Patent Publication Number: US-9894774-B2

Title: Printed circuit board, method of manufacturing the same and connection terminal

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. patent application Ser. No. 13/674,323, filed Nov. 12, 2012, which claims priority to U.S. Provisional Patent Application No. 61/601,266 filed Feb. 21, 2012, the disclosures of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to a printed circuit board, a method of manufacturing the same and a connection terminal. 
     (2) Description of Related Art 
     Actuators are used in drives such as hard disc drives. Such an actuator includes an arm provided rotatably around a rotation shaft, and a suspension board with a circuit for a magnetic head that is attached to the arm. The suspension board with the circuit is a printed circuit board for positioning the magnetic head at a desired track of a magnetic disc. 
     Signal lines and connection terminals are formed on the suspension board with the circuit. A head slider including the magnetic head is attached on the connection terminals of the suspension board with the circuit. The magnetic head is electrically connected to another electronic circuit through the connection terminals and the signal lines on the suspension board with the circuit. 
     The connection terminals are manufactured by etching described below. (See, for example, JP 8-283966 A.) Resists are formed on both sides of a metal substrate. Then, a desired resist pattern is formed by selective exposure, development and drying, etc. Thereafter, part of the metal substrate that is exposed on the resist pattern is etched. Finally, the resist pattern is removed from the metal substrate after the completion of the etching, whereby a product with the desired pattern (connection terminals in this example) is obtained. 
     BRIEF SUMMARY OF THE INVENTION 
     When rectangular connection terminals are formed by the etching described above, roundness is generated on the corners of the connection terminals. In this case, the bonding areas of the connection terminals are reduced when an electronic component such as a head slider and the connection terminals are bonded to each other by a solder. In recent years, printed circuit boards have been made finer, and thus, the bond strength of the connection terminals and the other electronic components weakens since the bonding areas of the connection terminal are reduced. As a result, the connection reliability of the connection terminals deteriorates. 
     An object of the present invention is to provide a printed circuit board having a connection terminal in which connection reliability can be secured and which can be made finer, a method of manufacturing the printed circuit board, and such a connection terminal. 
     (1) According to an aspect of the present invention, a printed circuit board includes an insulating layer, a wiring trace formed on the insulating layer and a connection terminal made of conductor that is electronically connected to the wiring trace, wherein the connection terminal has at least one corner with a radius of curvature of not larger than 35 μm. 
     In the printed circuit board, the connection terminal made of conductor is electronically connected to the wiring trace on the insulating layer. The radius of curvature of at least one corner of the connection terminal is not larger than 35 μm. In this case, the corner is hardly rounded. Therefore, the reduction of the bonding area due to the roundness of the corner can be suppressed. As a result, the connection reliability of the connection terminal can be secured and the suspension board can be made finer. 
     (2) The conductor may include stainless steel and the radius of curvature of the at least one corner of the connection terminal may be not larger than 30 μm. In this case, the reduction of the bonding area of the connection terminal including stainless steel can be more sufficiently suppressed. 
     (3) The conductor may include copper and the radius of curvature of the at least one corner of the connection terminal may be not larger than 10 μm. In this case, the reduction of the bonding area of the connection terminal including the copper can be more sufficiently suppressed. 
     (4) The connection terminal may have first and second sides parallel to each other and a third side orthogonal to the first and second sides, and at least one corner includes a first corner that is formed by the first and third sides, and a second corner that is formed by the second and third sides. 
     In this case, the first and second corners are hardly rounded. Therefore, the reduction of the bonding area of the connection terminal can be further sufficiently suppressed. 
     (5) The width between the first and second sides may be not larger than 70 μm. In this case, the suspension board can be sufficiently made finer while the reduction of the bonding area of the connection terminal is suppressed. 
     (6) The connection terminal may be configured to be electrically connectable to a magnetic head and transmittable of an electric signal between the magnetic head and an electronic circuit through the wiring trace and the connection terminal. 
     In this case, the connection terminal and the magnetic head are electrically connected and the electric signal is transmitted between the magnetic head and the electronic circuit through the printed circuit board. Because the reduction of the bonding area of the connection terminal is suppressed, a weakening of bond strength of the connection terminal and the magnetic head is suppressed. As a result, the connection reliability between the connection terminal and the magnetic head can be improved. 
