Flip-chip applications carriers

TABLE 4 Low-Cost, Heat-Sensitive Chip Carriers Utilized in Polymer Flip-Chip Applications Applications of polymer flip-chip bonding... 

FIG. 17 A schematic depicting flip-chip application utilizing chips with micromachined polymer bumps, (a) Process flow for creating micromachined polymer bumps in the wafer state, (b) Die attachment to a chip carrier. 

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. 

ACA Bumped Flip Chips for High Frequency Applications. The high frequency behavior of ACA interconnections has attracted much attention in the past several years. Sihlbom and co-workers demonstrated that ACA-bonded flip chips can provide performance equivalent to solder flip chips in the frequency range of 45 MHz to 2 GHz on FR4 chip carriers and 1 to 21 GHz on a high frequency Telfon-based chip carrier (Fig. 5). The different particle sizes and materials in the conductive adhesives gave little difference in high frequency behavior of ACA joints (22,23). 

A second flip chip technique is realized by the use of eutectic Pb-Sn or other low-melting-temperature solder bumps in place of high-Pb solder bumps. These chips can be directly reflow attached to the chip carrier using low temperature. In this application, the chip carrier can be either ceramic or an organic laminate. 

For aqueous clean fluxes, isomers of carboxylic acids, solvents, and other ingredients can be selected for heat stability in the temperature ranges of interest [154], tackiness, and aqueous solubility. For no-clean applications, the activators and solvents can be chosen so that by optimizing flux quantity, very little residue remains, and postreflow oxygen plasma treatment can offer an effective nonsolvent method for removing low-residue solids associated with a no-clean flux in assembling flip chips on organic carriers [155]. 

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