The present invention relates to an apparatus for the fabrication of a printed circuit board, and more particularly to a carrier plate positioning device for the fabrication of a printed circuit board. As shown in FIG. 1, the prior art printed circuit board (PCB) fabrication apparatus 10 is composed of an operation panel 100, a PC chassis 11, a power supply 20, and a plurality of processing equipments 11a, 11b, and 11c disposed within the PC chassis 11. As shown in FIG. 2, the PC chassis 11 is composed of a base 15 and a pair of side boards 14 and 15, disposed at the bottom side of the base 15. The power supply 20 is disposed on the base 15 and is electrically connected to the processing equipments 11a, 11b, and 11c. The processing equipments 11a, 11b, and 11c are arranged in a row along an Y-axis direction, as shown in FIG. 2, and are disposed at the front side of the base 15 of the PC chassis 11 and are electrically connected to the power supply 20 via cables 22. Each processing equipment 11a, 11b, and 11c is composed of a carrier plate 12 and a plurality of operating tools 11d. As shown in FIG. 3, the carrier plate 12 includes a main board 12a and a plurality of positioning seats 12b, and the plurality of operating tools 11d are disposed above the main board 12a. Each operating tool 11d is composed of a head portion 11e and a support seat 11f. The PC chassis 11 further includes a base plate 18 and a plurality of carrier plates 18a. The base plate 18 is mounted on the rear side of the PC chassis 11, and the carrier plates 18a are disposed on the base plate 18. As shown in FIG. 4, the base plate 18 includes a main board 18a and a plurality of supporting seats 18b, and each carrier plate 18a is disposed on the main board 18a. The PC chassis 11 further includes a tool tray 16. The tool tray 16 is disposed below the base plate 18 and is parallel to the base plate 18. As shown in FIGS. 4 and 5, the base plate 18 includes a base plate locating portion 18c. Each carrier plate 18a includes a carrier plate locating portion 18b. The carrier plate locating portion 18b is parallel to the base plate locating portion 18c. A pair of guide holes 11g, 11h are formed on each carrier plate locating portion 18b and a pair of positioning holes 13a, 13b are formed on each carrier plate 12. The positioning holes 13a, 13b are aligned with each other. The guide holes 11g, 11h are parallel to the positioning holes 13a, 13b. When assembling the carrier plates 12, the carrier plates 12 are placed in a tool tray 16, and each guide hole 11g and the corresponding positioning hole 13a are aligned with each other. The operator pushes each carrier plate 12 to the corresponding base plate locating portion 18c with the positioning holes 13a aligned with the guide holes 11g, thus enabling the carrier plate 12 to be mounted onto the base plate locating portion 18c. When the carrier plates 12 are mounted onto the base plate locating portion 18c, the carrier plates 12 are shifted by a distance of d along the Y-axis direction. Referring to FIG. 2 again, each processing equipment 11a, 11b, and 11c further includes a PC board transfer device 11i. As shown in FIG. 6, the PC board transfer device 11i includes a support arm 21 and a positioning device 30. The support arm 21 is mounted on the bottom side of the base 15. The support arm 21 is composed of a U-shaped main frame 22 and a positioning shaft 23. The main frame 22 is located on the base 15 and has a hole portion 22a disposed at the center thereof. The main frame 22 includes two side walls 221 and a lower wall 222, and the positioning shaft 23 is mounted inside the main frame 22 and extends out of the base 15. The lower wall 222 is spaced away from the Y-axis line. The positioning shaft 23 includes a first support wall 231, a second support wall 232, a positioning shaft rotating shaft 234, and a rotating shaft axle 235. The positioning shaft rotating shaft 234 is mounted inside the lower wall 222, and the rotating shaft axle 235 is mounted on the positioning shaft rotating shaft 234. The rotating shaft axle 235 is inserted into the hole portion 22a of the main frame 22 and is movable along the Y-axis direction. The first support wall 231 is located on the left side of the lower wall 222, and the positioning shaft rotating shaft 234 is mounted on the first support wall 231. The second support wall 232 is located on the right side of the lower wall 222 and faces the first support wall 231. A pair of rollers 241 are mounted on the right and left ends of the second support wall 232, and the