  FIG. 2 ). The end of the flexible printed circuit board  20  includes a base  21 , a gage side conductor pattern  22 , pre-solder  230 , and a cover glass  25 . The gage side conductor pattern  22  is provided on the upper surface of the base  21  and it is made of rolled copper foil. The pre-solder  230  is provided on a wide surface of an end of the gage side conductor pattern  22  to rise or bulge slightly. The cover glass  25  covers the base  21 , except for a portion at which the pre-solder  230  is formed to rise or bulge, in order to protect the gage side conductor pattern  22 . 
     Subsequently, an explanation will be given about a process for bonding the pre-solder  230  on the gage side conductor pattern  22  to the welding side land  120  of the flexible printed circuit board  1  with thermalcompression-bonding.  FIGS. 3 and 4  are illustrative views of a welding process.  FIG. 3  depicts a state immediately before the electrode of the flexible printed circuit board  1  is joined or connected to the counterpart electrode  220  with soldering by the cross-section taken along the line III-III in  FIG. 1 .  FIG. 4  depicts a state subsequent to the state depicted in  FIG. 3 , that is the state in which the flexible printed circuit board  1  is completely pressed against the counterpart electrode  220  with a pulse heater H. 
     When the welding side land  120  of the flexible printed circuit board  1  is welded to the counterpart electrode  220  via soldering, first, the lower side opening (the opening defined on the lower surface of the insulation layer  11 ) of the through hole  140  of the flexible printed circuit board  1  is allowed to approach the upper part of the pre-solder  230  on the counterpart electrode  220  of the gage side conductor pattern  22 , and positional adjustment thereof is performed, as depicted in  FIG. 3 . Next, the pulse heater H is moved downward in a state that the lower surface (a heating surface) thereof is pressed against the upper surface (a contact surface) of the heating side land  130  of the flexible printed circuit board  1 . Pressing the pulse heater H against the upper surface of the heating side land  130  allows the heat of the pulse heater H to be conducted to the welding side land  120  of the flexible printed circuit board  1  via the rolled copper foil  141  covering the inner circumferential surface of the through hole  140 . Subsequently, when the pulse heater H is moved downward further, a part of the welding side land  120  around the lower side opening of the flexible printed circuit board  1  comes into contact with the pre-solder  230  and then it is pressed against thereto. This allows the heat from the pulse heater H to be conducted to the pre-solder  230  so as to melt the pre-solder  230 . 
     As depicted in  FIG. 4 , a part of melted solder  230 A moves upward through the through hole  140  such that the through hole  140  is filled with the melted solder without void or cavity. In this situation, air in a space defined by the upper surface of the pre-solder  230 , the lower surface of the pulse heater H, and the inner circumferential surface of the through hole  140  is allowed to escape to the outside of the heating side land  130  (released to the outside of the heating side land  130 ) via the groove  131  of the heating side land  130  as the melted solder  230 A moves upward. After the melted solder  230 A has reached the lower surface of the pulse heater H, an excess solder  230 B flows into the groove  131  of the heating side land  130  through the upper side opening (the opening defined on the upper surface of the insulation layer  11 ) of the thorough hole  140 . 
     Next, the pulse heater H is moved away from the heating side land  130  to cool the melted solder  230 A (including the excess solder  230 B flowed into the groove  131 ). Then, cooled solder is securely fixed to the welding side land  120 , the inner circumferential surface of the through hole  140 , and a part of or entire groove  131  of the heating side land  130 , of the flexible printed circuit board  1 , and the counterpart electrode  220  of the gage side conductor pattern  22 . This results in the reliable electrical connection between the welding side land  120  and the counterpart electrode  220  of the gage side conductor pattern  22 . 
