Design for Flip-Chip and Chip-Size Package Technology

As new generations of electronic products emerge they often surpass the capability of existing packaging and interconnection technology and the infrastructure needed to support newer technologies. This movement is occurring at all levels: at the IC, at the IC package, at the module, at the hybrid, the PC board which ties all the systems together. Interconnection density and methodology becomes the measure of successfully managing performance. The gap between printed boards and semiconductor technology (wafer level integration) is greater than one order of magnitude in interconnection density capability, although the development of fine-pitch substrates and assembly technology has narrowed the gap somewhat. All viable efforts are being used in filling this void utilizing uncased integrated circuits (flip-chip) and incorporating more than one die or more than one part in the assembly process. This paper provides a comparison of different commonly used technologies including flip-chip, chip-size and wafer level array package methodologies detailed in a new publication, IPC-7094. It considers the effect of bare die or die-size components in an uncased or minimally cased format, the impact on current component characteristics and reviews the appropriate PCB design guidelines to ensure efficient assembly processing. The focus of the IPC document is to provide useful and practical information to those who are considering the adoption of bare die or die size array components. Introduction The flip-chip process was originally established for applications requiring aggressive miniaturization. Initially developed by IBM for the Solid Logic Technology (SLT) hybrid electronic circuitry for their System 360 computers, the die were very small and had very few contacts. The perimeter located bond sites were, as they are to a degree today, prepared with alloy bumps enabling the uncased semiconductor die to be bonded directly to a substrate using surface mount processes. In developing the IPC-7094, „Design and Assembly Process Implementation for Flip-Chip and Die Size Components’, a number of experts were assembled to address the industries need for practical and up-to-date information and guidance in how to employ the uncased as well as minimally packaged semiconductor elements being offered to the commercial market. A number of miniature package solutions have evolved as well that retain an outline that is the same or very near the same as the die element. The uncased die element, when processed for in the wafer format for board level attachment, is identified as a wafer level BGA (WLBGA). A family of die-size BGA (DSBGA) is a component that employs a package assembly process utilizing an intermediate interposer to add a level of physical robustness, enable more efficient electrical test, burn-in and board level assembly. Because of the rapid movement toward semiconductor package miniaturization, members of the Joint Electronic Device Engineering Council (JEDEC) developed a number of standards to help maintain a degree of uniformity for the semiconductor devices final configuration. Its membership includes semiconductor manufacturers, packaging services companies, material suppliers and users. The organizations JC-11 subcommittee is responsible for developing the mechanical outline requirements for solid state semiconductor packaging. The two standards that the IPC-7094 document has focused on are JEDEC 95 Publication „Design Guide 4.18‟ (WLBGA) and „Design Guide 4.7‟ for DSBGA. Their definition is as follows:  A Wafer Level Ball Grid Array (WLBGA) has an array of metallic balls on the underside of the package. The substrate of the package is the semiconductor die with or without a redistribution layer that may have a square or rectangular shape with metallic balls applied onto the circuit side of the die. The array pattern of metallized balls provides the mechanical and electrical connection from the package body to the next level component such as a printed circuit board or intermediate substrate.  A Die-size Ball Grid Array (DSBGA) also has an array of metallic balls on the underside of the package. The die elements are, however, attached to a dielectric substrate or carrier for redistribution of the die bond pads to a uniform contact array pattern. The package substrate may have a square or rectangular shape with a metalized circuit pattern applied to one or both sides of a dielectric structure. The size of the substrate or carrier is as close to the die size as practically possible. The contacts on the WLBGA and DSBGA are balls, bumps or other protruding terminals constructed from a variety of alloy and/or polymer materials. The alloy ball or bump features are added to each contact site to provide the mechanical and electrical connection from the package body to the next level component such as a printed circuit board. Typically, when balls are present, they consist of eutectic lead/tin solder or a lead-free solder alloy. For purposes of the JEDEC documents, these contacts of whatever final form are referenced generically as “balls”. In regard to accommodation for board level assembly processing, the perimeter bumping process is generally limited to mounting uncased die having a relatively few contact sites. Larger die requiring higher pin count, will likely be processed to redistribute the peripheral located wire-bond sites to a uniform area array format. When the contact sites are arranged in the array package format it furnishes a more uniform interface configuration that better accommodates component level test, better access for circuit routing and is more compatibility with established surface mount assembly processes. The profile height of the WLBGA and DSBGA devices are very thin. The finished devices are not encased in plastic typical of the coarser pitch BGA families, however, some suppliers may furnish these miniature components with a thin polymer coating to protect the exposed silicon surface. The actual height of the array device is defined as the distance from the seating plane (the surface of the mating circuit board) to the highest point on the package body. This distance is measured perpendicularly to the seating plane. Applications Hand-held communication and entertainment products will continue to dominate the consumer markets worldwide and, with each generation offering more and more features and/or capability, system level integration and miniaturization becomes more of a priority. And even though the actual applications and functionality of the new product offering expands, the customer is expecting each generation to be smaller and lighter that its predecessor. Semiconductor elements widely targeted for improving package performance and package size reduction include both logic and memory. High volume consumer products requiring greater memory component density include:  MP3 players, portable media players  Memory cards (mini SD, SD, MMC etc.)  Digital Still Cameras (DSC)  Handheld video games  Solid State Drives for mobile computing Consumer applications may include a number of the functions unique to the product. For wireless handsets, for example, companies are looking for miniature package solutions for all functions. Array packaging has evolved as the most direct method for package-to-board interface since there is no intermediate leadframe required. The die prepared for flip-chip applications enables increased circuit density which can contribute to improved electrical performance. When comparing the WLBGA and DSBGA semiconductor packaging to the QFP lead-frame package, the surface area differences are quite dramatic. Table 1 provides a comparison of different semiconductor packaging methods having a common 10 mm x 10 mm die element with 100 active contact features. Table 1: Comparative Table of Various Technologies for a 100 I/O, 10x10 mm Size Die