This application claims priority to Japanese Application No. 2010-189673 filed Aug. 26, 2010, the entire content of each of which is hereby incorporated by reference in this application.
1. Field of the Invention
The present invention relates to a circuit board, and specifically relates to a circuit board provided with a control circuit and an optical module.
2. Description of Related Art
In accordance with increase of the performance of computers, the speed of signals inputted into and outputted from a CPU (Central Processing Unit) mounted in a computer has been getting higher. Therefore, it is a problem how to take measures against attenuation of high-speed electric signals propagating on a circuit board and against generation of noise in the signals.
To solve this problem, in recent years, such a method has been considered that high-speed electric signals are inputted into and outputted from a CPU by executing electro-optical conversion with a photoelectric conversion module (referred to as an “optical module” hereinafter) and optical signals are propagated at high speeds. An optical module has a function of converting electric signals into optical signals and vice versa.
In a method for taking measures against attenuation of high-speed electric signals with this optical module, such a characteristic is utilized that attenuation of power of optical signals propagating within an optical fiber or an optical waveguide is considerably smaller than that of electric signals propagating on a circuit board. In other words, in part of a section in which high-speed electric signals inputted and outputted are transmitted, the electric signals are converted into optical signals with an optical module and transmitted, and thereby, attenuation of the high-speed signals on a circuit board is reduced. Moreover, in a method for taking measures against generation of noise in high-speed electric signals, such a characteristic is utilized that optical signals propagating within an optical fiber are not influenced by electromagnetic waves from outside.
Since signals inputted into and outputted from the CPU are electric signals, there is a need to propagate high-speed electric signals on a circuit board in a section between the optical module and the CPU. Therefore, in order to increase the effect of transmission of signals by conversion of high-speed electric signals into optical signals, there is a need to minimize attenuation of the electric signals and generation of noise in the electric signals on the circuit board. Accordingly, it is required to make the length of the section where transmission signals propagate in the form of electric signals on the circuit board as short as possible.    [Patent Document 1] JP2010-67731A
However, the abovementioned optical module, in which an optical element and an optical-element driving IC are mounted on a transparent resin board, has a problem on heat radiation performance. An optical module, which executes photoelectric conversion, generally radiates heat when operating. To be specific, since an optical module has, for example, a configuration that a laser diode used as a light source element is driven by a driver IC, the optical module then consumes electric power and radiates heat. Accordingly, in order to sufficiently take advantage of the property of the laser diode, there is a need to cool by efficiently diffusing the radiated heat with a heat sink or the like.
On the other hand, since an optical module shown in FIG. 29 of JP2010-67731A has a light input/output part provided with a planar micro lens array on the upper face of a package, it is impossible to place a heat sink for cooling on the upper face of the optical module. Consequently, such a problem occurs that heat radiation in the optical module is limited to radiation by heat conduction from the lower face of the optical module to a circuit board, and moreover, it is impossible to sufficiently radiate heat because the material of the circuit board is usually resin.
Further, as mentioned before, there is a need to make the optical module as close as possible to the CPU in order to increase the effect of transmission of signals by conversion of high-speed electric signals of the CPU into optical signals, whereas such a problem occurs that the temperature of the optical module rises because of the influence of heat radiation of the CPU and exhausted heat by the CPU, and therefore, even the optical module provided with the heat sink cannot cool down if it is adjacent to the CPU.