Package for integrated circuit with thermal vias and method thereof

[0002] The present invention relates generally to a package for an integrated circuit. In particular, the present invention relates to techniques for achieving a low thermal resistance package for an integrated circuit.

[0003] The temperature of a PN junction in a semiconductor device is an important parameter that affects the proper operation of the semiconductor device. Thus, the package for an integrated circuit is designed with much emphasis on its ability to dissipate the heat generated during device operation. The thermal performance of a package is typically characterized by several figures of merit: θ_JA, ψ_JT, and θ_JC. θ_JAis given by:

[0004] θ_JA=(T_J−T_A)/P_H

[0005] where T_Jand T_Aare respectively the junction and ambient air temperatures, and P_His the power dissipation of the device. Similarly, ψ_JTand θ_JCare given by:

[0006] ψ_JT=(T_J−T_T)/P_H

[0007] θ_JC=(T_J−T_C)/P_H

[0008] where T_Tand T_Care respectively the temperature at the top-center of the package, and the temperature of the case. ψ_JTis typically used to estimate junction temperature from a measurement of T_T. θ_JCis an important parameter in designing a package to be operated with a heat sink attached to its top surface.

[0009] In designing integrated circuits, often times an accurate estimate of the power dissipation of an integrated circuit is not available until after the integrated circuit design is substantially complete, or after the integrated circuit is first fabricated. However, at that stage, if one discovers that the intended package for the integrated circuit would not handle that power dissipation, the corrective options available are limited and expensive. For example, one may consider using a more expensive ceramic package, attach an external heat sink or fan, or redesign the integrated circuit to a lower current density. A redesign of the integrated circuit (e.g., by increasing the die size) is expensive not only because of the additional engineering and manufacturing tooling costs, there are also costs associated with the higher manufacture cost per die and time delay to market.

[0010] In one embodiment described below, the present invention provides enhanced heat dissipation performance in an integrated circuit package having multiple semiconductor dies. Integrated circuits having multiple semiconductor dies in one package are known in the prior art. For example, FIG. 1 shows prior art multi-chip module 100 having semiconductor die 104 bonded conventionally (e.g., using thermally conductive adhesive) to semiconductor die 102, which in turn is bonded to insulating substrate 101. Semiconductor die 104 may be, for example, a static random access memory (SRAM) integrated circuit and semiconductor die 102 may be, for example, a “flash” electrically programmable read-only memory (“flash EPROM”). Substrate 101 may be, for example, a printed circuit board coupled by electrical conductors (not shown) on the printed circuit board to external leads of multi-chip module 100. In FIG. 1, bond wires 105aand 105bcouple bonding pads of the electronic circuit in semiconductor die 102 to electrical conductors on the printed circuit board. Similarly, bond wires 103aand 103bcouple bonding pads of the electronic circuit in semiconductor die 104 to electrical conductors on the printed circuit board.

[0011] A method for enhancing thermal performance of an integrated circuit package attaches a dummy die to the semiconductor die of the integrated circuit. The dummy die is then thermally coupled through external terminals to the conductive layers of a printed circuit board. In one embodiment, the integrated circuit package includes an insulating substrate on which the dummy die is attached. Conductive vias thermally connect conductive terminals provided on one side of the insulating substrate to conductive terminals provided on the other side of the insulating substrate. In one embodiment, the integrated circuit package is provided as a ball grid array (BGA) package. In addition, multiple layers of conductors can be provided in both the insulating substrate of the integrated circuit and in the external printed circuit board.

[0012] Because one can increase the size of the dummy die without redesign to increase the surface area from which heat can dissipate, and to allow the number of thermal vias that can be in thermal contact with the dummy die, a power dissipation problem discovered at a late stage of integrated design can be thus corrected very inexpensively.

[0013] The present invention is better understood upon consideration of the detailed description below and the accompanying drawings.

[0014] FIG. 1 shows prior art multi-chip module 100 having semiconductor die 104 bonded conventionally to semiconductor die 102.