     (7) According to another aspect of the present invention, a method of manufacturing a printed circuit board includes the steps of forming a conductor trace that has a side extending in one direction on an insulating layer, forming a first etching resist on the insulating layer that has an edge crossing the side of the conductor trace and covers part of the conductor trace, and forming a connection terminal that has at least one corner by removing a portion of the conductor trace exposed on the first etching resist by etching. 
     In the method of manufacturing the printed circuit board, the conductor trace that has the side extending in one direction is formed on the insulating layer. The first etching resist that has the edge crossing the side of the conductor trace and covers part of the conductor trace is formed on the insulating layer. A portion of the conductor trace that is exposed on the first etching resist is removed by etching. As a result, the side that extends in another direction crossing the one direction mentioned above is formed on the conductor trace, and the connection terminal that has at least one corner is formed. 
     In this case, the corner is hardly rounded. Therefore, the reduction of the bonding area of the connection terminal due to the roundness of the corner can be suppressed. As a result, the printed circuit board can be made finer while the connection reliability of the connection terminal can be secured. 
     (8) A radius of curvature of the at least one corner of the connection terminal may be not larger than 35 μm. In this case, the reduction of the bonding area of the connection terminal due to the roundness of the corner can be suppressed. As a result, the printed circuit board can be made finer while the connection reliability of the connection terminal can be secured. 
     (9) The step of forming the conductor trace may include the steps of forming a second etching resist on the insulating layer that has an edge extending in the one direction to cover part of a conductor layer formed on the insulating layer, and forming the conductor trace by removing a portion of the conductor layer exposed on the second etching resist by etching. 
     In this case, the conductor trace that has the side extending in the one direction is formed by etching the conductor layer using the second etching resist, and the connection terminal that has at least one corner is formed by etching the conductor trace using the first etching resist. Thus, the connection terminal with hardly any roundness on its corner can easily be formed by two-time etching. 
     (10) The step of forming the conductor trace may include the step of forming the conductor trace that has the first and second sides parallel to each other, and the step of forming the first etching resist on the insulating layer may include the step of forming the first etching resist on the insulating layer that has an edge orthogonal to the first and second sides, the step of forming the connection terminal that has the at least one corner may include the step of forming the connection terminal that has a third side orthogonal to the first and second sides by removing a portion of the conductor trace exposed on the first etching resist by etching, and the at least one corner of the connection terminal may include a first corner formed by the first and third sides, and a second corner formed by the second and third sides. 
     In this case, the print circuit board that includes the connection terminal having the first and second corners is manufactured. The first and second corners of the connection terminal are hardly rounded. Therefore, the reduction of the bonding area of the connection terminal can be further sufficiently suppressed. 
     (11) According to a further aspect of the present invention, a connection terminal that is to be electrically connected to a wiring trace to transmit the electrical signal, comprising at least one corner having a radius of curvature of not larger than 35 μm. 
     In this connection terminal, the radius of curvature of at least one corner is not larger than 35 μm. In this case, the corner is hardly rounded. Therefore, the reduction of the bonding area of the connection terminal can be suppressed. As a result, the connection terminal can be made finer while the connection reliability of the connection terminal can be secured. 
     According to the present invention, it is possible to make the printed circuit board finer while securing the connection reliability of the connection terminal. 
     Other features, elements, characteristics, and advantages of the present invention will become more apparent from the following description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         FIG. 1  is a top view of a suspension board according to a first embodiment of the present invention; 
         FIGS. 2 ( a )  and  2  ( b ) are cross-sectional views of the suspension board illustrated in  FIG. 1 ; 
         FIGS. 3 ( a ) to 3( c )  are diagrams showing the steps of manufacturing the suspension board illustrated in  FIG. 1 ; 
         FIGS. 4 ( a ) to 4( d )  are diagrams showing the steps of manufacturing the suspension board illustrated in  FIG. 1 ; 
