1. Field of the Invention
The present invention relates to a printed circuit board for a ball grid array (BGA) semiconductor package and a method for molding a BGA semiconductor package using the printed circuit board. More particularly, the present invention relates to a printed circuit board for a BGA semiconductor package provided at one corner thereof with a degating opening serving as a mold runner gate and as a degating region and a method for molding a BGA semiconductor package using the printed circuit board.
2. Description of the Prior Art
Typically, BGA semiconductor packages have a configuration including a printed circuit board, at least one semiconductor chip mounted on one surface of the printed circuit board, and an array of solder balls attached to the other surface of the printed circuit board and adapted to electrically connect the semiconductor chip to a mother board. Such BGA semiconductor packages have been widely used as multi-pin devices having 200 pins or more, integrated circuits of very large scale integration (VLSI) and microprocessors.
Referring to FIG. 1, a typical BGA semiconductor package is illustrated. As shown in FIG. 11 the BGA semiconductor package, which is denoted by the reference numeral 100, includes a printed circuit board 130 having a die pad 132 at the central portion of its upper surface. A semiconductor chip 150 is mounted on the die pad 132 of the printed circuit board 130 by means of an epoxy adhesive. The semiconductor chip 150 is provided with bond pads (not shown) electrically connected to conductive traces 133 of the printed circuit board 130 by bonding wires 140. A resin seal 120 is molded on the printed circuit board 130 to protect the semiconductor chip 150 and wires 140 from the external environment. Conductive traces 135 are provided on the printed circuit board 130, and electrically connected to the solder ball pads 136 which are formed at opposite side surface thereof, through via holes 134. Solder balls 160 are fused on the solder ball pads 136 as input/output terminals, respectively.
Such a BGA semiconductor package is fabricated in accordance with the above-mentioned process steps, namely, the semiconductor chip mounting step, wire bonding step, seal molding step and solder ball fusing step.
Now, a conventional seal molding method used in the fabrication of a BGA semiconductor package will be described in conjunction with FIG. 5.
In accordance with this method, a mold 201 is used which includes an upper mold 202 and a lower mold 203. The mold 201 has a cavity 202a in which a printed circuit board 206 to be molded is disposed. The mold 201 also has a mold runner consisting of passages 202b and 202e. In the mold 201, a pot 203b is also defined in which a melt resin is received. The mold runner has a gate defined by a groove formed on the lower surface of the upper mold 202 and a corresponding upper surface portion of the printed circuit board 206 disposed in a recess 203a of the lower mold 203. The melt resin in the pot 203b is injected into the cavity 202a through the mold runner as it is pushed by a resin feed ram 204. At this time, the melted resin comes into contact with the upper surface portion of the printed circuit board 206 defining the mold runner gate. As a result, a surplus of resin is left on the upper surface portion of the printed circuit board 206 while being connected to a resin seal molded on the printed circuit board 206 after the curing of the resin seal. Accordingly, it is required to remove such a surplus resin. However, the printed circuit board 206 may be peeled off or damaged at its surface during the removal of the surplus resin. The damage of a solder mask may also occur. This may result in an exposure of the conductive traces.
In order to solve these problems, various methods have been proposed. This method is illustrated in FIG. 2. In accordance with this method, a printed circuit board 30', which is a constituting element of a BGA semiconductor package 10', is coated at one corner of its upper surface with a metal, such as platinum, palladium, nickel or silver, exhibiting a bonding strength lower than the upper surface of the printed circuit board 30'. Alternatively, the same corner of the upper surface of the printed circuit board 30' may be attached with a tape. By the coating or tape, a degating region 50 is defined on the printed circuit board 30'. Here, the degating region means a region for easily removing a cured surplus resin left at a mold runner gate which serves as a resin injection passage in a molding process, without any damage of the upper surface of the printed circuit board 30'.
Where a molding process is carried out using a mold shown in FIG. 5 after processing a printed circuit board, to be molded, in accordance with the above-mentioned method, it is possible to easily remove a cured surplus resin left at the degating region 50. In this case, accordingly, it is possible to considerably reduce the above-mentioned problems, namely, the peeling or damage of the upper surface of the printed circuit board or the exposure of the conductive traces caused by the damage of the solder mask.
However, these problems, namely, the peeling or damage of the upper surface of the printed circuit board or the exposure of the conductive traces caused by the damage of the solder mask, are still involved in the above method, even though they are reduced. This is because the degating region 50 defined at one corner of the upper surface of the printed circuit board is relatively large and because the method is only based on the provision of the degating region 50 exhibiting a bonding strength lower than the surface of the printed circuit board. In particular, where the degating region is formed by a tape attached to the printed circuit board, the tape may be melted during a molding process carried out at a high temperature, thereby degrading a stability of the molding process. Consequently, it is difficult to practically use the above-mentioned method.