1. Field of the Invention
The present invention relates to integrated circuit packages, and more particularly, to an interposer for a ball grid array (BGA) package having high thermal dissipation, a low coefficient of thermal expansion (CTE) and a high Young's modulus.
2. Description of the Related Art
In the last few decades, the electronics industry has literally transformed the world. Electronic products are used by, or affect the daily lives of, a large segment of the world's population. For example, telephones, televisions, radios, personal computers (PCs), laptop PCs, palmtop PCs, PCs with built-in portable phones, cellular phones, wireless phones, pagers, modems and video camcorders, are just a few of the electronic products that have been developed in recent years and which have been made smaller and more compact, while providing more functions than ever before and/or enhanced functions. The integrated circuit (IC) chip and the more efficient packaging of the IC chip have played key roles in the success of these products.
The IC chip is not an isolated island. It must communicate with other chips in a circuit through an Input/Output (I/O) system of interconnects. Moreover, the IC chip and its embedded circuitry are delicate, and must therefore be protected in a package that can both carry and protect it. As a result, the major functions of the IC package are: (1) to provide a path for the electrical current that powers the circuits on the chip; (2) to distribute the signals on to and off of the chip; (3) to remove the heat generated by the circuit; and (4) to support and protect the chip from hostile environments.
As ICs become more complex and printed circuit boards become more crowded, IC packages continually need more leads or pins while their footprints consume smaller and smaller areas. In an effort to meet these demands, developers created the ball grid array (BGA) package.
A typical BGA package includes an IC affixed to a flexible polyimide tape or interposer. A very thin conductor or wire bond connects a pad on the IC to a conductive trace on the polyimide tape. The conductive trace is routed to a solder ball. The solder ball is one of an array of solder balls mounted to the opposite side of the polyimide tape and protruding from the bottom of the BGA package. These solder balls interconnect with an array of pads located on a substrate, such as a printed circuit board. Accordingly, the typical BGA package electrically connects each pad on an IC to a pad on a printed circuit board.
A variation of the BGA package that has been introduced recently is the Area Tape Automated Bonding (ATAB) Ball Grid Array (BGA) package, or more commonly referred to as simply the Tape Ball Grid Array (TBGA) package. The TBGA package advantageously provides high lead counts, is thin, is lightweight, has high electrical and thermal performance, and has a BGA surface mount. The conventional TBGA package includes a tape containing a polyimide dielectric. At least one layer of the tape is formed into traces or conductors that interconnect a chip to a printed circuit board (PCB). See John H. Lau (Ed.), Ball Grid Array Technology, Chapter 14, “Area Tape Automated Bonding Ball Grid Array Technology” (McGraw-Hill, 1995), incorporated herein by reference.
One particular type of BGA package developed by Tessera is the micro-ball grid array (μBGA) package. The basic package typically includes a package interposer that is a 25 μm thick polyimide film with double-sided copper (Cu). One side of the Cu serves as a ground plane, which the other side has signal traces for I/O redistribution. A layer of silicone elastomer is positioned between the chip and the substrate. This compliant layer typically has a thickness of 150 μm. The first-level interconnects of the μBGA are flexible ribbons which are typically bonded on aluminum (Al) bond pads on the chip by a single-shot thermosonic process. The ribbons are typically 25 μm-wide soft gold (Au) leads with a thickness of 20 to 25 μm, bonded in a lazy-S shape so that they may accommodate any deformation due to thermal expansion. In order to protect the bonded leads, an encapsulant such as a silicone material is dispensed from the back side (between the chip and the interposer) after the lead bonding is completed. The package terminals of the μBGA may be plated bumps, solder balls or solid-core metal spheres. Further details describing the typical μBGA package may be found in John H. Lau, Chip Scale Package, Chapter 16, “Tessera's Micro-Ball Grid Array (μBGA)” (McGraw-Hill, 1999), incorporated herein by reference.
In a typical μBGA manufacturing process, the flexible tape interposer is first provided and tailored from a reel to mount as strips onto a metal frame. The elastomer layer is applied to the tape, and an adhesive material is deposited for die attachment. Die attachment is performed with an automated pick-and-place machine. Subsequently, ribbon leads are bonded to the Al die pads by a thermosonic process. Once the lead bonding is finished, a dry film resist is laminated to the interposer using a vacuum system. The encapsulant is dispensed from the back side, and the curing is performed to complete the encapsulation. The subsequent procedures include dry film exposure and developing, solder-ball attachment and reflow, cleaning, marking, and package singulation. Further details are described in Chapter 16 of John Lau's Chip Scale Package referenced above.
One problem with integrated circuits, including BGA packages, is that they require precise temperature control for efficient operation. Thus, if a package runs too hot, the heat can affect the performance and timing of the device. Accordingly, there is a need for an effective way to maintain control over the temperature of a device and keep it cool.
Another problem in BGA and similar packages is the mismatch in coefficient of thermal expansion (CTE) between the die and the tape or interposer containing the polyimide dielectric. The polyimide tape typically has a much higher coefficient than that of the die to which the tape is bonded. For instance, a die having a CTE of about 3 ppm/° C. may be coupled to a polyimide tape interposer having a CTE of about 20 ppm/° C. or more. This mismatch causes the tape to expand and shrink more rapidly than the die, thereby creating stress on the conductive leads connecting the solder ball array to the die. This stress can lead to breakage of the wire and a corresponding loss of electrical connection between the IC pads. The mismatch in CTE between the interposer and the die can also lead to delamination of the die attach or elastomer layer found therebetween. These problems result in lower yield rates and increase the overall cost of package manufacture.