         FIGS. 5 ( a ) to 5( c )  are diagrams showing the steps of manufacturing the suspension board illustrated in  FIG. 1 ; 
         FIG. 6  is an enlarged bottom view showing a tongue of a suspension board and its periphery; 
         FIGS. 7 ( a ) to 7( c )  are diagrams showing the steps of manufacturing a suspension board according to a second embodiment; 
         FIG. 8  is a bottom view showing one step of manufacturing a suspension board according to a third embodiment; and 
         FIGS. 9 ( a ) and 9( b )  are bottom views showing one step of manufacturing a suspension board according to a fourth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     [1] First Embodiment 
     A printed circuit board with connection terminals according to the first embodiment of the present invention and a method of manufacturing the same will be described below with reference to the drawings. A suspension board used for an actuator in a hard disc drive as the printed circuit board according to the first embodiment of the present invention will be described below. 
     (1) Configuration of Suspension Board 
       FIG. 1  is a top view of the suspension board according to the first embodiment of the present invention. As shown in  FIG. 1 , a suspension board  1  includes a suspension body  100  formed of a metallic and elongate support substrate. Write wiring traces W 1  and W 2  and read wiring traces R 1  and R 2 , as shown by thick dotted lines, are formed on the suspension body  100 . The write wiring trace W 1  and the write wiring trace W 2  constitute a signal line pair. The read wiring trace R 1  and the read wiring trace R 2  constitute a signal line pair. 
     At one end of the suspension body  100 , a magnetic head supporting portion (hereinafter referred to as a tongue)  12  is provided by forming a U-shaped opening  11 . The tongue  12  is bent along a broken line R to form a predetermined angle with respect to the suspension body  100 . On the lower surface of the tongue  12  at one end of the suspension body  100 , four connection terminals  21 ,  22 ,  23  and  24  are formed. A head slider that includes a magnetic head is attached to the connection terminals  21  to  24  on the lower surface of the tongue  12 . 
     In the present embodiment, each of the connection terminals  21  to  24  has a rectangular shape. The width of each of the connection terminals  21  to  24  (the width between sides L 1  and L 2  of  FIG. 6 , described later) is preferably not larger than 70 μm. In this case, the suspension board  1  can be made finer and smaller. 
     Four connection terminals  31 ,  32 ,  33  and  34  are formed on the upper surface of the other end of the suspension body  100 . An electronic circuit such as a preamplifier is connected to the connection terminals  31  to  34 . The connection terminals  21  to  24  of the tongue  12  and the connection terminals  31  to  34  of the other end of the suspension body  100  are electrically connected by the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2 , respectively. Further, a plurality of holes H are formed in the suspension body  100 . 
     The suspension board  1  is provided in a hard disc device. An electric current flows through the pair of the write wiring traces W 1  and W 2  at the time of writing information into a magnetic disc. Further, the electric current flows through the pair of the read wiring traces R 1  and R 2  at the time of reading information from the magnetic disc. 
     Next, the connection terminals  21  to  24  of the suspension board  1  and its peripheral portions will be described in detail below.  FIG. 2  is a cross-sectional view of the suspension board  1  of  FIG. 1 .  FIG. 2 ( a )  shows a cross sectional view taken along the line A-A of the suspension board  1  of  FIG. 1 , and  FIG. 2 ( b )  shows a cross sectional view taken along the line B-B of the suspension board  1  of  FIG. 1 . 
     As shown in  FIG. 2 ( a ) , an insulating layer  41  made of polyimide, for example, is formed on a metallic support substrate  10  made of stainless steel, for example. The write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  are spaced apart from and parallel to each other on the insulating layer  41 . The write wiring traces W 1  and W 2  extend along one lateral side of the insulating layer  41  and the read wiring traces R 1  and R 2  extend along the other lateral side of the insulating layer  41 . A cover layer  43  made of polyimide, for example, is formed on the insulating layer  41  to cover the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2 . 
     The write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  that extend along the one lateral side and the other lateral side of the insulating layer  41  bend inward at one end of the suspension body  100  of  FIG. 1 , and bend towards the tongue  12 , and then extend to the tongue  12  as shown in  FIG. 2 ( b ) . The write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  on the tongue  12  are respectively connected to the connection terminals  21  to  24  on the lower surface of the tongue  12 . 
     (2) Method of Manufacturing the Suspension Board 
