BGA/LGA with built in heat slug/spreader

A ball or land grid array plastic substrate portion is formed with a hole therethrough in the region on which the integrated circuit die is to be formed, with a copper heat slug inserted within the opening having a bottom surface substantially aligned with the bottom surface of the plastic portion to allow molding tooling for conventional ball or land grid array packages to be employed. The integrated circuit die is mounted on the heat slug, which has a solderable bottom surface and is directly soldered to the PCB. An additional copper heat spreader region is formed on an upper surface of the plastic portion.

TECHNICAL FIELD OF THE INVENTION

The present invention is directed, in general, to integrated circuit packaging and, more specifically, to heat dissipation within ball grid array integrated circuit packages.

BACKGROUND OF THE INVENTION

The following description is provided solely as background and without admission of “prior art” status of any of the structures and processes disclosed therein within the legal meaning of that term.

Conventional ball grid array (BGA) packages600are based on a plastic packaging substrate601as illustrated in FIG.6. Heat dissipation occurs mainly by conduction from the integrated circuit die602through the packaging substrate601and the solder balls603to the printed circuit board (PCB)604. The plastic packaging substrate601has a very low thermal conductivity (0.2 Watts per meter per degree Celsius or W/m/° C.) and is therefore generally poor in both heat conduction and heat spreading (arrows indicate heat conduction).

BGA and equivalent land grid array (LGA) packages are often compared to thermally enhanced lead frame packages such as the exposed pad (epad) thin quad flat package (TQFP)700illustrated inFIGS. 7A and 7B, in which the integrated circuit die701is attached to the die pad (or paddle)702, which is in turn directly soldered to the PCB703by solder regions704. The heat conduction path includes the copper lead frame die pad702, which has a high thermal conductivity such as copper (400 W/m/° C.) contributing to the low thermal resistance for the package. It would be desirable for a BGA/LGA package to have equivalent thermal performance to epad TQFP so that BGA/LGA packaging may be employed to accommodate the same electrical and thermal performance requirements.

To achieve such thermal enhancement, a copper heat slug/copper heat spreader BGA (C2BGA) package800has been proposed, as illustrated inFIG. 8. C2BGA package800employs a bi-layer substrate801including a non-conductive (e.g., plastic, fiberglass, or epoxy) portion802with an opening therethrough and covered by a copper heat spreading layer803. A copper heat slug804having a bottom with a solderable finish is attached to the substrate801by an adhesive805, preferably thermally conductive, such as an electrically conductive glue or solder. The integrated circuit die806is directly attached to the heat slug804, which in turn is soldered to the PCB807. Heat is dissipated outward toward the solder balls by the heat spreader layer803and directly conducted to the PCB807through the high thermal conductivity heat sink804and the solder balls.

C2BGA package800has a thermal performance equivalent to that of an epad TQFP package. However, since the C2BGA package800is not flat due to the attached slug, specialized wire bonding and molding tooling are required for packaging. In addition, there are challenges in the slug attachment to the substrate, including making a void free attachment with a consistent bond line and tight lateral (x-y) placement tolerance.

There is, therefore, a need in the art for an improved thermal performance ball grid array package that allows the same tooling to be employed for package assembly as conventional ball grid array packages, with consistent placement of the heat slug/spreader.

SUMMARY OF THE INVENTION

To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide, for use in an integrated circuit package, a ball or land grid array plastic substrate portion formed with a hole therethrough in the region on which the integrated circuit die is to be mounted, with a copper heat slug inserted within the opening having a bottom surface substantially aligned with the bottom surface of the plastic portion to allow molding tooling for conventional ball or land grid array packages to be employed. The integrated circuit die is mounted on the heat slug, which has a solderable bottom surface and is directly soldered to the PCB in some applications. An additional copper heat spreader region is formed on an upper surface of the plastic portion.

The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art will appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art will also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1depicts a high thermal conductivity ball grid array or land grid array package compatible with conventional package assembly tooling for such packages according to one embodiment of the present invention. The package100employed a dual-material substrate101including a plastic portion102having an opening or cavity therein and a high thermal conductivity metal (e.g., copper) plate103which serves as both a die pad and a heat spreader. Metal plate103, which may be stamped, has a down set104at the location of the opening through the plastic portion102serving as a die pad/heat slug. Metal plate103is laminated to the plastic portion102. The bottom surface of the down set die pad/heat slug portion104of metal plate103, which has a solderable finish, is at the same level as the bottom of plastic portion102(on which solder balls or lands are formed) and is soldered directly to the PCB105. The bottom surface of heat slug104may extend within the opening through the plastic substrate portion102or may project slightly out of that opening, provided the projection does not interfere with the use of conventional ball grid array or land grid array molding tools. In that regard, the bottom surface of the heat slug or heat spreader104on which the integrated circuit die is mounted is “substantially aligned” with the bottom surface of the plastic portion of the substrate if the difference (between the level of the bottom of the heat slug and the level of the bottom of the plastic portion) does not interfere with use of conventional package assembly tooling for packaging conventional ball or land grid array packages which do not include a heat slug or heat spreader below the integrated circuit die.

The integrated circuit die106is mounted on the down set die pad/heat sink104using, for example, an adhesive107. Metal plate103includes slots exposing the bond fingers of conductive traces on plastic substrate (not shown inFIG. 1) for connection with bond wires from the integrated circuit die106.