     Conventionally, there has been the following problem. That is, air in the space defined by the upper surface of the pre-solder, the lower surface of the pulse heater, and the inner circumferential surface of the through hole remains therein during the welding of pre-solder by the aid of the heat from the pulse heater. Thus, a layer of air is formed between the solder and the counterpart electrode of the gage side conductor pattern and/or between the solder and the inner circumferential surface of the through hole. As a result, any failure of solder welding are caused, and consequently the conduction failure between the welding side land of one of the flexible printed circuit boards and the counterpart electrode of the other of the flexible printed circuit boards are caused. In this embodiment, however, since the flexible printed circuit board  1  has the above structure or configuration, the air in the space does not remain between the solder and the parts to which the solder is welded and the above problem can be avoided. 
     In addition to the above embodiment, as depicted in  FIGS. 7 and 8 , a portion of the thin wire pattern for electrical connection  125  (wiring) which is formed on the other surface of the flexible insulation layer  11  and which is connected to the welding side land  120  may not be covered with the insulation coating (resist)  129 , the portion having certain length and being defined adjacent to the welding side land  120 . The area which is not covered with the insulation coating  129  (the recess which is obtained by the lack of the insulation coating  129  and has the depth corresponding to the thickness of the insulation coating  129 ) may be used as a solder relief part (solder escape part)  132 . When such solder relief part  132  is provided, a part of excess solder flows into not only the groove  131  of the heating side land  130  of the flexible printed circuit board  1  as described above but also the solder relief part  132 , during the process in which the pre-solder  230  is melted by the pulse heater H. Accordingly, air is more reliably prevented from accumulating between the solder and the parts to which the solder is welded. 
       FIG. 5  is a plan view depicting an exemplary flexible printed circuit board  2  having a plurality of welding side lands  120  and a plurality of heating side lands  130 , the lands  120  and  130  corresponding to those depicted in  FIG. 1 . Although, the heating side lands  330  (each of which is numbered with  331 ,  332 ,  333 ,  334 ,  335 , and  336 ) having grooves, provided on the upper surface of the flexible printed circuit board  2  have the same shape as that of the heating side land  130  in the above embodiment, it is characteristic that the heating side lands  330  are disposed in a zigzag pattern along the width direction of the flexible printed circuit board  2  (the longitudinal direction in  FIG. 5 ). Further, welding side lands  320  (each of which is numbered with  321 ,  322 ,  323 ,  324 ,  325 , and  326 ), which are conductor patterns and each have a circular shape in plan view, are formed on the lower surface of the flexible printed circuit board  2  at positions corresponding to the heating side lands  330  respectively. That is, the welding side lands  320  are also disposed in the zigzag pattern along the width direction of the flexible printed circuit board  2  in a similar manner to the heating side lands  330 . 
     As clearly depicted in  FIG. 5 , the heating side lands  330  disposed in the zigzag pattern and thin wire patterns for electrical connection  320   a  corresponding the heating side lands  330  are formed such that the longitudinal direction (extending direction) of the grooves  131  formed in the heating side lands  330  is parallel with the extending direction of the thin wire patterns for electrical connection  320   a.  In the present teaching, it is preferred that adjacent heating side lands  330 , among the heating side lands  330  disposed in the zigzag pattern as depicted in  FIG. 5 , have grooves  131  arranged such that extension lines extending in the longitudinal directions of the grooves  131  do not overlap with each other. The reason thereof is as follows. If the extension lines extending in the longitudinal direction of the grooves  131  of adjacent heating side lands  330  overlap with each other, there is fear that the solder flowing from the outlets of the grooves  131  facing each other might harden, in a space between adjacent heating side lands  330 , in a state of sticking together at the time of the melting of solder. This could cause the short circuit in the heating side lands  300 . 