     A method of manufacturing the suspension board  1  of  FIG. 1  will be described below.  FIGS. 3 to 5  are schematic diagrams showing the steps of manufacturing the suspension board  1  of  FIG. 1 . In  FIGS. 3 ( a )  to  3  ( c ), cross sectional views taken along the line B-B of the suspension board  1  are shown in the left and top views of the tongue  12  of the suspension board  1  of  FIG. 1  and its periphery are shown in the right. In  FIGS. 4 ( a )  to  5  ( c ), cross sectional views taken along the line B-B of the suspension board  1  of  FIG. 1  are shown in the left and bottom views of the tongue  12  and its periphery of the suspension board  1  of  FIG. 1  are shown in the right. The same hatched and dotted patterns applied to the members of the cross sectional views are applied to the members of the top view of  FIGS. 3 ( a )  to  3  ( c ) and the bottom views of  FIGS. 4 and 5  in order to facilitate understanding of the configuration. 
     First, as shown in  FIG. 3 ( a ) , the insulating layer  41  made of polyimide is formed on the support substrate  10  made of stainless steel. The thickness of the support substrate  10  is not less than 10 μm nor more than 50 μm, for example. The thickness of the insulating layer  41  is not less than 5 μm nor more than 15 μm, for example. Here, the insulating layer  41  is formed in the same shape as the suspension board  1  of  FIG. 1 . Further, a plurality of openings  41   h  (four in the example illustrated in  FIG. 3 ( a ) ) are formed in the insulating layer  41 . As a result, part of the support substrate  10  is exposed in the openings  41   h.    
     Then, as shown in  FIG. 3 ( b ) , the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  having predetermined patterns are formed on the insulating layer  41  and the support substrate  10  exposed in the openings  41   h . The thickness of each of the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  is not less than 6 μm nor more than 18 μm, for example. Further, each of the width of the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2  is not less than 8 μm nor more than 50 μm, for example. Moreover, each of the spacing between the write wiring traces W 1  and W 2  and the spacing between the read wiring traces R 1  and R 2  is not less than 8 μm nor more than 100 μm, for example. 
     Next, as shown in  FIG. 3 ( c ) , a cover layer  43  made of polyimide is formed on the insulating layer  41  to cover the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2 . The thickness of the cover layer  43  is not less than 2 μm nor more than 10 μm, for example. 
     Then, as shown in  FIG. 4 ( a ) , a resist film  18  is formed using a photosensitive dry film resist, for example, on the lower surface of the support substrate  10 . Next, as shown in  FIG. 4 ( b ) , the resist film  18  is exposed in a predetermined pattern, and then developed by using a development liquid such as sodium carbonate whereby an etching resist  18   a  is formed. Here, the etching resist  18   a  has a plurality of (four in the example illustrated in  FIG. 4 ( b ) ) rectangular insular parts IR in a rectangular opening OP. Each insular part IR has edges  18   b  extending in one direction. 
     Thereafter, as shown in  FIG. 4 ( c ) , the support substrate  10  is etched using ferric chloride solution and cupric chloride solution as etching solution, whereby a plurality of conductor traces  10   a ,  10   b ,  10   c  and  10   d  (four in the example illustrated in  FIG. 4 ( c ) ) made of stainless steel are formed on the lower surface of the insulating layer  41 . The conductor traces  10   a  to  10   d  are electrically connected to the write wiring traces W 1  and W 2  and the read wiring traces R 1  and R 2 , respectively, and separated from one another. 
     Each of conductor traces  10   a  to  10   d  has a substantially rectangular shape. The four corners of each insular part IR of the etching resist  18   a  are slightly rounded and the etching solution penetrates beneath the four corners of the etching resist  18   a . Therefore, it is difficult to form each corner of the conductor traces  10   a  to  10   d  so as to have a radius of curvature of not larger than 35 μm. 
     Then, after removing the etching resist  18   a , as shown in  FIG. 4 ( d ) , a resist film  19  is formed using the photosensitive dry film resist, for example, on the lower surfaces of the conductor traces  10   a  to  10   d  and the support substrate  10 . Next, as shown in  FIG. 5 ( a ) , the resist film  19  is exposed in a predetermined pattern, and then developed using the development liquid such as sodium carbonate, whereby the etching resist  19   a  is formed. 
     Here, the etching resist  19   a  has a rectangular opening OP 1  that includes the corners on one end of each of the conductor traces  10   a  to  10   d , and a rectangular opening OP 2  that includes the corners on the other end of each of the conductor traces  10   a  to  10   d . One side of each of the openings OP 1  and OP 2  crosses (i.e., in the example of  FIG. 5 ( a ) , is orthogonal to) each of the lateral sides of the conductor traces  10   a  to  10   d . This causes both ends of the conductor traces  10   a  to  10   d  in a longitudinal direction to be exposed on the etching resist  19   a . Further, the other part of the etching resist  19   a  is formed in the same shape as the suspension board  1  of  FIG. 1 . 
     Next, the conductor traces  10   a  to  10   d  and the support substrate  10  are etched using the ferric chloride solution and the cupric chloride solution as the etching solution. Here, as shown in  FIG. 5 ( b ) , both ends in the longitudinal direction of the conductor traces  10   a  to  10   d  are removed. Thereafter, as shown in  FIG. 5 ( c ) , the etching resist  19   a  is removed and then the rectangular connection terminals  21  to  24  are formed. As a result, the suspension board  1  is completed. 
