Page 314 - System on Package_ Miniaturization of the Entire System
P. 314

288    Cha pte r  F i v e


                          1.5
                          1.0
                          0.5
                        Capacitance change (%)  −0.5 −60  −40  −20  0  20  Pure BCB  60  80  100  120
                          0.0
                                                             40

                         −1.0
                         −1.5
                                                         20% Ta O
                                                               2
                                                         30% Ta O
                                                               2
                         −2.0
                                                         40% Ta O 5 5 5
                                                               2
                         −2.5
                         −3.0
                                                      Temperature (°C)
                    FIGURE 5.24  Temperature dependence of capacitance of BCB-based composites with various
                    Ta 2 O 5  contents. [55]
                    characteristics. Ta O  was therefore added to BCB polymer to improve the temperature
                                   2
                                     5
                    stability of capacitance, along with improving the capacitance density as shown in
                    Figure 5.24.
                    5.4.6 Resistors

                    Resistor Technologies
                    In general, the RF resistors can be achieved by three major process technologies: screen-
                    printing of polymer thick film (PTF), electroless plating, and direct foil lamination.
                       The polymer thick films have instability problems in the microwave frequency
                    range due to their moisture absorption. However, these thick films offer much higher
                    resistance that is suitable for the pull-up, pull-down, and circuit isolation. As such, PTF
                    is a mature technology that has been applied into numerous products [56]. For example,
                    Motorola uses carbon-phenolic polymer thick-film (PTF) ink and a screen-printing
                    process to form resistors in the inner layers of high-density interconnect printed wiring
                    boards. The PTF ink is screened on to copper termination pads that are treated with
                    proprietary interface metallurgy to improve reliability and environmental stability. In
                    Motorola product applications, between 8000 to 20,000 resistors have been printed on
                    an 18 in × 24 in panel in a single screening step, resulting in significant economies of
                    scale and associated cost savings. PTF resistors can be trimmed at the inner-layer stage
                    to within 1 percent tolerance.
                       In contrast to the above thick-film resistors, the thin-film resistors are achieved by
                    electroless plating and direct foil lamination [57]. The electroless plating involves
                    surface preparation of the dielectric medium followed by chemical treatment in order
                    to deposit a thin (usually 0.3 to 1 μm) resistive layer that then can be patterned, and the
                    stubs can be plated to define resistors. Figure 5.25 shows such a resistor comprised of
                    NiWP on an epoxy dielectric [58].
                       The high-frequency measurements are typically performed with an HP 8510C
                    vector network analyzer and ground-signal-ground (GSG) coplanar waveguide 200-
                    μm-pitch probes. The results of structures with NiP/NiWP fabricated without a ground
   309   310   311   312   313   314   315   316   317   318   319