Abstract:
A method for fabricating a printed circuit board includes the steps of: fabricating a printed circuit board having at least one collapsed portion; depositing a first solder resist in the collapsed portion; exposing the first solder resist-coated printed circuit board at a pressure lower than atmospheric pressure for a predetermined time; coating a second solder resist on the entire surface of the printed circuit board; and drying and hardening the first and the second solder resists. With this method, when a solder resist is coated, since an air space does not remain in a blind via hole, the reliability of the attachment between a printed circuit board and the solder resist layer is increased.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printed circuit board, and more particularly to a method for fabricating a printed circuit board in which when a solder resist is coated, an air space is not allowed to remain in a blind via hole, thereby heightening the reliability of the attachment between the printed circuit board and a solder resist layer. 
     2. Description of the Background Art 
     FIG. 1 is a sectional view of a printed circuit board in accordance with a conventional art. 
     As shown in the drawing, a first conductive circuit pattern  3   a  is formed at the upper surface of a substrate  1 , and a first insulation resin layer  5   a  is formed at the upper surface of the first conductive circuit pattern  3   a . Via holes  7  are formed at the first insulation resin layer  5   a , through which the upper surface of the first conductive circuit pattern  3   a  is exposed. 
     A second conductive circuit pattern  3   b  is formed at the upper surface of the first insulation resin layer  5   a , and a first plated layer  9   a  is formed at the upper surface of the second conductive circuit pattern  3   b , at the inner wall face of the via hole  7  and at the upper surface of the first conductive circuit pattern  3   a  exposed through the via hole  7 . The second conductive circuit pattern  3   b  formed at the upper surface of the first insulation resin layer  5   a  and the first conductive circuit pattern  3   a  formed at the lower surface of the first insulation resin layer  5   a  are electrically connected. 
     A second insulation resin layer  5   b  is formed at the upper surface of the second plate layer  9   a  and at the upper surface of the first insulation resin layer  5   a.    
     Via holes  7  are formed at the upper surface of the second insulation resin layer  5   b  with the upper surface of the first plated layer  9   a  partially exposed. 
     A third conductive circuit pattern  3   c  is formed at the upper surface of the second insulation resin layer  5   b.    
     A second plated layer  9   b  is formed at the upper surface of the third conductive circuit pattern  3   c , the inner wall of the via hole  7  and the first plate layer  9   a.    
     The second conductive circuit pattern  3   b  and the third conductive circuit pattern  3   c  are electrically connected by the second plated layer  9   b.    
     A solder resist  11  is coated at the upper surface of the second plated layer  9   b  and of the second insulation resin layer  5   b . The exposed portion  12  of the second plated layer  9   b  which has not been coated with the solder resist  11  is connected with an outer lead of an electronic part. 
     The printed circuit board in the above described structure has the following problems. 
     In fabricating the printed circuit board, the following problems arise when the solder resist layer is finally coated at the upper surface of the plated layer. For example, the surface of the plated layer of the printed circuit board is rough, not level. In other words, the surface of the plated layer at the portion where the via holes are formed is concave compared with the other portion where a via hole is not formed. In the case where the solder resist layer is coated at the upper surface of the plated layer having a level portion and a collapsed portion, the solder resist does not entirely fill the collapsed portion, forming a void. The reason for this is that, when the solder resist is applied, the air in the via hole is not completely removed, whereby the air remaining in the via hole forms an air space. The air space prevents the solder resist from completely filling the via hole. 
     FIG. 2 illustrates a via hole  7 , a solder resist  11  and a void  20  formed in the via hole  7 . The same elements as in FIG. 1 are given the same reference numerals. In addition, a plated layer  9   b  is formed at the surface of the inner wall at the bottom surface of the via hole  7  and at the upper surface of the conductive circuit pattern  3   c . The air fills the void  20  within the via hole  7 , and the solder resist  11  covers the upper surface of the void  20 . 
     The void formed in the via hole  7  as shown in FIG. 2 causes the following problems. When the printed circuit board is exposed to a high temperature process such as in the case where parts are mounted on the printed circuit board, the air present in the void  20 , results in a rupturing of the solder resist  11  which covers the void  20 . Then, the solder resist  11  falls out and the plated layer  9   b  and the conductive circuit pattern  3   c  also break down, degrading the reliability of the printed circuit board. 
     SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a method for fabricating a reliable printed circuit board. 
     Another object of the present invention is to provide a method for fabricating a printed circuit board in which an air space does not result in a portion of a printed circuit board collapsing, thereby improving the adhesion reliability between the printed circuit board and a solder resist. 
     To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for fabricating a printed circuit board including the steps of: fabricating a printed circuit board having at least one collapsed portion; depositing a first solder resist in the collapsed portion; exposing the first solder resist-coated printed circuit board at a pressure lower than atmospheric pressure for a predetermined time; coating a second solder resist on the entire surface of the printed circuit board; and drying and hardening the first and the second solder resists. 
     To achieve the above objects, the method for fabricating a printed circuit board further comprises a step of mounting a window screen having a window larger than the collapsed portion at a position corresponding to the collapsed portion on the upper surface of the printed circuit board, before the step of coating the first solder resist. 
     To achieve the above objects, in the step of exposing the printed circuit board to a pressure lower than atmospheric pressure in the method for fabricating a printed circuit board, the pressure is about {fraction (1/10)} of atmospheric pressure. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification, illustrate embodiments of the invention and together with the description serve to explain the principles of the invention. 
     In the drawings: 
     FIG. 1 is a sectional view of a printed circuit board in accordance with a conventional art; 
     FIG. 2 is an enlarged view of a collapsed portion coated with the solder resist of FIG. 1 in accordance with the conventional art; 
     FIGS. 3A through 3F are vertical sectional views of the printed circuit board illustrating sequential fabricating processes of the printed circuit board in accordance with the present invention; 
     FIGS. 4A through 4D are enlarged view of the collapsed portion and its vicinity of the circled portions A, B, C and D of the FIGS. 3C through 3F in accordance with the present invention; and 
     FIGS. 5A and 5C are alternatives of FIGS. 4A and 4C in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. 
     FIG. 2 is an enlarged view of a collapsed portion coated with the solder resist of FIG.  1 . 
     The present invention is directed to a method for filling a resin layer in a collapsed portion without a void. With reference to FIG. 2, the ‘collapsed portion’ is defined as follows. The collapsed portion is a portion of the surface of the printed circuit board which is dented due to a via hole formed to electrically connect the upper and lower layer conductive patterns or a through hole formed to penetrate the printed circuit board. 
     FIGS. 3A through 3F are vertical sectional views of the printed circuit board illustrating sequential fabricating processes of the printed circuit board, and FIGS. 4A through 4D are enlarged views of the collapsed portion in the vicinity of the circled portions A, B, C and D of the FIGS. 3C through 3F in accordance with the present invention. 
     With reference to FIG. 3A, a printed circuit board ( 30 ) is fabricated without a coated solder resist layer. The printed circuit board of FIG. 3A is fabricated by adopting the conventionally known method, detailed descriptions of which are thus omitted. 
     The printed circuit board includes a resin layer  31 , a plurality of via holes  32  formed penetrating the upper and the lower surface of the resin layer, a conductive circuit pattern  33  formed at the upper surface of the resin layer  31 , and a plated layer  34  plated at the upper surface of the conductive circuit pattern  33  and at the inner wall face and bottom surface of the via hole  32 . Accordingly, in the case of this printed circuit board, since the via hole  32  is collapsed, it becomes the collapsed portion. 
     Next, as shown in FIG. 3B, a window screen  40  having a plurality of windows  41  formed penetrating its upper and lower faces is mounted at the upper surface of the printed circuit board  30 . The window  41  of the window screen  40  is formed to correspond to the collapsed portion  32  of the printed circuit board  30 , and is larger than the entrance of the collapsed portion  32 . 
     Referring to the formation of the window  41  of the window screen  40 , oil is coated on a textile with uniform holes formed, and in this state, oil on a portion having a larger diameter than the upper portion of the collapsed portion  32  is removed through a typical exposure and development method, thereby forming the window  41  at the position corresponding to the collapsed portion  32 . 
