Flip Chip Bonding Technology

Next to the reflow of solder bumps, two other processes have been developed for bonding bumped integrated circuits to MCM substrates. A first approach uses 

A conductive epoxy adhesive is screen printed over the connecting pads of the wiring board on top of which the integrated circuit carrying gold stud bumps is flip 

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Torii, A. Takizawa, M. Sawano, M. The application of flip chip bonding technology using anisotropic conductive film to the mobile communication terminals. Proceedings of the International Electronics Manufacturing Technology/International Microelectronics Conference, Tokyo, Japan, April 1998 94-99. 

The most advanced implementation of cofired-ceramic-packaging technology is the thermal conduction module (TCM) used in large-scale computers (IBM) (4, 72, 74). This package can accommodate over 100 flip-chip-bonded ICs on a 90 by 90 mm cofired ceramic substrate. The multilayer ceramic substrate contains 33 metal layers for chip pad redistribution, signal interconnection, and power distribution (Figure 14). Each chip contains 120 bonding pads, and 1800 pins are brazed to the bottom of the substrate for connection to a PWB. 

Several approaches using electrically conductive adhesives instead of solder have been explored and are proving successful. Anisotropic adhesives (See Chapter 1), for example, have been used to connect flex circuits and cables from the separate driver circuits to the panel, avoiding the use of solder connections. More importantly, integrated-circuit chips can be bonded directly to the ITO conductor traces on the panel, a technology called chip-on-glass (COG). IC chips can be flip-chip bonded, then underfilled with a stress-free underfill adhesive if necessary. For protection, the chips may then be encapsulated with epoxy (glob-topped). 

Other electrical interconnection techniques such as wafer bumping and flip-chip bonding have emerged [13]. As shown in Fig. 3 (courtesy of Amkor Technology, Inc.), small metal balls... 

Several high density, but reliable termination technologies such as Ball Grid Array (BGA) and Flip Chip Bonding have been intrO duced for the high density rigid/flex. 

Other electric interconnection techniques such as wafer bumping and flip-chip bonding have emerged [11]. As shown in Fig. 3 (Courtesy of Amkor Technology, Inc.), small metal balls (Ag/Sn or Sn/Pb) are directly electroplated on the metal pads of the device chip. The chips are then flipped over and bonded to a printed circuit board (PCB) or a substrate with prefabricated metal connections. An epoxy underflll is then used to fill the space... 

Vuorela, M. Holloway, M. Fuchs, S. Stam, F. Kivilahti, J. Bismuth-filled anisotropically conductive adhesive for flip-chip bonding. Proceedings of the 4th International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Helsinki, Finland, June 2000 147-152. 

Connell, G. Zenner, R.L.D. Gerber, J.A. Conductive adhesive flip-chip bonding for bumped and unbumped die. Proceedings of the 47th Electronic Components and Technology Conference, San... 

Novel Approaches. A number of groups have focused on silicon substrates for thin-film interconnections (89, 91-93, 97). The most advanced technology of this type is AT T s advanced VLSI package (AVP), which uses flip-chip solder bonding of ICs onto a silicon substrate containing high-... 

Packaging (and Wire Bonding), Fig. 3 Flip-chip technology for electrical interconnection... 

ACA flip chip technology has been employed in many applications where flip chips are bonded to rigid chip carriers (13). This includes bare chip assembly of ASICs in transistor radios, personal digital assistants (PDAs), sensor chip in digital cameras, and memory chip in lap-top computers. In all the applications, the common feature is that ACA flip chip technology is used to assemble bare chips where the pitch is extremely fine, normally less than 120 /rm. For these fine applications, it is apparently the use of ACA flip chip instead of soldering which is more cost effective. 

Flip-chip technology, as shown in Fig. 11.14, is similar to TAB technology in that successive metal layers are deposited on the wafer, ending up with solder-plated bumps over the device contacts. One possible configuration utilizes an alloy of nickel and aluminum as an interface to the aluminum bonding pads. A thin film of pure nickel is plated over the Ni/Al, followed by copper and solder. The copper is plated to a thickness of about 0.0005 in., and the solder is plated to about 0.003 in. The solder is then reflowed to form a hemispherical bump. The devices are then mounted to the substrate face down by reflow solder methods. During reflow, the face of the device is prevented from contacting the substrate metallization by the copper bump. This process is sometimes referred to as the controlled collapse process. 

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