The present invention relates to a printed circuit board and a method of manufacturing same, and more particularly, to a printed circuit board provided with a conductor pattern having superior adhesion and a method of manufacturing same.
A prior art printed circuit board 9 is shown in FIG. 5(A). The printed circuit board 9 includes an insulative substrate 2, a plurality of conductor patterns 93, and an insulative protection film 94 coating the substrate 2 and the patterns 93. The conductor patterns 93 function as wiring and external terminals. The bottom surfaces 931 of the conductor patterns 93 are adhered to the upper surface of the substrate 2. The width a of the bottom surface 931 is equal to the width b of the top surface 932 of the conductor pattern 93. In other words, the conductor patterns 93 have rectangular cross-sections. The side surfaces 935 of the conductor patterns 93 are completely coated by the protection film 94. The conductor patterns 93 are formed after forming the protection film 94 on the substrate 2.
Since the conventional conductor patterns 93 have a rectangular cross-section in which the width a of the bottom surface 931 is equal to the width b of the top surface 932, the area of contact between the conductor patterns 93 and the substrate 2 is relatively small. Hence, the adhesion between the substrate 2 and the conductor patterns 93 is relatively weak. Accordingly, as shown in FIG. 5(B), when an external force, such as thermal stress, is applied to the printed circuit board 9, the conductor patterns 93 may be separated from the upper surface of the substrate 2.
To solve this problem, as shown in FIG. 6(A), the conductor patterns 93 may have a trapezoid cross-section in which the width a of the bottom surface 931 is greater than the width b of the top surface 932. However, in this case, the material forming the conductor patterns 93 forces the protection film 94, which is shaped in correspondence with the conductor patters 93, upward. Thus, as shown in FIG. 6(B), the protection film 94 may be separated from the upper surface of the substrate 2. Further, as shown in FIG. 6(C), during formation of the conductor patterns 93, a portion 939 of the conductor patterns 93 may enter the space between the bottom surface of the protection film 94 and the upper surface of the substrate 2 causing a short-circuit with the adjacent conductor pattern 93.
Further, as shown in FIG. 7(A), in the conventional printed circuit board 9, the side surfaces 935 of the conductor pattern 93 are completely coated by the protection film 94. Thus, for example, when a connection terminal plating 95 and a solder ball 96 are joined to the conductor pattern 93, the solder ball 96 is riot hooked to the side surfaces 935 of the conductor pattern 93. Accordingly, if a lateral force, such as that indicated by an arrow, is applied to the solder ball 96, the solder ball 96 may be separated from the conductor pattern 93.
To solve this problem, as shown in FIG. 7(B), the side surfaces 935 of the conductor pattern 93 may be completely exposed to allow the solder ball 96 to enter the space between the side surfaces 935 of the conductor pattern 93 and the protection film 94. However, in this case, the solder ball 96 contacts the substrate 2, which mechanical strength is relatively weak. Due to the surface tension of the solder ball, the area of the contact portion between the solder ball 96 and the substrate 2 is relatively small. Therefore, when a lateral force, such as that indicated by the arrow, is applied to the solder ball 96, the force is locally applied to the contact portion of the substrate 2. As a result, a crack 99 may be produced in the substrate 2, and the substrate 2 may be damaged.
It is an object of the present invention to provide a printed circuit board and its manufacturing method that provides strong adhesion between a substrate and a conductor pattern and prevents damage of the substrate.
To achieve the above object, a first aspect of the present invention provides a printed circuit board including a substrate, a conductor pattern formed on the substrate, and a protection film coating the substrate and the conductor pattern. The conductor pattern includes a bottom surface contacting the substrate, a top surface opposite to the bottom surface, and a pair of side surfaces. Each of the side surfaces have a lower side surface covered by the protection film and an upper side surface exposed from the protection film. The width of the bottom surface is greater than the width of the top surface.
It is preferred that the width of the bottom surface of the conductor pattern be greater than the width of the top surface, the lower side surface be coated by the protection film, and the upper side surface be exposed from the protection film.
In the printed circuit board according to a first aspect of the present invention, the width of the bottom surface of the conductor pattern is greater than the width of the top surface, and the area of the portion adhered to the substrate is relatively large. Accordingly, in comparison with a conductor pattern having a rectangular cross-section, the adhesion between the substrate and the conductor pattern is stronger. Thus, separation of the conductor pattern from the upper surface of the substrate is prevented.
The upper side surfaces of the conductor pattern are exposed from the protection film. Thus, when joining, for example, a solder ball, to the conductor pattern, the solder ball enters the upper side surfaces of the conductor pattern. Thus, even if a lateral force is applied to the solder ball subsequent to the joining, the solder ball has a portion hooked to the side surfaces thereby preventing the solder ball from being separated from the conductor pattern.
Further, the lower side surface of the conductor pattern is coated by the protection film. Accordingly, for example, the solder ball contacts the conductor pattern, which mechanical strength is greater than that of the substrate, without contacting the substrate. Thus, even if a lateral force is applied to the solder ball subsequent to the joining, the force acts on the conductor pattern and not on the substrate thereby preventing damages, such as cracking, of the substrate.
It is preferred that the top surface of the conductor pattern be coated by a plating. In this case, the solder ball is easily joined to the conductor pattern.
It is preferred that the solder ball contact the conductor pattern at the upper side surfaces of the conductor pattern. In this case, even if a lateral force is applied to the joined solder ball, the solder ball is in contact with the side surfaces of the conductor pattern and thus separation of the solder ball from the conductor pattern is prevented.
A second aspect of the present invention provides a method for fabricating a printed circuit board. The method includes the steps of etching a substrate including a conductor to form a conductor pattern, applying an insulative protection film to the conductor pattern and the substrate, and removing part of the protection film. In the etching step, the conductor pattern is formed so that a width of a bottom surface contacting the substrate is greater than a width of a top surface, which is opposite the bottom surface. In the etching step, an upper portion of the conductor pattern is exposed.
The conductor pattern is formed before the protection film. Accordingly, the protection film does not exist when the conductor pattern is formed. Thus, the conductor pattern does not force the protection film upward. Further, part of the conductor pattern does not enter the space between the bottom surface of the protection film and the top surface of the substrate. As a result, short-circuiting between adjacent conductor patterns is prevented.