PRINTED WIRING BOARD

Abstract: A printed wiring board includes an insulated base plate (1), a circuit pattern at least on one surface of the base plate (1), and solder resist film (6) containing silicon series and/or fluorine series resin and covering the base plate (1) except desired soldering area of the circuit pattern. Such solder resist film (6) has desired property to prevent bridging of solder across adjacent circuits. However, the solder repelling property apts to form dots on the solder resist film (6). Accordingly, at least one flux accumulating means is formed on said one surface of the base plate (1) to accumulate flux repelled by the solder resist film. The flux accumulating means may be recess and/or through hole means arranged on the surface of the solder resist film.

Description: Technical field to which the invention relates The present invention relates to a printed wiring board. More particularly, the invention relates to a printed wiring board including an insulated base plate, a circuit pattern at least on one surface of the base plate, and solder resist film containing silicon series and/or fluorine series resin and covering the base plate except desired soldering area of the circuit pattern. Description of the prior art In general, a printed wiring board is formed by forming necessary circuit pattern on an insulated base plate by electric conductor, and assembling necessary electronic parts with the circuit pattern. Assembling the electronic parts is completed such that, after the electronic parts are mounted on the printed wiring board, the circuit pattern surface of the printed wiring board is dipped in a melted solder bath or a jet stream solder bath to attach the solder on the circuit pattern surface to electrically and mechanically connect the circuit pattern with leads of the electronic parts. To perform the soldering the circuit pattern with the leads of the electronic parts, previously all the surface of the circuit pattern is covered by solder-resist except the land portions to be soldered, to attach solder only on the necessary portion on the circuit pattern. As the circuit pattern is smaller, distance between adjacent circuits is also very narrow and distance between connection lands is narrow, to result in bridging between the lands by soldering, to invalid the function of the solder-resist. This results in repair operation afterwards. To prevent the bridging of the solder, especially to prevent the bridging of solder between narrow distance land portions, an improved printed wiring board manufacturing process is disclosed such that, electrically conductive pattern is printed on the insulated base plate, all the surface of the conductive pattern is covered by first layer of solder resist layer except land portions to be soldered, and second layer of bridge resisting solder resist layer is covered on the first layer at least between narrow distance land portions to prevent bridging of solder. The process is disclosed in Japanese Patent Publications No. 41162/1979. However, the process must be performed by forming the first layer of the solder resist layer, and forming the second layer of the bridge resisting solder resist layer. As the first layer of the solder resist layer covers all the surface of the conductive pattern except land portions to be soldered, highly acculate alignment accuracy is required to expose the land portions to be soldered. Further as the second bridge resisting solder resist layer is formed on the first solder resist layer after the first solder resist layer is formed, another highly acculate alignment accuracy is required, and also solder resist ink of screen printing to form solder resist layer apts to spread on the land portions to be soldered. Accordingly, the applicant of the present invention disclosed in Japanese Patent Application No. 37647/1987 to mitigate the above mentioned disadvantages of the known printed wiring board, an improved printed wiring board having solder-resist layer having necessary solder resist function without adding any complication to technical operations of conventional solder resist covering. More particularly, the solder resist film on the printed wiring board has positively preventing function from adhering flux or solder on the surface by characteristics of silicone and/or fluorine series resin contained in the film. Further, when the film is formed by solder resist printing ink on another surface of a base plate having circuit pattern on one surface, invasion of flux or solder through through holes or parts insert holes formed in the printed wiring board is also positively prevented. Fig. 1 is an enlarged sectional view of a portion of the printed wiring board having the above described solder resist film. In the drawing, 1 designates an insulated base plate, 2 is a circuit pattern formed by electrically conductive copper film on one surface 1a of the insulated base plate, 3 is a connection land in the circuit pattern 2, 4 is a through hole opened at the connection land, 6 is a solder resist film coated on the surface of the circuit pattern 2, and 7 is solder resist film coated on the other surface 1b of the insulated base plate 1. The solder resist film 6 covers all the surface of the circuit pattern 2 except the connection land 3, and is formed with the solder resist film 7 by means e.g. silk screen printing process utilising printing ink containing insulating silicone or