In recent years, demand for such high density, high speed and multi-terminals on a semiconductor package is increasing more and more with the high integration of a semiconductor chip, so that a BGA (Ball Grid Array) type of package is needed.  The BGA package is a kind of a surface mount type of a package in which many contact terminals including solder balls are arrayed on the bottom surface.  In order to cope with the increment of heat generation of the semiconductor chip due to high performance in recent years, a thermal enhanced type of BGA package having a cavity-down structure, equipped with a heat sink on a plastic circuit board, has been developed. 
As shown in FIG.  1, a conventional thermal enhanced type of BGA package 1a, forms a cavity 5 in a manner that a heat sink 3, made of copper or copper alloys, is adhesively joined to a plastic circuit board 2, having a cutout space in its central portion, with a bonding agent 4, such as a prepreg and an adhesive by means of heating and pressurizing. 
As shown in FIG.  2, in another conventional thermal enhanced type of BGA package 1b, an additional metal plate 6, having the same size in plan view as the plastic circuit board 2 and being made of copper or copper alloys, is provided between the plastic circuit board 2 and the heat sink 3.  The plastic circuit board 2 and the metal plate 6, and the metal plate 6 and the heat sink 3, are adhesively joined with a bonding agent 4, such as the prepreg and an adhesive, by means of heating and pressurizing. 
The thermal enhanced type of BGA packages are used for a semiconductor device in a manner that a semiconductor chip is die-bonded to the cavity 5, wire-bonded to the plastic circuit board, and sealed with sealing resin, and a solder ball is attached on a surface of the plastic circuit board. 
A surface roughening process such as an oxidation treatment known as a black oxide treatment is applied to the bonding surface of the heat sink in order to increase adhesive strength, so that mechanical strength is increased by an anchor effect. 
However, there are unsolved problems of these thermal enhanced type of BGA packages which are as follows:                (I) The adhesive strength could be decreased by a void trapped in the bonding agent or uneven thickness of the adhesive agent layer.         (II) Since the bonding is accomplished by heating and pressurizing the bonding agent, the bonding agent could run into the cavity, so that yield is reduced.         (III) When a solder resist film is formed by charging the resin into a through hole formed in the plastic circuit board, moisture taken into the resin or the bonding agent loses all means of escape between the heat sink due to the heating of reflow soldering.  Then, because it generates a so-called popcorn phenomenon, the thermal enhanced type of BGA package could be destroyed.         (IV) It is important to take action against noise generated in the package or noise coming from outside of the package, due to increase a clock frequency in recent years.  However, in the conventional thermal enhanced type of BGA packages, any preventive action against the noise in the heat sink is not taken. 