1. Field of the Invention
The invention relates to an image sensor, and in particular to a package structure of an image sensor having shortened wires.
2. Description of the Related Art
Referring to FIG. 1, a conventional image sensor includes a substrate 10, a frame layer 18, a photosensitive chip 26, a plurality of wires 28, and a transparent layer 34. The substrate 10 has a first surface 12 on which signal input terminals 15 are formed, and a second surface 14 on which signal output terminals 16 are formed. The frame layer 18 has an upper surface 20 and a lower surface 22 adhered to the first surface 12 of the lower surface 22 to form a cavity 24 together with the substrate 10. The photosensitive chip 26 is placed within the cavity 24 formed by the substrate 10 and the frame layer 18 and is mounted to the first surface 12 of the substrate 10. Each of the wires 28 has a first terminal 30 electrically connected to the photosensitive chip 26, and a second terminal 32 electrically connected to a corresponding signal input terminal 15 of the substrate 10. The transparent layer 34 is placed on the upper surface 20 of the frame layer 18.
According to the above-mentioned wire bonding structure, the lengths of the wires 28 from the bonding pads of the photosensitive chip 26 to the signal input terminals 15 of the substrate 10 are quite long. Therefore, the signal transmission speed is slow and the image sensor is impractical.
Referring to FIG. 2, which is a cross-sectional view showing an image sensor disclosed in a U.S. patent application Ser. No. 10/146,997, filed on May 15, 2002. The lengths of the wires 88 are also long, and the signal transmission speed of the image sensor is slow and the signal transmission efficiency thereof is also low.
An object of the invention is to provide a practical image sensor with shortened wires so as to enhance the signal transmission efficiency.
To achieve the above-mentioned object, the invention provides an image sensor having shortened wires. The image sensor includes a substrate composed of a plurality of metal sheets arranged in a matrix, a photosensitive chip, a plurality of bonding pads, a frame layer, a plurality of wires and a transparent layer. Each of the metal sheets has a first board and a second board located at a height different from that of the first board. A chamber is defined under a bottom surface of the substrate. The photosensitive chip is arranged within the chamber. The bonding pads are formed on the photosensitive chip at positions corresponding to the first boards of the substrate. The bonding pads and the first boards of the substrate are staggered so that each of the bonding pads is arranged between adjacent two of the first boards. The frame layer is formed on a periphery and the bottom surface of the substrate to encapsulate the photosensitive chip. Upper faces of the first boards and bottom faces of the second boards are exposed out of the frame layer, and the bottom faces of the second boards are electrically connected to the printed circuit board. Each of the wires has a first terminal and a second terminal. The first terminals are electrically connected to the bonding-pads of the photosensitive chip, respectively, and the second terminals are electrically connected to the corresponding staggered first boards of the substrate, respectively. The transparent layer is placed on the frame layer to cover the photosensitive chip so that the photosensitive chip may receive optical signals passing through the transparent layer.
Accordingly, the signal transmission distance can be shortened, and the signal transmission speed may be increased.