1. Field of the Invention
The present invention relates to a printed wiring board (PWB) having a land shape which prevents solder bridges from occurring between neighboring lands when electronic parts are soldered by dipping into melting solder, and this invention has advantage in the number of wiring pattern between the lands.
2. Description of the Related Art
There is a dip soldering method for inserting electronic parts' leads into through holes provided on the PWB. The PWB is dipped into melting solder and electrically connected to the electronic parts' leads by solder.
Typically, the dip soldering has a problem that solder bridges easily occur so that solder ranges between the neighboring electronic parts' leads and it takes much time and cost to remove the bridges.
There are conventional techniques for preventing solder bridges from occurring between neighboring lands. One of them, there is a method for alternately arranging standard lands 70 each having a through hole 72 at its center and longitudinal lands 80 each having a through hole 82 at a deviated position from its center, and leading solder from the standard land 70 into the longitudinal land 80 in an arrow direction of numeral 78 as shown in FIG. 8 (see Japanese Patent Application Laid-Open No. 11-68298).
As another conventional technique, there is a method for forming lands 90, each having a through hole 92 at its center, in a rectangular shape (such as rhomboid) and arranging the lands 90 such that a long diagonal of the land matches with a wave solder carrying direction (in an arrow direction of numeral 95), thereby preventing solder bridges from occurring between neighboring lands as shown in FIG. 9 (see Registered Japanese Utility Model No. 3003062). Since the solder flows in an arrow direction of numeral 93 along the lands 90, the bridges easily break. The wave soldering is one of the dip soldering methods. The wave soldering uses a solder wave in a melting solder bath. The solder wave is brought into contact with a PWB, when the PWB is moved over the solder bath.
As still another conventional technique, there is a method for preventing solder bridges from occurring between neighboring lands by forming solder resist openings or lands in a protruded shape as shown in FIGS. 10A and 10B (see Japanese Patent Application Laid-Open No. 5-304353), or forming the same in a tapered and cutout shape as shown in FIG. 11A (see Japanese Patent Application Laid-Open No. 7-254774).
In FIGS. 10A and 10B, protrusions 66 are provided in the neighboring lands 60, and in FIG. 11A, tapers 106a, 106c and cutouts 106b, 106d are provided in the lands. As shown in FIG. 10A, the protrusions 66, 66 are provided in the lands 60, 60. As shown in FIG. 10B, an electrode pin 65 as electronic parts is inserted into a through hole 62. Then, the PWB 2 is dipped in the solder bath to be soldered to the lands 60. The protrusions 66 are provided so that an angle formed between solder 63 and the land 60 becomes smaller, and the solder bridges are prevented.
The above techniques are used for preventing solder bridges from occurring between neighboring lands, but have the following problems.
In the method shown in FIG. 8, longitudinal lands are needed and the area for mounting increases, which has disadvantage in achieving high component mounting density.
In the method shown in FIG. 9, since the carrying direction of PWB and the arrangement of the lands always forms correct angle, there is a problem that the arrangement direction of the parts is fixed relative to the carrying direction.
In the method shown in FIGS. 10A and 10B, the protrusions 66 make the gap between the lands 60 narrow, and thus it has disadvantage in the number of wiring patterns between the lands.
In the method shown in FIGS. 11A and 11B, the diagonal of the solder resist opening matches with the arrangement direction of lands. The ends of the square are furthest from the center and a gap between the neighboring solder resist openings is narrow. As shown in FIG. 11B, the conductive gaps 119 tends to be narrower, because they are defined by 4 factors (the accuracy of drill machining, the accuracy of solder resist, an amount of cutouts of solder resist, the amount of over-resist), and thus it has disadvantage in the number of wiring patterns between the lands.