[0015] FIG. 2 shows a cross section of a packaged integrated circuit 200 installed on an external printed circuit board 205, in accordance with one embodiment of the present invention.

[0016] To facilitate cross-reference among the figures and to simplify the detailed description, like elements in the figures are provided like reference numerals.

[0017] The present invention provides enhanced thermal performance of an integrated circuit by dissipating heat generated by an electronic circuit in a semiconductor die through thermal vias. FIG. 2 shows a cross section of a packaged integrated circuit 200 installed on external printed circuit board 205, in accordance with one embodiment of the present invention.

[0018] As shown in FIG. 2, packaged integrated circuit 201 includes semiconductor die 208 which contains an electronic circuit that dissipates significant amount of heat during operation. For example, in one embodiment, semiconductor die 208 includes a high speed (e.g., 10 gigabits per second) transceiver dissipating power at 1.1 watts (W). In addition, dummy die 209 is provided bonded in a conventional manner to semiconductor die 208. Preferably, dummy die 209 is bonded to semiconductor die 208 using thermally conductive bonding agent, such as a thermally conductive adhesive. Dummy die 209 need not include an operating electronic circuit. In this embodiment, packaged integrated circuit 201 is provided in a ball grid array (BGA) package which attaches to external printed circuit board 205 by an array of solder balls.

[0019] Semiconductor die 209 is isolated from the solder ball array by insulating substrate 211. Insulating substrate 211 may be, for example, a polyimide tape or a printed circuit board having one or more conductor layers (e.g., oxygen-free copper) embedded in or laminated on insulating substrate 211. Insulated substrate 211 has surfaces 211aand 211b. Surface 211aof insulated substrate 211 is provided conductive terminals that are in thermal contact with a surface of dummy die 209. Similarly, surface 211bof insulating substrate 211 is provided conductive terminals that are in contact with the solder ball array. The conductive terminals on opposite sides of insulating substrate 211 can be selectively connected by vias 210 in insulating substrate 211. Bond wires 207 connect bonding pads to designated conductive terminals on surface 211aof insulating substrate 211. These conductive terminals couple their respective signals from the electronic circuit on semiconductor die 208 to their respective solder balls in the ball grid array through vias in insulating substrate 211.

[0020] As shown in FIG. 2, printed circuit board 205 includes conductive terminals at its top surface and conductive layers 204a, 204band 204c, which can be interconnected by vias 206a, 206b, 206cand 206d. Under this arrangement, heat generated in semiconductor die 208 can dissipate through dummy die 209, which typically has a larger surface area than semiconductor die 208, to the external through insulating substrate 211, such as through vias 210 in insulating substrate 211. In addition, as shown in FIG. 2, the solder balls coupled to dummy die 209 by vias 210 can further dissipate the heat to the conductive layers 204a, 204b, 204cand 204dthrough vias 206a, 206b, 206cand 206d.

[0021] In one embodiment, packaged integrated circuit 201 is provided in a plastic BGA package that achieves a θ_JAof 27° C./W, which is formerly believed unachievable for a plastic BGA package of its dimension. In that embodiment, semiconductor die 208 is a 3.57 mm by 3.7 mm, 0.8 mm thick semiconductor die. The size of dummy die 209, and hence the number of thermal vias that can be accommodated, is traded-off against the bond wire lengths that are required to make connections to terminals on surface 211a. In that embodiment, the bond wire lengths are kept short to achieve the required high speed transceiver operations. As a result, dummy die 209 is a 6 mm by 6 mm, 0.8 mm thick semiconductor die. The 144 solder balls on the ball grid array are arranged in a square configuration at 0.8 mm pitch, so that there are 85 thermal vias that can be provided underneath dummy die 209, after providing adequate room for bonding electrical input or output signals between the electronic circuit on semiconductor die 208 and the solder balls according to a designated pin-out. The maximum wire length in this embodiment is under 2.2 mm.

[0022] The above detailed description is provided to illustrate specific embodiments of the present invention and is not intended to be limiting. Numerous variations and modifications within the scope of the present invention are possible. The present invention is set forth in the following claims.