FIGS. 2A and 2Bdepict various views of a substrate for a high thermal conductivity ball grid array or land grid array package compatible with conventional molding tooling for such packages according to one embodiment of the present invention. One embodiment101aof substrate101is shown in a side cross-sectional view inFIG. 2A, whileFIG. 2Bis a split partial plan view of the substrate101, with a partial bottom view shown on the left side and a partial top view shown on the right side.

The plastic (e.g., epoxy, fiberglass, etc.) portion102includes an opening therethrough at the location on which the die is to be mounted, and metal (e.g., copper or copper alloy) plate103aor103bis laminated to the plastic portion102with a downset104a/104bwithin and filling the opening and having a bottom surface level with the bottom surface of the plastic portion102, on which solder balls or lands are to be formed. The integrated circuit die will be mounted on the upper surface of downset104a/104b, while the bottom surface201has a solderable finish and for soldering directly to the PCB. A gap or slot200through metal plate103exposes bond fingers for later connection with wire bonds.

In the embodiment ofFIG. 2A, the downset portion104ais thinner than the total thickness of substrate101a(i.e., thinner than the combined thickness of plastic portion102and the portion of the plate103aoverlying the plastic portion102). For example, the substrate101amay have a total thickness of about 0.45 millimeters (mm), where the plastic portion102(which may be multiple layers) has a thickness of about 0.22 mm and a laminate layer (not explicitly shown) between the plastic portion102and the heat slug/spreader103ahas a thickness of about 0.1 mm. The metal heat slug/spreader plate103ahas a uniform thickness of about 0.127 mm (stamped and with a total distance of about 0.45 mm between furthest opposing surfaces in the thickness direction).

FIG. 3is a diagram illustrating a process for forming a high thermal conductivity ball grid array or land grid array package utilizing conventional molding tooling for such packages according to one embodiment of the present invention. Process300begins with parallel formation of the plastic portion of the substrate and the heat slug/spreader. The substrate process starts with a core material (step301). Via drilling, cavity routing and copper plating are performed (step302), followed by patterning, solder mask application(s), and plating to selected areas (e.g., bond sites and solder ball or land contact regions) that are required (step303).

Meanwhile a copper sheet is sized appropriately according to the substrate panel (step304) and plated to, for example, create a solderable finish on the downset bottom (step305). The sheet is then stamped to form the slots for exposing the bond sites or fingers and the downset (step306). Finally, the stamped copper sheet is laminated to the panel of plastic substrate cores (step307). As a result, heat slug attachment is performed at the panel level at lower unit cost and with higher slug position precision.

FIGS. 4A through 4Cdepict various views of another substrate for a high thermal conductivity ball grid array or land grid array package compatible with conventional molding tooling for such packages according to an alternative embodiment of the present invention. In this embodiment, substrate101bincludes a plastic portion102bwith an opening therethrough and metal plate103bwithin and filling the opening. Metal plate103bis illustrated, at the interface with plastic portion102bon the left side of plate103b, as slightly overlapping peripheral portions of the plastic portion102baround the opening. While better interlocking is achieved by such overlap, fabrication of such a structure would be difficult. Accordingly, a simple friction fit as illustrated at the right interface with plastic portion102bmay be utilized, or, alternatively, overlap at the interface on only one major surface (not shown). Normally, both interfaces between plastic portion102band metal slug103bwould be fitted similarly.

The bottom surface203of heat slug104bis solderable, and is soldered directly to the PCB during mounting of the packaged integrated circuit. The integrated circuit die is mounted on the opposite surface of the heat slug104b. While the bottom surface of heat slug104bis shown as level with the bottom portion, as noted above a slight difference in level may be tolerated provided the two surfaces are substantially aligned.

Bond fingers and an optional ground ring are formed on an upper surface of plastic portion102bas shown in FIG.4B. If a ground ring is employed, the ground ring may be electrically connected to the heat slug103b, which will then serve as a grounding contact.

FIG. 4Cshows a packaged integrated circuit utilizing the alternate embodiment of the packaging substrate. Integrated circuit die106is attached to the heat slug103bby an adhesive107, and encapsulated (together with the bond wires) by an encapsulating material108. The packaging substrate is then affixed at the heat slug103bto the PCB105by an adhesive109, with conductive traces on the plastic portion102belectrically connected to conductive traces on the PCB105by solder balls or lands110.

FIGS. 5A through 5Fillustrate stepwise formation of a substrate for a high thermal conductivity ball grid array or land grid array package compatible with conventional molding tooling for such packages according to the alternative embodiment of the present invention. In this process, a substrate core (FIG. 5A) is provided, again typically in the form of a panel including a number of substrates rather than by a single substrate as shown.

The substrate core is first patterned (FIG. 5B) to form the opening in which the heat slug will be inserted and any other specific physical characteristics required (e.g., shelf for the overlap or trenches for signal traces). A metal heat slug is then inserted in the opening (FIG.5C), and swaged (FIG.5D), with the slug held in place by contact pressure with the sides of the opening through the plastic substrate portion. As noted earlier, a ground ring electrically connected to the heat slub may be formed on the substrate through plating and patterning.

A dry film process is then used to pattern materials on an upper surface of the substrate, and a solder mask is employed to form the balls or lands on the bottom (FIG.5E). After nickel and/or silver plating, the dry film is stripped off (FIG.5F).

The present invention provides good heat slug placement precision, and may be employed with standard transfer molding tools. Use of land grid array contacts is also enabled, reducing solder costs.

Although the present invention has been described in detail, those skilled in the art will understand that various changes, substitutions, variations, enhancements, nuances, gradations, lesser forms, alterations, revisions, improvements and knock-offs of the invention disclosed herein may be made without departing from the spirit and scope of the invention in its broadest form.