     Next, as shown in FIGS. 3C and 4A, the first solder resist  35  is applied to the window  41  and the collapsed portion  32  of the printed circuit board. At this time, since the collapsed portion  32  is filled with air, the solder resist  35  does not totally fill the collapsed portion  32 , forming a void  36  in the collapsed portion  32  covered with the first solder resist  35 . FIG. 4A shows the enlarged view of the portion indicated by a circle ‘A’, representing the portion around the collapsed portion  32 , of FIG.  3 C. 
     The printed circuit board  30  of FIG. 3C is then put in a vacuum chamber and exposed to a vacuum atmosphere. At this time, the pressure in the vacuum chamber is set to be about {fraction (1/10)} of atmospheric pressure P o , and the time for exposing the printed circuit board is set to be approximately 1-2 minutes. 
     When the printed circuit board  30  is exposed to the vacuum atmosphere, since the internal pressure within the void  36  is in a state of atmospheric pressure while the outer side is in a state of being pressured below atmospheric pressure, the air within the void  36  breaks the first solder resist  35  causing it to flow out into the vacuum environment, as shown in FIGS. 3D and 4B. FIG. 4B is an enlarged view of the portion indicated as a circle ‘B’ of FIG.  3 D. 
     Consequently, as shown in FIGS. 3E and 4C, the first solder resist  35 , which is in a half-hardened state, flows into the void  36  where the air has vacated, completely filling the collapsed portion  32 . FIG. 4C is an enlarged view of the portion indicated by a circle ‘C’ of FIG.  3 E. 
     Then, as shown in FIGS. 3F and 4D, the window screen  40  is removed, and a second solder resist  37  is coated on the entire upper surface of the solder resist  35  of the plated layer  34  and the collapsed portion  32  at atmospheric pressure. FIG. 4D is an enlarged view of the portion indicated by a circle ‘D’ of FIG.  3 F. 
     In this manner, the coating process of the solder resist is performed twice, whereby the air space is prevented from forming in the collapsed portion of the printed circuit board and thus a printed circuit board having an even surface can be obtained. As a result, the attachment reliability between the printed circuit board and the solder resist is heightened. 
     After the solder resist layer  37  is coated, the resulting structure is subject to drying and hardening, thereby completing the fabrication of the printed circuit board. 
     Referring to the method as shown in FIGS. 4A and 4D, the window  41  of the window screen  40 , used for coating the solder resist  35 , is formed larger than the entrance of the collapsed portion  32 , and the collapsed portion  32  is entirely exposed through the window  41 . 
     Alternatively, as shown in FIGS. 5A and 5C, a window screen where the window is formed only at the perimeter of the collapsed portion  32  and the central part of the collapsed portion  32  that shields the window screen, may be used. 
     Namely, with reference to FIG. 5A, the window screen is installed in a manner that the central part of the collapsed portion  32  is shielded by the window screen  40   a  and the outer periphery of the entrance of the collapsed portion  32  is exposed through the window  41   a . In this case, when the solder resist  35  is coated, as shown in FIG. 5B, the solder resist  35  is coated only at the side wall of the collapsed portion  32  while the central part is not coated. In this state, where the printed circuit board is put in the vacuum chamber to expose it to the vacuum atmosphere, when the air flows out of the collapsed portion  32 , the solder resist  35  is not broken. After the air flows out, as shown in FIG. 5C, the half-hardened solder resist  35  fills the inside of the collapsed portion  38 . 
     As so far described, the method for fabricating a printed circuit board according to the present invention has the following advantage. That is, since the air space having a different heat characteristic to that of the material of the printed circuit board is prevented from occurring, the attachment reliability between the printed circuit board and the solder resist can be improved. Namely, the solder resist will not wear off or break down during the process of coating the solder resist or during the use of the printed circuit board coated with the solder resist, so that the quality of a product adopting the printed circuit board can be highly improved. 
     As the present invention may be embodied in several forms without departing from the spirit or essential characteristics thereof, it should also be understood that the above-described embodiments are not limited by any of the details of the foregoing description, unless otherwise specified, but rather should be construed broadly within its spirit and scope as defined in the appended claims, and therefore all changes and modifications that fall within the meets and bounds of the claims, or equivalence of such meets and bounds are therefore intended to be embraced by the appended claims.