Polymer flip chip

To reduce the cost of flip chip interconnection, an organic process was developed at Epoxy Technology Inc. [81]. The polymer flip chip technology (PFc ) a polymer dielectric paste and an electrically conductive adhesive. 

As with virtually all flip-chip processes, the A1 bond pads must be protected to eliminate the formation of nonconductive aluminum oxide. This insures a low and stable resistance at bond-bond pad interface. The polymer flip-chip process utilizes an electroless plating technique, Ni/Au... 

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

Seidowski, T. Kriebel, F. Neumann, N. Polymer flip chip technology on flexible substrates— development and applications. Proceedings of the 3rd International Conference on Adhesive Joining and Coating Technology in Electronics Manufacturing, Binghamton, NY, September 1998 240-243. 

Properties of nanofillers recently developed nano materials are reported to display greater mechanical strength, greater thermal conductivity and improved electrical performance when compared to materials of normal particle sizes. Nano dimensional materials are being studied as fillers in polymer matrices in a variety of formulations for electrically conductive adhesives, thermally conductive adhesives, encapsulants, printed circuit boards, coatings, catalysts, underfills for flip-chip-attached devices and wafer-level connections. ... 

In a similar process, known as polymer-film interconnect (PFI), an insulative thermoplastic film is laminated over the devices at the wafer stage, and vias are opened over the bonding pads using a laser. At that point, either the normal solder bumps can be formed or a silver-filled conductive adhesive can be stencil printed into the vias to form polymer bumps. After printing, the epoxy is B-staged and the flip-chip devices are diced. In assembly, the devices are heated to a temperature that completes the cure of the B-staged bumps and simultaneously reflows the thermoplastic underfill material. 

Electrically conductive adhesives, primarily silver-filled epoxies, are finding uses as replacements for solder in surface-mounting components on printed-circuit boards and in flip-chip attachments. There are several driving forces for this application, a major one being the trend to eliminate lead and tin-lead solders because they may be health hazards. Also associated with the use of solder, is the need to eliminate ozone-depleting solvents presently used to clean and remove flux residues. Electrically conductive polymer... 

Yim and co-workers developed a microwave frequency model for ACF-based flip chip joints based on microwave network analysis and S-parameter measurements. By using this model, high frequency behavior of ACF flip chip interconnections with two filler particles, Ni and Au-coated polymer particles, was simulated. It was predicted that Au-coated polymer-particle-fllled ACF flip chip interconnections exhibited comparable transfer and loss characteristics to solder bumped flip chips up to about 13 GHz and thus they can be used for up to 13 GHz, but Ni-filled ACF joints can only be used for up to 8 GHz because the Ni particle has a higher inductance compared to the Au-coated particle. Polymeric resins with a low dielectric constant and conductive particles with low inductance are desirable for high resonance frequency applications (24). 

Conductive Columns. Nitto Denko Corp. developed an ACF for fine pitch flip chip applications (27). The features of this ACF were (1) connectability between bumpless chips and fine pitch PCB (2) high electrical conductivity (3) repairabil-ity (easy to peal off chips from a printed circuit board at elevated temperatures) (4) high reliability and (5) potential storage at room temperature. There are other notable features too (7) ACF is usable at pitches down to 25 fim, (2) the conductive elements are micrometallic columns as opposed to random-shaped particles, and (3) this adhesive matrix consists of a thermoplastic polymer resin, conductive columns coated with an insulator, and a high Tg polymer, which completely separates the columns from the adhesive (Fig. 6). 

Underfill. An underfill is then injected into the gap between the chip and chip carrier and then cured to complete the flip chip process. The function of the underfill or encapsulation as it is sometimes referred to is to provide mechanical integrity and environmental protection to a flip chip assembly. Studies have demonstrated that both thermoset and thermoplastic ICAs can offer low initial joint resistances of less than 5 mS2 and stable joint resistances (Au-to-Au flip chip bonding) during all the accelerated reliability testing listed in Table 1. The reliability results have indicated that there is no substantial difference in the performance of thermoset and thermoplastic bumps and both types of polymers apparently offer reliable flip chip electrical interconnections (53). 

Fig. 14. Schematic depicting a flip chip technology utilizing chips with micromachined poljrmer bumps, (a) Process flow for creating micromachined polymer bumps in the wafer state, (b) Die attachment to a chip carrier.

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