fluorine series resin which can prevent flux or solder from adhering on the surface. Examples of formulation of the solder resist printing ink which is used to form the solder resist films 6 and 7 are as follows: In the above mentioned formulations, substantial market obtainable examples of the silicon series and fluorine series resins are follows: Silicon series resin Shinetu Chemical Ind. Co. Ltd. Japan Polon L, Polon T, KF 96, KS-700, KS-701, KS-707, KS-705F, KS 706, KS-709, KS-709S, KA-711, KSX-712, KS-62F, KS-62M, KS-64, Silicolube G-430, Silicolube G-540, Silicolube G-541. Tore Silicon Co. Ltd. Japan SH-200, SH-210, SH-1109, SH-3109, SH-3107, SH-8011, FS-1265, Syli-off 3, DC pan Glaze 620. Fluorine series resin Daikin Ind. Co. Ltd. Japan Dai-free MS-443, MS-543, MS-743, MS-043, ME-413, ME-810. Sumitomo 3-M Co. Ltd. Japan Fulorard FC-93, FC-95, FC-98, FC-129, FC-134, FC-430, FC-431, FC-721. Sumitomo Chemical Ind. Co. Ltd. Japan Sumi-fulunon FP-81, FP-81R, FP-82, FP-84C, FP-84R, FP-88. Seibi Chemical Ind. Co. Ltd. Japan Surflon SR-100, SR-100X. In the above described formulations, silicon series or fluorine series high molecular resin was solely formulated. However, silicon series and fluorine series high molecular resins may be both formulated, and the quantity is preferably within 2 - 5 wt. part, to attain desired effect. To increase the formulated quantity, economical problem may accompanies, however, the functional effect may be expected. Further, as the conventional solder resist ink has contact angle about 60 - 70 degree, contact angle of the solder resist ink of the above described formulations is more than 90 degree. The printed wiring board of such construction has on both sides of the insulated base plate 1, solder resist films 6 and 7 which are formed by coating the printing ink containing the silicon and/or fluorine series resin. Thus, according to the characteristics of the silicon and fluorine series resin, flux or solder cannot wet the film so that adherence of the flux or solder outside the electric connection portions e.g. the connection land 3, is prevented positively. Consequently, bridging between adjacent lands caused by high density circuit pattern 2 can be prevented. The solder resist films 6 and 7 can be formed by means e.g. silk screen printing process which is similar with the conventional process to form conventional solder resist film so that it is very advantageous. As the other surface 1b of the insulated base plate 1 which has the circuit pattern 2 on one side surface 1a, also forms the solder resist film 7 so that invasion of the flux or solder from the through hole 4 formed in the printed wiring board is effectively prevented. In the above mentioned embodiment, one surface 1a of the insulated base plate 1 forms circuit pattern 2, however, circuit patterns may be formed on both surfaces of the base plate 1. Further, the solder resist film 6 only may cover the circuit pattern 2 on one side surface 1a of the insulated base plate 1 as desired. Also, in the embodiment shown, the solder resist films 6 and 7 cover nearly all surfaces of the insulated base plate 1, however, the solder resist film may cover locally as desired e.g. high density circuit portions and/or portions about the through hole 4. The above mentioned printed wiring board has solder resist film which itself has property of preventing wetting by flux or solder, and is effective to prevent bridging between adjacent circuits or terminals while operations of soldering to mount electronic parts, or connection between circuit terminals on the printed wiring board. Thus, production accuracy is improved, bridge repair work is eliminated, and workability is improved. However, the printed wiring board having the above mentioned silicon and/or fluorine series resin solder resist film has disadvantage such that when flux is covered on the solder resist film, the flux is adhered on the solder resist film as dots so that production accuracy and external appearance is impaired. Technical Problem The object of the present invention is to mitigate the above mentioned disadvantage of the printed wiring board having solder resist film of silicon and/or fluorine series resin containing material, and to provide improved printed wiring board which effectively prevent dots of flux on the solder resist film from producing. Solution of the Technical Problem A printed wiring board includes an insulated base plate, a circuit pattern at least on one surface of the base plate, and solder resist film containing silicon series and/or fluorine series resin and covering the base plate except desired soldering area of the circuit pattern. The printed wiring board includes, according to the present invention, at least one flux accumulating means formed on the one surface of the base plate to accumulate flux repelled by the solder resist film. Preferably, the flux accumulating means includes at least one flux accumulating recess formed on the one surface of the base plate and is not covered by the solder resist film. Preferably, the flux accumulating means includes at least one flux accumulating through hole formed through the base plate. The flux accumulating through hole may be formed concentric with the flux accumulating recess. Preferably, the flux accumulating recess further includes flux absorbing layer on the recess. When the other surface of the insulated base plate also has the solder resist film, the