A window ball grid array (WBGA) semiconductor package employs an advanced type of BGA packaging technology, wherein at least one opening is formed through a substrate, and a semiconductor chip is mounted on the substrate in an upside-down manner that an active surface of the chip faces downwards and covers the opening of the substrate, allowing the chip to be electrically connected to a lower surface of the substrate via a plurality of gold wires received in the opening. Such package structure can effectively reduce the length of gold wires and improve the quality of electrical communication between the chip and substrate, which thus has been widely applied to DRAM (dynamic random access memory) chips having central pads.
U.S. Pat. No. 6,218,731 discloses a WBGA semiconductor package. As shown in FIG. 1, this semiconductor package 3 comprises a substrate 30 having a central opening 304 therethrough; a chip 31 mounted on the substrate 30, with bond pads 310a on an active surface 310 of the chip 31 being exposed to the opening 304 of the substrate 30; a plurality of gold wires 33 received in the opening 304, for electrically connecting the bond pad 310a of the chip 31 to a lower surface of the substrate 30; a first encapsulant 340 and a second encapsulant 341 formed on an upper surface and the lower surface of the substrate 30 respectively, for encapsulating the chip 31 and filling the opening 304; a plurality of solder balls 35 implanted on the lower surface of the substrate 30 not having the second encapsulant 341, for establishing electrical connection with external electronic devices.
Conventionally due to cost concerns for fabricating the above semiconductor package, a molding process is performed in a batch manner to encapsulate a substrate strip comprising a plurality of substrates, and then a sawing process is carried out to separate apart the individual substrates. As shown in FIG. 2, after the chip-mounting and wire-bonding processes, the substrate strip 30 (designated with the same reference numeral as substrate) is placed between an upper mold and a lower mold of a transfer mold 37. After engaging the upper and lower molds, injecting a molding compound and performing a curing step, which are known in the art, the first encapsulant 340 and the second encapsulant 341 are respectively formed on the upper surface and the lower surface of the substrate 30. Finally, after the ball-implanting process, the package structure is sawed to form a plurality of individual WBGA semiconductor packages 3.
Such molding method is relatively cost-effective and suitable for mass production. However, since loops of the gold wires and the second encapsulant for encapsulating the gold wires protrude from the lower surface of the substrate, in order to fabricate appropriate second encapsulants, it needs to prepare different types of molds corresponding to different sizes and structures of openings in the substrates, which would undesirably increase the fabrication cost. Moreover, in order to completely encapsulate the gold wires, the second encapsulant may occupy relatively much area on the substrate, thereby limiting the density and number of solder balls that can be implanted on the substrate. In addition, since the first encapsulant and the second encapsulant are not completely symmetric to each other, the upper and lower molds may not firmly clamp the substrate, thereby leading to flash of the second encapsulant on the lower surface of the substrate. This not only affects the appearance of the package but also may cover ball pads on lower surface of the substrate, which would adversely affect the ball-implanting process and degrade the electrical performance of the solder balls formed on the ball pads. As a result, an extra step of using a solvent to remove the encapsulant flash is required. The flash problem is thus considered as a significant drawback in the prior art.
Therefore, the problem to be solved here is to provide a semiconductor package and a method for fabricating the same, which can increase the density of implanted solder balls and solve the flash problem, so as to improve the overall yield and electrical performance.