     In this case, in the step illustrated in  FIG. 5 ( b ) , the linear edges  19   b  of the openings OP 1  and OP 2  are orthogonal to the lateral sides of the conductor traces  10   a  to  10   d . Further, the etching solution penetrates along the linear edges  19   b  of the openings OP 1  and OP 2  of the etching resist  19   a . Therefore, the radius of curvature of each corner of the connection terminals  21  to  24  can be not larger than 35 μm. When the connection terminals  21  to  24  are made of stainless steel, the radius of curvature of each corner of each of the connection terminals  21  to  24  is preferably not larger than 30 μm and more preferably 0 μm. 
     (3) Effect 
       FIG. 6  is an enlarged bottom view showing the tongue  12  of the suspension board  1  and its periphery. Because the insular parts IR of the etching resist  18   a  of  FIG. 4 ( b )  have the edges  18   b  arranged in one direction, each of the connection terminals  21  to  24  has two sides L 1  and L 2  parallel to each other as shown in  FIG. 6 . Further, because the openings OP 1  and OP 2  of the etching resist  19   a  of  FIG. 5 ( a )  have the edges  19   b  extending in the direction orthogonal to the one direction, each of the connection terminals  21  to  24  has two sides L 3  and L 4  orthogonal to the sides L 1  and L 2  as shown in  FIG. 6 . 
     A corner A 1  is formed between the sides L 1  and L 3 , a corner A 2  is formed between the sides L 2  and L 3 , a corner A 3  is formed between the sides L 1  and L 4 , and a corner A 4  is formed between the sides L 2  and L 4 . The radius of curvature of each corner A 1  to A 4  is formed to be not larger than 35 μm. That is, each corner A 1  to A 4  is hardly rounded. Therefore, the reduction of the bonding area of the connection terminals  21  to  24  due to the roundness of the corners A 1  to A 4  can be suppressed. As a result, this enables the suspension board  1  to be made finer while securing the connection reliability of the connection terminals  21  to  24 . 
     Furthermore, as can be understood from other embodiments described later, because the radius of curvature of the corners A 1  to A 4  is reduced, the maximum plastic strain of the connection terminals  21  to  24  becomes smaller when stress is applied to the connection terminals  21  to  24 . 
     [2] Second Embodiment 
     While the connection terminals  21  to  24  are formed to have the radius of curvature of not larger than 35 μm in the first embodiment, the invention is not limited to this. The connection terminals  21  to  24  may be formed such that the radius of curvature of part of the corners is not larger than 35 μm. 
       FIG. 7  is a schematic diagram showing the steps of manufacturing the suspension board according to the second embodiment. The method of manufacturing suspension board  1  according to the present embodiment has the steps similar to those of the method of manufacturing the suspension board  1  illustrated in  FIGS. 3 ( a )  to  4  ( d ) according to the first embodiment. Description will be made of the method of manufacturing the suspension board  1  according to the present embodiment by referring to differences from the method of manufacturing the suspension board  1  according to the first embodiment will be described. 
     Following the step illustrated in  FIG. 4 ( d ) , as shown in  FIG. 7 ( a ) , the etching resist  19   a  having a predetermined pattern is formed by processing the resist film  19 . Here, the etching resist  19   a  has the rectangular opening OP 2  that includes the corners on one end of each of the conductor traces  10   a  to  10   d . The edges  19   b  of the opening OP 2  cross (i.e., in the example of  FIG. 7 ( a ) , is orthogonal to) each of the lateral side of the conductor traces  10   a  to  10   d . This causes one ends of the conductor traces  10   a  to  10   d  in the longitudinal direction to be exposed on the etching resist  19   a.    
     Then, the conductor traces  10   a  to  10   d  and the support substrate  10  are etched. Here, as shown in  FIG. 7 ( b ) , one ends of the conductor traces  10   a  to  10   d  in the longitudinal direction are removed. Thereafter, the etching resist  19   a  is removed, thereby forming the rectangular connection terminals  21  to  24 , as shown in  FIG. 7 ( c ) . As a result, the suspension board  1  is completed. In this case, the radius of curvature of the corners on the one end side of each of the connection terminals  21  to  24  (the corners A 1  and A 2  of  FIG. 6 ) can be made to be not larger than 35 μm. 
     [3] Third Embodiment 
     While in the step illustrated in  FIG. 4 ( b )  according to the first embodiment, the insular parts IR are formed in a rectangular shape, the invention is not limited to this. The insular parts IR may be formed in another shape that has the edges  18   b  extending in one direction.  FIG. 8  is a bottom view showing the method of manufacturing the suspension board according to the third embodiment. 