positioning shaft rotating shaft 234 is mounted on the pair of rollers 241. As shown in FIG. 7, each roller 241 includes a gear wheel 24 formed on its end surface and a pair of springs 25 formed on the right and left side thereof and located on the right and left side thereof. The springs 25 are located inside the main frame 22. The roller 241 is rotatable along the rotating shaft axle 235. As shown in FIG. 6 again, each positioning tool 11d includes a locating portion 21d and a support shaft 27. The locating portion 21d has a hole portion 21c located at the center thereof. The support shaft 27 is mounted inside the locating portion 21d and includes a rotating shaft 27a, a rotating shaft axle 27b, and a pair of guides 27c. As shown in FIG. 7, the rotating shaft 27a is disposed inside the locating portion 21d and is mounted on the first support wall 231 of the positioning arm 21, and the rotating shaft 27a is moved upwardly or downwardly along the rotating shaft axle 27b. The rotating shaft 27a is aligned with the guide holes 11g and 11h of the carrier plates 12. The rotating shaft 27a is rotated around the rotating shaft axle 27b and is parallel to the first support wall 231 of the positioning arm 21. The rotating shaft 27a is operated by a motor (not shown) and driven by a first rotating shaft. The rotating shaft 27a of each positioning tool 11d is used to locate the carrier plate 12. In order to correctly locate the carrier plate 12 at the correct position on the base plate 12, the second support wall 232 of the positioning arm 21 and the first support wall 231 of the positioning arm 21 are required to be located on the Y-axis line during the locating operation. However, if the second support wall 232 is located on the Y-axis line, the carrier plate 12 will collide with the second support wall 232 and be damaged when rotating the rotating shaft 27a. Accordingly, as shown in FIG. 6, the rotating shaft 27a is separated from the second support wall 232. At this moment, the locating portion 21d is fixed at a specific location on the Y-axis line, such that the carrier plate 12 is capable of moving along the Y-axis line with respect to the locating portion 21d. At the same time, the positioning device 30 is used to perform a locating operation. The locating portion 21d of each positioning tool 11d is located at the base plate 18. When the locating portion 21d is located at the base plate 18, the guide hole 11g of the carrier plate 12 is aligned with the guide hole 11h of the carrier plate 12, as shown in FIG. 7. At this moment, the rotating shaft 27a of each positioning tool 11d is moved upwardly or downwardly along the rotating shaft axle 27b of the locating portion 21d. The rotating shaft 27a drives the support shaft 27. The rotating shaft 27a and the support shaft 27 are aligned with the guide hole 11g and the guide hole 11h of the carrier plate 12, respectively. In order to locate the carrier plate 12 correctly, the locating portion 21d is positioned at the center of the positioning hole 13a of the carrier plate 12 by using the positioning device 30. When the locating portion 21d is positioned at the center of the positioning hole 13a of the carrier plate 12, the guide hole 11g of the carrier plate 12 is aligned with the guide hole 11h of the carrier plate 12. As a result, the rotating shaft 27a can rotate freely and is parallel to the guide hole 11h, and thus will not collide with the support shaft 27. Therefore, when operating the rotating shaft 27a, the guide holes 11g and 11h of the carrier plate 12 do not collide with each other and the carrier plate 12 is accurately located on the base plate 12. The prior art apparatus for the fabrication of a printed circuit board is installed on a base plate, and each processing operation is located on a positioning arm. As a result, the apparatus occupies a large volume, and if the apparatus is installed in a limited space, each processing operation is likely to interfere with the adjacent processing operations and cause collision therebetween. Moreover, the prior art apparatus has a large number of processing equipments, resulting in a heavy loading. As a result, the prior art apparatus cannot be installed in a space with a limited height, such as a space below the ceiling. Therefore, it is desired to provide an improved apparatus for the fabrication of a printed circuit board in order to overcome the above-described shortcomings. The object of the present invention is to provide an apparatus for the fabrication of a printed circuit board, comprising a base plate having