1. Field of the Invention
The present invention relates to a semiconductor device, and in particular, to a semiconductor device having a semiconductor element of large power consumption mounted thereto.
2. Description of the Related Art
In recent years, electronics including information communication devices, office electronics, household electronics, measuring devices, industrial electronics such as assembling robots, medical electronics, and electronic toys have become increasingly smaller and lighter, and it is therefore strongly desired to reduce an actual area for a semiconductor device to be mounted. Examples of devices that have developed in response to such a demand include a BGA (Ball Grid Array) semiconductor device. A semiconductor element mounted on this type of a semiconductor device is desired to realize higher integration and downsizing of the chip, and to respond to high heat generation.
FIG. 5 shows the BGA semiconductor device (see Japanese Laid-Open Patent Application No. 2001-28410) that responds to the high heat generation. A semiconductor element 3 is mounted on a wiring substrate 1 via a heat sink 2, where the surface layer wiring 1a of the wiring substrate 1 and the semiconductor element 3 are electrically connected with a wire 4. External electrodes 6 connected to the surface layer wiring 1a or the heat sink 2 via through holes 5 are formed on the back surface of the wiring substrate 1, and the through holes 5 connected to the heat sink 2 is connected to a solid wiring 7 in the wiring substrate 1, so that high heat generation of the semiconductor element 3 is allowed to escape to the mounting substrate (not shown), to which the external electrodes 6 are connected via the heat sink 2, the through hole 5, the solid wiring 7, and the external electrode 6. The solid wiring is a wiring pattern having an area larger than the normal surface layer wiring (signal wiring). Reference character 8 refers to a mold resin for sealing the element mounting surface of the wiring substrate 1.
In the above semiconductor device, such as the solid wiring and a copper plate disposed on the surface of the wiring substrate 1 are used as the heat sink 2, on which the semiconductor element 3 is directly mounted, and the semiconductor element 3 and the heat sink 2 are at the same potential at the bonding surface. Thus, due to the electrical demand of the semiconductor element 3, that is, since the back surface of the semiconductor element 3 is generally desired either not to be electrically connected or to be at GND potential, the inner solid wiring 7 to which the through hole 5 that is connected to the heat sink 2 is connected is uniquely determined as the GND layer. That is, the through hole 5 connected to the heat sink 2 can only be connected to the GND layer, and the power supply layer cannot be used for heat dissipation even if the power supply layer provided in the wiring substrate 1 similarly as the solid wiring is equal to or greater than the GND layer and is able to efficiently dissipate the heat.