Polymer chip, bonding

Although the current membranes are synthesized on glass chips to allow easy inspection and testing, the same technology can be transferred to hard polymer chips, which should be cheaper to produce and easier to bond. This remains the final obstacle to commercialization. 

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). 

As a biocompatible coating [16], parylene C has recently been explored as a structural and packaging material for miniaturized biomedical devices. Parylene protects the sensors and serves as electrical isolation. In order to package an array of sensors in a catheter, a microelectric interconnect has been established via a flexible polymer (Fig. 10). The electrical leads are attached to sensors embedded in parylene film (Fig. 10b). Platinum or gold wires are sandwiched in the parylene film to make the connections to the sensors either by wire bonding or flip-chip bonding (Fig. 10c). 

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

Although Parylene-N possesses an outstanding combination of physical, electrical, and chemical properties, the benzylic C—H bonds present are potential sites for thermal and oxidative degradation. It is well known that replacing a C— bond with a C—F bond not only enhances the thermal stability of the resulting polymer, but also reduces the dielectric constant. Because incorporation of fluorine is known to impart thermal and oxidative stability, it became of interest to prepare poly(a,a,a, a -tetrafluoro- p -xylylene), Parylene-F Joesten reported that the decomposition temperature of poly(tetrafluoro-j9-xylylene) is ca. 530°C. Thus, it seemed that the fluorinated analog would satisfy many of the exacting requirements for utility as an on-chip dielectric medium. 

The above concept of forming adhesive bonds in the solid state has been used to demonstrate the possibility of the parallel processing of a multi-chip module substrate, consisting of a multilayered polymer substrate with circuitry embedded on each polymer layer via lithographic processing [43], In this case, it is essential that the polymeric layer retains its dimensional stability so that registration and interconnections between the layers can be achieved using a Pb-Sn solder (see Fig. 28). A copolyester which appears to be ideally suited for this purpose is the 4/1 PHBA/BPT which melts at 320 °C in the randomized form... 

For chips mounted face up, heat is transferred to the substrate by conduction through the interconnection layers, and because the polymer dielectric has poor thermal conductivity, heat conduction is often promoted by an array of metallized vias through the interconnection layers (Figure 16) (100). For face-down-mounted chips, the heat may be removed from the back side by using pistons (as in the TCM) or conductive fluids, or heat may be conducted through the solder bonds to the interconnection substrate (98). 

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