Substrates LTCC

Figure 1.6 Passive components embedded in substrates [LTCC (a), and printed resin board (b)].

As the processing is compatible with LTCC processing, development of fuel cells cofired into LTCC substrates are feasible. 

MCM-C interconnect substrates are produced from either low-temperature cofired ceramic (LTCC) or high-temperature cofired ceramic (HTCC). Either process can produce multilayer substrates having high numbers of conductor layers (up to 100), although for most applications 2-20 layers are sufficient. The fabrication of MCM-C involves thick-film processes that have wider lines and spacings (5-20 mils) than MCM-D, but are lower in cost. 

The reaction between Si02 and PbO is relevant for PZT films on Si containing substrates like 96% Alumina, Si wafer and LTCC. The existence of silicon oxide in tire substrate and in commercial electrode materials causes the diffusion into the PZT thick film and the reaction to Pb based silicates, which deteriorate ftie ferroelectric behavior of the PZT thick film. Therefore a special Au electrode was developed by Fraunhofer KTS with modified composition preventing silicate formation during firing of the PZT thick film. For ZtOr and AI2O3 substrates a commercial Au electrode (Heraeus 5789) fired at 850°C/ 30 min can bee used. 

Kyocera has been conducting research and development on AIN and LTCC for more than 10 years, and has produced many kinds of AlN and LTCC products, such as Cer-Quad and multilayer packages, in addition to thin film substrates. Now, we have developed three materials for packages. The first is a novel AlN material (AN75W) that can co-fired at low temperature to reduce cost. The second is a novel LTCC that has a high thermal coefficient of expansion close to that for FR-4. The third is also a novel LTCC that has low permittivity... 

Transmission characteristics of the LTCC substrate were compared to that of alumina with tungsten metallization. The substrate used in this measurement has a microstrip line configuration (Figure 1.1.18). Figure 1.1.19 shows the measured results of insertion loss (S21). The frequency range was from 0.1 to 50 GHz. The insertion loss of the LTCC was —0.35 dB/cm at 30 GHz, and was... 

The discussion of lead properties of course does not apply to leadless devices such as leadless ceramic chip carriers (LCCCs). Design teams using these and similar packages must understand the better heat transfer properties of the alumina used in ceramic packages and must match coefficients of thermal expansion (CTEs or TCEs) between the LCCC and the substrate since there are no leads to bend and absorb mismatches of expansion. Use of ceramic devices may lead the design team to consider the use of Low Temperature Co-fired Ceramic (LTCC) substrates and assembly technologies XXX. 

Photographs of a thin-film interconnect module on a 14-layer LTCC Substrate, (a) Partially assembled completed substrate and (b) with an assembled large BGA microprocessor on the substrate. Note the attached passive devices as well as sites for the FPGA, ASIC, and memory chips. Each end of the substrate is composed of -1000 sites for z-axis interconnects. 

Figure 1.3 shows an example module with thin-film interconnects on a low-temperature cofired ceramic (LTCC) substrate [32]. This module measures 81 X 55 X 1.88 mm and has a 14-layer cofired LTCC substrate with silver conductor and 6-layer thin-film interconnects (three on the top and three on the bottom) using Ti/Cu and benzocyclobutane (BCD) structure. A microprocessor is connected to 2 memory chips and over 140 passive components through about 4000 nets. 

Figure 1.4 shows the surface profiles of multilayer LTCC substrates imder different conditions as-fired, lapped, and polished, and in contrast to a Si wafer. 

Surface profiles of LTCC substrates (a) as-fired, (b) lapped, (c) polished, and (d) in contrast to a Si wafer. AH vertical scales are in angstroms. 

This module (Figure 1.23) was an early application of LTCC for wireless products and demonstrated the cost-effectiveness of LTCC in commercial production volumes. The module is a VCO/synthesizer with surface-mounted components, an embedded resonator, and embedded inductors and capacitors. The substrate is about V2-in. square and has eight conductor... 

It has not been unusual for LTCC substrate manufacturers to be presented with designs in which the overall part size cannot be changed. The cost of LTCC multilayer circuits is very dependent on overall part dimensions and the relationship to the foundry panel size, panel tooling, and the usable area within the panel. It is important to maximize the total number of parts within the usable area to achieve the lowest possible unit cost. A small dimensional increase could eliminate a row or column of parts or produce excess waste... 

The dielectric constant of LTCC materials is somewhat lower than for the standard substrate materials because of the high glass content. This feature minimizes stray capacitance and cross-coupling of signals. 

HTCC is an all-inclusive term to describe ceramic substrates that are consolidated at temperatures above about 1000°C. Applied to electronic packaging, this descriptor includes aluminum oxide, aluminum nitride (AIN), and a variety of other developmental or seldom-used materials. Until recently, discriminating between HTCC and low-temperatme cofired ceramics (LTCC) was elementary, as the firing temperatures differed by roughly 600°C. To confoimd that difference, an intermediate-firing multilayer ceramic, or medimn-temperature cofired ceramic (MTCC), has recently been introduced. Details on the processing and properties of this material will be discussed in Section 6.2 and Section 6.4. 

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