Description Brushless DC Spindle Motor Technical Field of the Invention The present invention relates to a brushless DC spindle motor for driving data recording discs (hereinafter referred to as "optical discs") to rotate, such as CD, DVD, CD-ROM and DVD-ROM. Background of the Invention With the development of information technology, the transmission of information and the read/write of data become more and more frequent. As a major way of data transmission, optical discs are used more and more frequently, and spindle motors driving the optical discs to run in high speed are also used more widely. However, in the prior art, the spindle motors driving the optical discs to run have the defects of more parts, complicated structure, more machining procedures and high manufacture cost. Summary of the Invention The present invention provides a brushless DC spindle motor with the advantages of reasonable design, simple structure, less parts, less machining procedures and low manufacture cost. The above technical object of the present invention is mainly solved by the following technical solutions. A brushless DC spindle motor is provided, including a rotating shaft that is supported rotatably, a rotor hood that rotates together with the rotating shaft, and a PCB and a PCB soleplate provided at the lower part of the rotor hood, characterized in that a damping tray is provided at one end of the rotating shaft, and a core rotating together with the damping tray is provided on the damping tray. The damping tray is integrally 1 made of nylon, thereby replacing three parts in the prior art, i.e., cushion, optical disc plate and tray, and reducing the number of parts and the machining procedures in the present invention. As a result, the structure of the brushless DC spindle motor becomes simpler, and the manufacture cost thereof is reduced. Preferably, an oil-contained bearing is provided outside the rotating shaft and a concave surface is provided on the inner surface of the oil-contained bearing; and the concave surface is conical or arc-shaped, with an included angle of 0.5o-1.5' formed between the conical concave surface and the axis of the rotating shaft. Providing a concave surface on the inner surface of the bearing may reduce the contact surface between the rotating shaft and the bearing, thereby decreasing the friction and heat between the rotating shaft and the bearing, and prolonging the service life of the brushless DC spindle motor. Preferably, the damping tray is provided with a circumferentially-closed annular groove, a supporting plate being provided below the annular groove, roll balls being provided in a cavity formed by the supporting plate and the annular groove. Therefore, the present invention has the advantages of reasonable design, simple structure, less parts, less machining procedures and low manufacture cost. Brief Description of the Drawing Fig. 1 is a structure diagram of the present invention; Fig. 2 is a top view of Fig. 1; Fig. 3 is a bottom view of Fig. 1; and Fig. 4 is a partially enlarged view of the riveting of a PCB and PCB soleplate. 2 Detailed Description of the Invention The technical solutions of the present invention will be further described in details as below with reference to the accompanying drawings by embodiments. Embodiment 1: As shown in Fig. 1, Fig. 2, Fig. 3 and Fig. 4, a brushless DC spindle motor is provided, including a rotating shaft 1 that is supported rotatably and a rotor hood 6 that rotates together with the rotating shaft. A damping tray 3 (see Fig. 1, Fig. 2 and Fig. 3) is provided at the upper end of the rotating shaft. The damping tray is integrally made of nylon, thereby replacing three parts in the prior art, i.e., cushion, optical disc plate and tray, and reducing the number of parts and the machining procedures in the present invention. As a result, the structure of the brushless DC spindle motor becomes simpler, and the manufacture cost thereof is reduced. A core 2 rotating together with the damping tray is provided on the damping tray 3. A PCB 13 and a PCB soleplate 14 are provided at the lower part of the rotor hood 6. A convex hollow bump 16 is provided on the PCB soleplate. A through hole 15, from which the hollow bump 16 may pass, is provided on the PCB. An end of the hollow bump, passing through the through hole 15 on the PCB, is provided with a turnover structure 17. The hollow bump 16, the through hole 15 and the turnover structure 17 may be formed by punching via a punch having a rolling head, so that the PCB and the PCB soleplate are riveted together (see Fig. 4). Magnetic steel 7 is provided below the rotating shaft 1. The magnetic steel 7 may be magnetically sucked with the rotating shaft to draw the rotating shaft downward, thereby effectively suppress the axial vibration of the rotating shaft during the operating, and greatly the stability of reading from or writing data into the optical discs. The damping tray 3 is provided with a circumferentially-closed annular 3 groove 4, and a supporting plate 10 is provided below the annular groove 4. Nine roll balls 11, the diameter of which is 3mm, are provided in a cavity formed by the supporting plate 10 and the annular groove 4. Both the upper end surface and the lower end surface of the rotating shaft 1 are spherical. An oil-contained bearing 8 is provided outside the rotating shaft and a conical concave surface 9 is provided on the inner surface of the bearing, with an included angle of 51'30" formed between the conical concave surface and the axis of the rotating shaft (see Fig. 1). A spring (not shown) is provided between the damping tray 3 and a pressing plate disposed on the damping tray. 4