FIELD OF THE INVENTIONThe present invention relates to semiconductor packages and fabrication methods thereof, and more particularly, to a window ball grid array (WBGA) semiconductor package with an improved yield, and a method for fabricating the semiconductor package.
BACKGROUND OF THE INVENTIONA 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 inFIG. 1, thissemiconductor package3 comprises asubstrate30 having a central opening304 therethrough; achip31 mounted on thesubstrate30, withbond pads310aon anactive surface310 of thechip31 being exposed to the opening304 of thesubstrate30; a plurality ofgold wires33 received in the opening304, for electrically connecting thebond pad310aof thechip31 to a lower surface of thesubstrate30; afirst encapsulant340 and asecond encapsulant341 formed on an upper surface and the lower surface of thesubstrate30 respectively, for encapsulating thechip31 and filling the opening304; a plurality ofsolder balls35 implanted on the lower surface of thesubstrate30 not having thesecond encapsulant341, 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 inFIG. 2, after the chip-mounting and wire-bonding processes, the substrate strip30 (designated with the same reference numeral as substrate) is placed between an upper mold and a lower mold of atransfer mold37. After engaging the upper and lower molds, injecting a molding compound and performing a curing step, which are known in the art, thefirst encapsulant340 and thesecond encapsulant341 are respectively formed on the upper surface and the lower surface of thesubstrate30. Finally, after the ball-implanting process, the package structure is sawed to form a plurality of individualWBGA semiconductor packages3.
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.
SUMMARY OF THE INVENTIONAccordingly, a primary objective of the present invention is to provide a semiconductor package and a method for fabricating the same, without having an encapsulant protruding out of a substrate in the semiconductor package.
Another objective of the present invention is to provide a semiconductor package and a method for fabricating the same, which can increase the density of implanted solder balls on a substrate in the semiconductor package.
Still another objective of the present invention is to provide a semiconductor package and a method for fabricating the same, without the occurrence of flash of an encapsulant.
A further objective of the present invention is to provide a semiconductor package and a method for fabricating the same, which only require the use of simple molds.
A further objective of the invention is to provide a semiconductor package and a method for fabricating the same, which can enhance the mechanical strength and supportability of bonding wires in the semiconductor package.
Another objective of the invention is to provide a semiconductor package and a method for fabricating the same, which can improve the yield of the bonding wires and the electrical performance of the semiconductor package.
In order to achieve the foregoing and other objectives, the present invention proposes a method for fabricating a semiconductor package, comprising the steps of: preparing a substrate having a first circuit layer, a second circuit layer, and a core layer formed between the first circuit layer and the second circuit layer; forming at least one second opening on the second circuit layer, and forming at least one first opening on the first circuit layer at a position corresponding to the second opening; forming a plurality of finger holes in the core layer at positions corresponding to a plurality of bond fingers formed on the first circuit layer; forming a through opening in the core layer, allowing the through opening to communicate with the first opening of the first circuit layer and the second opening of the second circuit layer; mounting at least one chip on the first circuit layer of the substrate, allowing the chip to cover the first opening and allowing an active surface of the chip to be exposed to the first opening; forming a plurality of bonding wires to electrically connect the active surface of the chip to the plurality of bond fingers on the first circuit layer through the finger holes; forming an encapsulant on the substrate to fill the first and second openings and the through opening and encapsulate the chip and the bonding wires; and implanting a plurality of solder balls on the substrate.
A semiconductor package fabricated by the above method according to the present invention comprises: a substrate having a first circuit layer, a second circuit layer, and a core layer formed between the first circuit layer and the second circuit layer, wherein at least one second opening is formed on the second circuit layer and at least one first opening is formed on the first circuit layer at a position corresponding to the second opening, and wherein a plurality of finger holes are formed in the core layer at positions corresponding to a plurality of bond fingers formed on the first circuit layer, and a through opening is formed in the core layer and communicates with the first and second openings; at least one chip mounted on the first circuit layer of the substrate to cover the first opening, allowing an active surface of the chip to be exposed to the first opening; a plurality of bonding wires for electrically connecting the active surface of the chip to the plurality of bond fingers on the first circuit layer through the finger holes; an encapsulant for filling the first and second openings and the through opening and encapsulating the chip and the bonding wires; and a plurality of solder balls implanted on the substrate.
The above finger holes in the core layer are formed by laser drilling. By a material selectivity characteristic of laser, the laser drilling technique can avoid damage to the bond fingers on the first circuit layer. The through opening in the core layer is formed by using a router. And the first opening of the first circuit layer and the second opening of the second circuit layer are formed by a conventional etching technique.
In addition, the core layer is further formed with a plurality of conductive vias for electrically connecting the first and second circuit layers to each other. A nickel (Ni)/gold (Au) layer is plated on the bond fingers respectively so as to enhance the bonding reliability between the bonding wires and the bond fingers.