lower edge of the through hole may simply pass through the solder resist film, or concentric flux accumulating recess may be formed on the other surface. Advantageous Effects of the Invention As the flux accumulating means accumulates flux repelled by the silicon series and/or fluorine series resin contained solder resist film so that dots producing problem is solved and desired quality and accuracy are maintained. Brief Description of the Drawings Figure 1 is an enlarged sectional view of a portion of known printed wiring board, Figure 2 is an enlarged sectional view of a portion of first embodiment of the printed wiring board, according to the present invention, Figure 3 is an enlarged sectional view of a portion of second embodiment of the printed wiring board, Figure 4 is an enlarged sectional view of a portion of third embodiment of the printed wiring board, Figure 5 is an enlarged sectional view of a portion of fourth embodiment of the printed wiring board, Figure 6 is an enlarged sectional view of a portion of fifth embodiment of the printed wiring board, and, Figure 7, 8 and 9 are enlarged sectional views each of a portion of modification of the printed wiring board, according to the present invention. Description of the Preferred Embodiments Fig. 2 is an enlarged sectional view of a portion of first embodiment of the printed wiring board according to the present invention. When a solder resist film 6 is formed by silk screen printing process on a circuit pattern 2 on an upper surface 1a of an insulated plate 1, a recess 8 for accumulate flux is formed without covering by the solder resist film 6. The recess 8, according to the present invention, may be only one as the embodiment shown. Preferably, a plurality of recesses 8 are arranged on the film 6 to accumulate flux which is repelled by the repelling function of the film 6, to prevent spots of the flux on the surface 1a. Other construction is similar with the printed wiring board shown in Fig. 1 so that same reference numeral is used to represent same or similar part or portion. The printed wiring board shown in Fig. 2 attains functional effect of the before mentioned printed wiring board shown in Fig. 1, and also has advantage such that the flux covered on the surface 1a of the insulated base plate 1 is repelled by the solder resist film 6 and is accumulated in the flux accumulation recess 8. Thus, the flux does not form dots on the solder resist film 6. Accordingly, disadvantage of unevenness of the surface of the printed wiring board caused by dots of flux is eliminated, appearance of the printed wiring board is improved, and desired printed wiring board having improved quality and accuracy is provided. Fig. 3 shows second embodiment of the printed wiring board according to the present invention. As shown in Fig. 3, a flux accumulating through hole 9 is formed through the insulated base plate 1 at desired position of the solder resist film 6. As before, a plurality of the through holes 9 may be preferably formed relating to the area of the solder resist film 6 to acertain to flow out the flux to prevent dots on the solder resist film 6. Fig. 4 shows third embodiment of the printed wiring board according to the present invention. As shown in Fig. 4, a flux accumulating recess 8 is formed on the upper surface 1a of the insulated plate 1 without being covered by solder resist ink, and is similar with that shown in Fig. 2. Further, a flux accumulating through hole 9 is formed concentric with the recess 8, and the through hole 9 is similar with that shown in Fig. 3. By combining the constructions shown in Figs. 2 and 3, flux repelled from the surface of the solder resist film 6 is accumulated into the recess 8 and flows off from the through hole 9. Fig. 5 shows fourth embodiment of the printed wiring board according to the present invention. As shown in Fig. 5, the printed wiring board is finished in addition to the solder resist film 6 on the upper surface of the insulated plate 1, solder resist film 60 preferably same formulation with the film 6, covers the rear surface 1b of the insulated plate 1. According to the invention, a flux accumulating recess 8a is formed without being covered by the solder resist film 6, and a flux receiving through hole 9 is formed concentric with the recess 8a both generally similar with that shown in Fig. 4. Further, a flux accumulating recess 8b is formed on rear side surface 1b of the insulated plate 1, concentric with the through hole 9, and without being covered by the solder resist film 60. Thus, flux on both surfaces 1a and 1b on the solder resist films 6 and 60 of the insulated plate is easily accumulated in the recesses 8a and 8b and flows out through the through hole 9. Fig. 6 shows fifth embodiment of the printed wiring board according to the present invention. As shown in Fig. 6, a flux accumulating recess 8 is formed on the upper surface of the insulated plate 1 without being covered by the solder resist film 6 generally similar with that shown in Fig. 2. Further, a flux absorbing layer 10 covers the recess 8. The flux absorbing layer 10 is preferably formed by painting carbon material. The flux absorbing layer 10 positively absorbs flux so that flux repelled from the solder resist film 6 is easily and positively accumulated by the layer 10. Fig. 7 shows a modification of the printed wiring board according to the present invention. As shown in Fig. 7, the printed wiring