     In the present embodiment, instead of the step illustrated in  FIG. 4 ( b )  in the first embodiment, the etching resist  18   a  that has a plurality of (four in the example illustrated in  FIG. 8 ) insular parts IR in the rectangular opening OP is formed as shown in  FIG. 8 . Here, the insular parts IR have the edges  18   b  extending in the one direction, and both ends of the insular parts IR in a longitudinal direction have semicircular shape. As a result, in the step illustrated in  FIG. 4 ( c ) , the conductor traces  10   a  to  10   d  with both ends having the semicircular shape in the longitudinal direction are formed. 
     Thereafter, similarly to the steps illustrated in  FIGS. 4 ( d )  to  5  ( c ), both ends of the conductor traces  10   a  to  10   d  in the longitudinal direction are removed, thereby forming the rectangular connection terminals  21  to  24  that have the corners with their radius of curvature of not larger than 35 μm. 
     [4] Fourth Embodiment 
     While in the step illustrated in  FIG. 5 ( a )  in the first embodiment, the edges  19   b  of the etching resist  19   a  are formed to be orthogonal to each lateral side of each of the conductor traces  10   a  to  10   d , the invention is not limited to this.  FIG. 9  is a bottom view showing one step of manufacturing the suspension board according to the fourth embodiment. 
     In the present embodiment, as shown in  FIG. 9 ( a ) , the edges  19   b  of the etching resist  19   a  are formed to cross each of the lateral sides of the conductor traces  10   a  to  10   d  at a desired angle different from a right angle. In this case, both ends of the conductor traces  10   a  to  10   d  in the longitudinal direction are removed and then the etching resist  19   a  is removed, so that the connection terminals  21  to  24  with an arbitrary quadrangle can be formed as shown in  FIG. 9 ( b ) . 
     [5] Other Embodiments 
     (1) While in the above-mentioned embodiment, the connection terminals  21  to  24  are made of stainless steel, the invention is not limited to this. The connection terminals  21  to  24  may be made of another metal such as copper, gold (Au), silver or aluminum, or alloy such as copper alloy, gold alloy, silver alloy or aluminum alloy. When the connection terminals  21  to  24  are made of copper, the radius of curvature of each corner of each of the connection terminals  21  to  24  is preferably 15 μm, and more preferably not larger than 10 μm and still more preferably 0 μm. 
     (2) While in the above-mentioned embodiment, all of the connection terminals  21  to  24  are formed to have the corners with the radius of curvature of not larger than 35 μm, the invention is not limited to this. Part of the connection terminals  21  to  24  may be formed to have the corners with a radius of curvature of not larger than 35 μm. 
     (3) While in the above-mentioned embodiment, the conductor traces  10   a  to  10   d  are formed by etching using the etching resist  18   a , the invention is not limited to this. The conductor traces  10   a  to  10   d  may be formed by other methods such as a semi-additive method or the like. 
     (4) While in the above-mentioned embodiment, the connection terminals  21  to  24  are formed on the lower surface of the suspension board  1 , the invention is not limited to this. The connection terminals  21  to  24  may be formed on the upper surface of the suspension board  1 . 
     (5) In the suspension board  1  of  FIG. 1 , part or all of the connection terminals  31  to  34  may be manufactured similarly to the method of manufacturing the connection terminals  21  to  24  in the first to fourth embodiments. Further, the connection terminals  31  to  34  may be formed on the lower or upper surface of the suspension board  1 . 
     [6] Correspondences Between Elements in the Claims and Parts in Embodiments 
     In the following paragraphs, non-limiting examples of correspondences between various elements recited in the claims below and those described above with respect to various preferred embodiments of the present invention are explained. 
     The insulating layer  41  is an example of an insulating layer, the read wiring traces R 1  and R 2  or the write wiring traces W 1  and W 2  are examples of a wiring trace, the connection terminals  21  to  24  are examples of a connection terminal, the corners A 1  to A 4  are examples of a corner, the suspension board  1  is an example of a printed circuit board, the sides L 1  to L 3  are examples of first to third sides respectively, the corners A 1  and A 2  are examples of first and second corners respectively, and the etching resists  19   a  and  18   a  are examples of first and second etching resists respectively. 
     As each of various elements recited in the claims, various other elements having configurations or functions described in the claims can be also used. 
     [7] Inventive Examples 
     In inventive examples 1 to 4 and a comparative example 1 described below, the maximum plastic strain was evaluated by simulation after stress was applied to a plurality of connection terminals with the corners that were made of stainless steel and had different radii of curvature. The results are shown in Table 1. Here, the connection terminals according to the inventive examples 1 to 4 and the comparative example 1 had a 50 μm-wide squareshape. Further, the radii of curvature of the corners of the connection terminals according to the inventive examples 1 to 4 and the comparative example 1 were respectively 0 μm, 10 μm, 20 μm, 30 μm and 50 μm. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 RADIUS OF 
                 MAXIMUM 
               