Accordingly, by provision of the first and second openings of the first and second circuit layers respectively and the plurality of finger holes in the core layer in the present invention, the bonding wires are completely received in the through opening of the substrate, such that the encapsulant for encapsulating the bonding wires does not protrude out of the substrate. This allows the density of solder balls implanted on the substrate to be increased, and eliminates the drawbacks of encapsulant flash and difficulty in standardizing the mold used for fabricating the encapsulant. Moreover, in the present invention, the mechanical strength and supportability of the bonding wires can be improved. Thus the problems in the prior art can be solved by the present invention.
BRIEF DESCRIPTION OF THE DRAWINGSThe invention can be more fully understood by reading the following detailed description of the preferred embodiments with reference made to the accompanying drawings, wherein:
FIG. 1 (PRIOR ART) is a schematic cross-sectional view of a WBGA semiconductor package disclosed by U.S. Pat. No. 6,218,731;
FIG. 2 (PRIOR ART) is a flow chart showing a molding process and a sawing process for fabricating conventional WBGA semiconductor packages;
FIGS. 3A to 3I are schematic diagrams showing procedural steps of a method for fabricating a substrate used in a semiconductor package according to the present invention; and
FIGS. 4A to 4D are schematic diagrams showing procedural steps of a method for fabricating the semiconductor package according to the present invention using the substrate shown inFIG. 3I.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTSPreferred embodiments of a semiconductor package and a method for fabricating the same proposed in the present invention are described in detail as follows with reference toFIGS. 3A to 3I andFIGS. 4A to 4D, whereinFIGS. 3A to 3I show a fabrication method of a substrate used in the semiconductor package.
First, referring toFIG. 3A, a dual-layer substrate10, such as a copper clad laminate (CCL) substrate, is prepared. Thissubstrate10 comprises a firstcopper circuit layer100; a secondcopper circuit layer101; aninsulating core layer102 formed between the first andsecond circuit layers100,101, making the first andsecond circuit layers100,101 separated by thecore layer102; and a plurality ofconductive vias107 formed in thecore layer102, for electrically connecting the first andsecond circuit layers100,101 to each other. Then, referring toFIG. 3B, the first andsecond circuit layers100,101 are subjected to a patterning process including exposure, development, etching, etc. to respectively form predetermined circuit patterns. As a result, thefirst circuit layer100 is formed with a plurality ofbond fingers104 and a central first opening100a. Thesecond circuit layer101 is formed with a centralsecond opening101acorresponding in position to thefirst opening100aof thefirst circuit layer100, wherein thefirst opening100ais smaller than thesecond opening101a, and predetermined portions of thecore layer102 are exposed via thefirst opening100aand thesecond opening101a. As shown inFIGS. 3B and 3C (FIG. 3C is a top view ofFIG. 3B), thefirst opening100ais surrounded and defined by the plurality ofbond fingers104 of thefirst circuit layer100, and thesecond opening101ais surrounded and defined by conductive traces (not shown) of thesecond circuit layer101.
Subsequently, referring toFIG. 3D, asolder mask18 is applied on thefirst circuit layer100 and thesecond circuit layer101 respectively to protect the circuit patterns thereof. A plurality ofopenings180 are formed in thesolder mask18 covering thesecond circuit layer101 to expose predetermined portions of the circuit patterns of thesecond circuit layer101.
Referring toFIG. 3E, a laser drilling technique is adopted to drill a plurality offinger holes105 on the portion of thecore layer102 exposed via thesecond opening101aof thesecond circuit layer101, and thefinger holes105 correspond in position to the plurality ofbond fingers104 of thefirst circuit layer100. The finger holes105 are made penetrating thecore layer102 such that thebond fingers104 can be partially exposed via the finger holes105. This process is accomplished by a material selectivity characteristic of laser to remove only the material ofcore layer102 without damaging the material ofbond fingers104 by adjusting the energy of laser. As shown inFIG. 3F, which is a top view ofFIG. 3E, areas with oblique lines in thefinger holes105 represent the portions of thebond fingers104 exposed via the finger holes105.
Referring toFIG. 3G, a plating process is performed to form a nickel (Ni)/gold (Au)layer16 on the exposed portions of thebond fingers104 and acopper layer103 of the circuit patterns exposed from theopenings180 of thesolder mask18, so as to allow bonding wires and solder balls (not shown) to be subsequently bonded to the Ni/Au layer16 that can enhance the bonding reliability. Referring toFIGS. 3H and 3I, a router is used to form a throughopening102ain thecore layer102, and the through opening102acommunicates with thesecond opening101aof thesecond circuit layer101 and thefirst opening100aof thefirst circuit layer100. As shown inFIG. 3H, the through opening102aalso communicates with the finger holes105, such that the subsequently formed bonding wires can electrically connect a chip (not shown) to thebond fingers104 through thefirst opening100a, the through opening102aand thefinger holes105 where thebond fingers104 are exposed. This completes the fabrication of thesubstrate10 in the present invention.FIG. 3I is a cross-sectional view ofFIG. 3H taken along line3I-3I through the finger holes105, which allows the relative sizes and locations of the through opening102aandfinger holes105 to be observed.
