Challenges in Bare Die Mounting

Traditionally, the evolution of advanced IC assemblies has been due to defense and aerospace applications, where reliability, size and weight were at a premium, and cost was a secondary consideration. In the 1980’s, high performance computing became the advanced packaging development stimulus. That world changed again in the late 1990’s with the emergence of ubiquitous digital content, providing impetus to the consumer markets of digital cameras, cellular phones, portable computers, PDAs and other similar high volume applications; these have now become the “driver” for advanced assemblies of semiconductors. Assemblers of state of the art consumer products are developing sophisticated packaging and interconnect approaches that more and more rely on the use of die products for these types of applications. Die products are defined as bare die, bumped die or “wafer level packaged die” that change the overall “footprint” on the mounting substrate when the die size changes. This paper reviews several of the die products technologies, and outlines mounting challenges to their use. Introduction Bare die mounting on multi-device substrates has been in use in the microelectronics industry since the 1960s. The aerospace industry’s hybrid modules and IBM’s Solid Logic Technology were early implementations that were developed in the 1960’s. The technologies progressed on a steady level until the mid 1990’s when, with the advent of BGA packaging and chip scale packages, the microelectronics industry started a wholesale move to area array packaging. This paper outlines the challenges for both traditional wire-bond die attached to a printed wiring board (pwb), to the more recent applications of bumped die attached to a high performance substrate. Die Product Technologies The several technologies that are considered die products are bare die with aluminum wire bondable I/O and power pads, bumped die with interconnections structures on the I/O and power pads, and wafer level packaged die that have a relaxed pitch,, mechanical protection on the surface of the die, optional rerouting of I/O and power pads, and formed with interconnection structures on the I/O and power pads. These are termed die products because they are sold to customers as a product and are available through normal IC distribution channels. Bare Die The most readily available form of die products are termed bare die. These die are identical to the die that are used in the vast majority of single chip packages today as they are wire bonded to a lead frame or interconnecting substrate. The use of wire bonded die mounted directly to the pwb is termed chip-on-board, (COB). COB is the most mature and largest part of the bare die market. Chip on board technology is characterized by the following factors: • Bare die mounted on interconnecting substrate or pwb • Die are mechanically attached to the substrate using conductive or non-conductive epoxies • Die are electrically connected to the substrate using wire bonding • Die are encapsulated with a protective shell Figure 1 shows a pwb with 4 chips mounted directly on the board. Figure 1: Bare die mounted directly on the surface of low cost laminate substrates is the highest use of bare die in products presently.