               
                   
                 CURVATURE OF 
                 PLASTIC 
               
               
                   
                 CORNER 
                 STRAIN 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                   
                 INVENTIVE 
                  0 μm 
                 0.0092 
               
               
                   
                 EXAMPLE 1 
               
               
                   
                 INVENTIVE 
                 10 μm 
                 0.0093 
               
               
                   
                 EXAMPLE 2 
               
               
                   
                 INVENTIVE 
                 20 μm 
                 0.0093 
               
               
                   
                 EXAMPLE 3 
               
               
                   
                 INVENTIVE 
                 30 μm 
                 0.0124 
               
               
                   
                 EXAMPLE 4 
               
               
                   
                 COMPARATIVE 
                 50 μm 
                 0.0163 
               
               
                   
                 EXAMPLE 1 
               
               
                   
                   
               
            
           
         
       
     
     As shown in Table 1, the maximum plastic strains of the connection terminals according to the inventive examples 1 to 3 were respectively 0.0092, 0.0093 and 0.0093. Further, the maximum plastic strain of the connection terminals according to the inventive example 4 was 0.0124. On the other hand, the maximum plastic strain of the connection terminals according to the comparative example 1 was 0.0163. 
     From the results in the inventive examples 1 to 4 and the comparative example 1, it was confirmed that when the connection terminals were made of stainless steel and had a 50 μm-wide squareshape, the increase in the maximum plastic strain was suppressed by setting the radius of curvature of the corners to be not larger than 30 μm. In particular, it was confirmed that the increase in the maximum plastic strain was largely suppressed by setting the radius of curvature of the corners to be not larger than 20 μm. 
     While preferred embodiments of the present invention have been described above, it is to be understood that variations and modifications will be apparent to those skilled in the art without departing the scope and spirit of the present invention. The scope of the present invention, therefore, is to be determined solely by the following claims.