Accordingly, the above fabricatedsubstrate10 can be used to fabricate a semiconductor package according to the present invention by a method illustrated inFIGS. 4A to 4D. InFIGS. 4A to 4D, thesubstrate10 is turned upside down, that is to allow thefirst circuit layer100 to face upwards.
First, referring toFIG. 4A, anactive surface110 of achip11 is mounted via an adhesive12 on thesolder mask18 covering thefirst circuit layer100 of thesubstrate10 in a manner that, thefirst opening100ais covered by thechip11, andbond pads111 formed on thechip11 are exposed to thefirst opening100a. Then, referring toFIG. 4B, a wire-bonding process is performed to form aplurality bonding wires13, such as gold wires, for electrically connecting thebond pads111 of thechip11 to thebond fingers104 on thefirst circuit layer100, wherein thebonding wires13 are completely received in the through opening102aof thesubstrate10 and connected to the Ni/Au layer16 plated respectively on thebond fingers104 through thefinger holes105 where thebond fingers104 are exposed (FIG. 3H); that is, thebonding wires13 are inserted in thefinger holes105 to be connected to thebond fingers104. The supportability of thebonding wires13 is enhanced by the surroundingcore layer102, thereby improving the reliability and yield of the wire-bonding process. Referring toFIG. 4C, anencapsulant14 is formed on thesubstrate10 to encapsulate thechip11 and thebonding wires13 and fill the through opening102a, the first andsecond opening100a,101aand thefinger holes105 of thesubstrate10. Since thebonding wires13 are completely received in the through opening102a, theencapsulant14 for encapsulating thebonding wires13 does not protrude out of thesubstrate10. In other words, the height of theencapsulant14 filling the first andsecond openings100a,101aand the through opening102ais equal to or smaller than the thickness of thesubstrate10. This thus eliminates the prior-art problems of encapsulant flash and limitation on density of solder balls arranged on the substrate, and only requires a simple encapsulation mold, for example comprising an upper mold with a cavity and a flat lower mold, for fabricating theencapsulant14 in the present invention. Finally, referring toFIG. 4D, a plurality ofsolder balls15 are implanted at the Ni/Au layer16 plated on thesecond circuit layer101 of thesubstrate10, and the overall structure is sawed to completely form the semiconductor package according to the present invention.
Therefore, the semiconductor package in the present invention is shown inFIG. 4D, comprising: asubstrate10, at least onechip11, a plurality ofbonding wires13, anencapsulant14, and a plurality ofsolder balls15.
Thesubstrate10 comprises afirst circuit layer100, asecond circuit layer101, and acore layer102 formed between thefirst circuit layer100 and thesecond circuit layer101. At least onefirst opening100ais formed on thefirst circuit layer100, and at least onesecond opening101ais formed on thesecond circuit layer101. A plurality offinger holes105 are provided in thecore layer102 at positions corresponding to a plurality ofbond fingers104 formed on thefirst circuit layer100. A through opening102ais formed through thecore layer102 and communicates with thefirst opening100aand thesecond opening101a(FIG. 3I). Thechip11 is mounted via itsactive surface110 on thefirst circuit layer100 of thesubstrate10 in a manner that, thechip11 covers thefirst opening100a, and a plurality ofbond pads111 formed on thechip11 are exposed to thefirst opening100a. Thebonding wires13 electrically connect thebond pads111 of thechip11 to thebond fingers104 on thefirst circuit layer100 through the finger holes105. Thesolder balls15 are implanted on thesecond circuit layer101 of thesubstrate10 and can be electrically connected to an external device such as a printed circuit board. Theencapsulant14 encapsulates thechip11 and thebonding wires13 and fills the through opening102a, the first andsecond opening100a,101aand the finger holes105.
In summary, the semiconductor package and the method for fabricating the same provided by the present invention allow the encapsulant not to protrude out of the substrate, such that the density of solder balls implanted on the substrate can be increased, and the prior-art problems of encapsulant flash and difficulty in standardizing the encapsulation mold are eliminated. Moreover, by provision of the finger holes with the surrounding core layer, the mechanical strength and supportability of the bonding wires can be enhanced strengthened, thereby improving the reliability and yield of the wire bonding process as well as the electrical performance of the semiconductor package.
The invention has been described using an exemplary preferred embodiment. However, it is to be understood that the scope of the invention is not limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements. The scope of the claims, therefore, should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.