The industry trend to shrink semiconductor device packages while increasing I/O count results in demands on the printed circuit boards to which these devices are assembled. An increase in wiring density is required at the board level, often equating to higher costs and/or layer counts. In many cases, designers are looking to multilayer flexible circuits as the next level of interconnect for a wide range of packaging technologies from fine-pitch surface mount to chip-scale packages to direct-chip attach. Challenges associated with a flex-based solution include assembly compatibility, material costs and reliability. This paper reviews work by Teledyne Electronic Technologies (TET) to address these board level interconnect challenges for several custom applications. An approach employing flexible multilayer substrates which incorporate fine-line circuitry, small vias and flexible soldermask is examined. A test vehicle designed and built to accommodate micro-BGA packages is used to illustrate the density achieved and to provide a platform for demonstrating assembly and reliability attributes. This test vehicle is also being used to evaluate materials and process technologies applicable to high density SMT assemblies. (Keywords: rigid-flex, microBGA, flex innerlayer, via-in-pad, microSMT, MCM-L) TET is a major supplier of electronic components, assemblies and systems to a wide variety of applications in the military and commercial sectors. These products are generally characterized by a high level of complexity, in terms of both functionality and physical packaging. Examples of customer end products include avionics, medical implantables and high capacity computer disk drives. Although it draws upon a broad spectrum of enabling technologies, two of TET’s most widely used, internally developed technologies are ceramic-based hybrids/multichip modules (MCM-C) and rigid-flex printed circuits and assemblies. Figure 1 shows a current high volume SMT product- an eight-layer rigid-flex with an average pad density of 115 pads/in 2 (18 pads/cm 2)
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