     Thin wire patterns for electrical connection  320   a  (each of which is numbered with  321   a,    322   a,    323   a,    324   a,    325   a,  and  326   a ) extend from the welding side lands  320  in the extending direction of the flexible printed circuit board  2  (the left-right direction in  FIG. 5 ). Since the welding side lands  320  are disposed in the zigzag pattern, the thin wire patterns for electrical connection  321   a,    322   a,  and  323   a,  which respectively extend from the welding side lands  321 ,  322 , and  323  disposed on the side of the front end of the flexible printed circuit board  2  (right side in  FIG. 5 ), run close to adjacent welding side lands  320  disposed on the side of the base end of the flexible printed circuit board  2  (the left side in  FIG. 5 ) or run between the welding side lands  324 ,  325 , and  326 , without being electrically connected to the welding side lands  324 ,  325 , and  326 . This configuration or arrangement of the heat side lands  330 , the welding side lands  320  and thin wire patterns for electrical connection  320   a  increases the package density of the flexible printed circuit board  2 , and thereby making it possible to reduce the width of the flexible printed circuit board  2  and consequently to downsize the flexible printed circuit board  2 . 
     The above advantages will become clearer through the comparison between the present teaching and the following conventional example. An exemplary flexible printed circuit board related to the conventional example is a flexible printed circuit board having comb-like electrode. This flexible printed circuit board is joined, with soldering via the comb-like electrode, to counterpart electrodes connected to a strain gage. 
     Each comb tooth of the comb-like electrode in this flexible printed circuit board has a protrusion shape which is thin, long, and narrow. Thus, this flexible printed circuit board related to the conventional example is configured to have lands formed in the comb tooth respectively, and the pitch between adjacent electrodes cannot be reduced unlike the embodiment of the present teaching in which the flexible printed circuit board  2  has the heating side lands  330  and the welding side lands  320  disposed in the zigzag pattern. Therefore, the conventional example still has the problem that the integration degree of the flexible printed circuit board cannot be improved. The flexible printed circuit board  2  related to the present teaching, however, has the above configuration, which makes it possible to improve the integration degree of the flexible printed circuit board and consequently to downsize the flexible printed circuit board while providing the same number of electrodes as the flexible printed circuit board related to the conventional example. 
     In addition to the above, since each tooth of the comb-like electrode has the protrusion shape which is thin, long, and narrow, each tooth is more likely to be bent or deformed. Thus, the comb-like electrode is required to be carefully handled so that no extra external force is applied on the comb-like electrode at the time of solder joint. The flexible printed circuit board related to the present teaching, however, does not have such a configuration, and thus handling thereof at the time of solder joint is much easier than the conventional example. 
     The present teaching is not limited to the above embodiment, and the action and effect of the present teaching can be also obtained through the following embodiments. For example, it is allowable to employ a heating side land  430  as depicted in  FIG. 6B . The heating side land  430  may have a circular or annular shape (doughnut shape) in which a hole  435  is formed in the center to communicate with the through hole  140 . A groove (recess)  431  may be formed in the heating side land  430  so that a part of the heating side land  430  is left in its depth direction. When the groove  431  is formed so that the depth of the groove  431  is smaller than the height of the heating side land  430 , a communicating hole is provided on the bottom of the groove  431  to let the groove  431  communicate with the through hole  140 . 
     Instead of using the heating side land  130 , of the above embodiment, which is formed of two semicircular plates and the groove  131  provided therebetween, a heating side land having only one semicircular plate may be used so that excess solder being melted is allowed to flow to an area excluding the semicircular plate of the heating side land. It is not indispensable to form the groove  131  over the entire area of the circular plate-shaped heating side land  130  in its radial direction, as in the above embodiment. The groove  131  may be formed only in an area which ranges from the upper side opening of the through hole  140  to a point on the outer circumference of the heating side land  130  (an area corresponding to a radius of the circular plate). In this case, the groove  131  may be formed so that the depth thereof reaches the bottom of the heating side land, like the above embodiment, or the groove  131  may be formed so that a part of the heating side land is left in its depth direction, like the modified embodiment. 