board has solder resist films 6 and 61 on both surfaces 1a and 1b of the insulated base plate 1. In this case, as shown in Fig. 3, a plurality of flux accumulating through holes 9 are formed through the base plate 1 and solder resist films 6 and 61. The flux accumulating function of the through hole 9 is similar with that described in Fig. 3. Fig. 8 shows another modification of the printed circuit board according to the present invention. As shown in Fig. 8, the printed circuit board has solder resist films 6 and 62 on both surfaces 1a and 1b of the insulated base plate 1, as before. In this case, at least one flux accumulating through hole 9 is formed in the insulated plate 1 as described in Fig. 7. Further, concentric with the flux accumulating through hole 9, flux accumulating recess 8 is formed on the other side surface 1b of the insulating plate 1 without being covered by solder resist film 62. Flux accumulating function is improved compared with that shown in Fig. 7, as the flux on the lower surface tends to spread. Also, the flux accumulated recess 8 can be formed as desired without considering the space of circuit pattern. Fig. 9 shows further modification of the printed wiring board according to the present invention. As shown in Fig. 9, the printed wiring board has solder resist films 6 and 61 on both surfaces 1a and 1b of the insulated plate 1, and includes at least one flux accumulating recess 8 without being covered by the solder resist film 6, and flux accumulating through hole 9 concentric with the recess 8 shown in Fig. 4. The flux accumulating function is generally similar with that shown in Fig. 4. The printed wiring board shown in Figs. 2 to 9 attains functional effect of the before mentioned printed wiring board shown in Fig. 1, and also has advantage such that the flux covered on the surface 1a of the insulated base plate 1 is repelled by the solder resist film 6 and is accumulated in the flux accumulating recess 8 and/or flux accumulating through hole 9. Thus, the flux does not form dots on the solder resist film 6. Accordingly, disadvantage of unevenness of the surface of the printed wiring board caused by dots of flux is eliminated, appearance of the printed wiring board is improved, and desired printed wiring board having improved quality and accuracy is provided.

Claims: A printed wiring board including an insulated base plate (1), a circuit pattern at least on one surface of the base plate (1), and solder resist film (6) containing silicon series and/or fluorine series resin and covering the base plate (1) except desired soldering area of the circuit pattern (2), wherein the board further comprises at least one flux accumulating means formed on said one surface of the base plate (1) to accumulate flux repelled by the solder resist film (6). A printed wiring board as set forth in claim 1, characterized in that said flux accumulating means includes at least one flux accumulating recess (8) formed on said one surface and is not covered by said solder resist film (6). A printed wiring board as set forth in claim 1, characterized in that said flux accumulating means includes at least one flux accumulating through hole (4) formed through said base plate (1). A printed wiring board as set forth in claim 2, characterized in that said flux accumulating means further includes a flux accumulating through hole (9) formed through said base plate (1) concentric with said flux accumulating recess. A printed wiring board as set forth in claim 2, characterized in that said flux accumulating means further includes flux absorbing layer means covering said flux accumulating recess. A printed wiring board as set forth in claim 4, characterized in that said base plate (1) having said solder resist film on each surface, in which said flux accumulating means includes at least one flux accumulating recess concentric with said through hole and without being covered by said solder resist film. A printed wiring board as set forth in claim 3, characterized in that said base plate (1) having said solder resist film on each surface, in which said flux accumulating through hole passes through both solder resist films. A printed wiring board as set forth in claim 3, characterized in that said base plate (1) having said solder resist film on each surface, in which said flux accumulating means further includes a flux accumulating recess concentric with each flux accumulating through hole and without being covered by said solder resist film. A printed wiring board as set forth in claim 4, characterized in that said base plate (1) having said solder resist film on each surface, in which said flux accumulating through hole passes through solder resist film on other surface. A printed wiring board as set forth in claim 1, characterized in that said flux accumulating means includes at least two sorts of, flux accumulating through hole through said base plate, flux accumulating recess formed on said one surface and without being covered by said solder resist film, flux accumulating recess formed on said one surface of the base plate and covered by flux absorbing layer, and flux accumulating recess formed on said one surface and having concentric flux accumulating through hole.

Inventors: HARUYAMA, SATOSHI, KAWAKAMI, SHIN, MACHIDA, HIDEO, MUKAI, NORITO, NIKAIDO, KATUTOMO, OKONOGI, HIROTAKA
Applicants: NIPPON CMK CORP.
Application year: 1987
Main classification: H05K3
IPC codes: H05K3