     As another modification, only one protrusion may be formed on the upper surface of the heating side land of the conventional type which has a perfectly circular or annular shape and has the same height in its circumferential direction. Alternatively, protrusions having the same height in its protruding direction may be formed on the upper surface of the heating side land in its circumferential direction. 
     Further, the height of the heating side land having the circular or annular shape may be periodically changed in its circumferential direction like a sine curve. Alternatively, steps having the same height may be periodically formed on the heating side land having the circular or annular shape in its circumferential direction. That is, the case, in which the heating side land includes a part having the perfectly circular or annular shape (that is, a circular or annular shape of which height (thickness) is constant), may be also included in the scope of the present teaching, provided that the upper surface of the heating side land against which the pulse heater is pressed has different heights, unlike the heating side land related to the conventional example. In other words, any heating side land having a shape or structure by which an air in a through hole can be released to the outside of the heating side land, when a pulse heater is pressed against the heating side land, is included in the scope of the present teaching. 
     As described above, the recess related to the present teaching includes the groove, which is formed to reach the bottom surface of the heating side land (which is formed to penetrate the upper and bottom surfaces of the heating side land) so that the circular plate-shaped heating side land is divided into two semicircular plates with a small space provided therebetween. Further, for example, when a plurality of cylindrical protrusions having the same height are provided around the upper side opening of the through hole, and each of the protrusions and the welding side land are thermally connected via a wiring extending from each protrusions to the through hole and a rolled copper foil in the through hole, the recess related to the present teaching also includes gaps defined between protrusions. That is, the recess related to the present teaching includes various forms. Further, the heating side land  130  may include a communication hole which connects the through hole  140  and the outer circumferential surface of the heating side land  130 . That is, a communication hole penetrating through a side wall of the heating side land  130  may be provided at a position lower than the top of the heating side land  130 . In this specification, the “recess” and the “communication hole” described above are collectively called “a channel which connects the through hole and the outside of the heating side land (a first land)”. 
     More specifically, as is clear from the above embodiment in the present description, the present teaching relates to the flexible printed circuit board including, a flexible insulation layer having a first surface and a second surface, a first land which is conductive and which is provided on the first surface of the flexible insulation layer, and a conductive member which is provided on the second surface of the flexible insulation layer, wherein a recess (groove) is formed on the first land. In this flexible printed circuit board, a through hole is formed in the flexible insulation layer, an edge of an opening, of the through hole, defined in the first surface of the flexible insulation layer is in contact with the first land, and the recess fluid-communicates with the through hole and an outer circumferential surface of the first land. The present teaching, however, is not limited to the above embodiment. As is clear from the modified embodiments in the present description, the present teaching also includes the embodiment or aspect in which the recess is provided at a part of the periphery of the opening of the through hole not to include the protrusion constituting a part of the land, the embodiment or aspect in which the protrusion has a substantially semicircular shape in plan view, and the embodiment or aspect in which the protrusions having the same height are formed around the opening of the through hole at a regular interval in the circumferential direction of the opening of the through hole. 
     In the above embodiment, the counterpart electrode to which the flexible printed circuit board is connected is the electrode of the strain gage. The present teaching, however, is not limited to this. It is needless to say that the electrode of the flexible printed circuit board related to the present teaching can be reliably welded, with soldering, to the counterpart electrode of any electric or electronic part. 
     Instead of providing the recess (groove) in the heating side land as described above, the following configuration may be adopted. That is, the heating side land may be formed to have the circular or annular shape in which the upper surface thereof has the same height, like the conventional example. In this case, the pulse heater may be provided with the recess (groove), which has a shape similar to the recess (groove) related to the above embodiment or the modified embodiments. In this configuration, melted solder is allowed to flow into the recess (groove) on the side of the pulse heater via the hole in the center of the circular-shaped heating side hand. 
     According to the embodiments of the present teaching, there can be provided the flexible printed circuit board having the electrode, which is connected or joined to the counterpart electrode via the solder joint with sufficient strength so as to provide satisfactory conductivity for a long time.