Multilayer ceramics firing

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. 

S. Tosaka, S. Hirooka, N. Nishimura, K Hoshi, and N. Yamaoka, Properties ofa low temperature fired multilayer ceramic substrate , ISHM Proc. (1984), pp. 358. 

W. A. Vitriol andj. I. Steiaberg, "Development of a Low Fire Cofired Multilayer Ceramic Technology," 1983, pp. 593—598. 

The most significant commercial product is barium titanate, BaTiO, used to produce the ceramic capacitors found in almost all electronic products. As electronic circuitry has been rniniaturized, demand has increased for capacitors that can store a high amount of charge in a relatively small volume. This demand led to the development of highly efficient multilayer ceramic capacitors. In these devices, several layers of ceramic, from 25—50 ]lni in thickness, are separated by even thinner layers of electrode metal. Each layer must be dense, free of pin-holes and flaws, and ideally consist of several uniform grains of fired ceramic. Manufacturers are trying to reduce the layer thickness to 10—12 ]lni. Conventionally prepared ceramic powders cannot meet the rigorous demands of these appHcations, therefore an emphasis has been placed on production of advanced powders by hydrothermal synthesis and other methods. 

Ceramic boards are currently widely used in high-performance electronic modules as interconnection substrates. They are processed from conventional ceramic precursors and refractory metal precursors and are subsequently fired to the final shape. This is largely an art a much better fundamental understanding of the materials and chemical processes will be required if low-cost, high-yield production is to be realized (see Chapter 5). A good example of ceramic interconnection boards are the multilayer ceramic (MLC) stractures used in large IBM computers (Figure 4.11). These boards measure up to 100 cm in area and contain up to 33 layers. They can interconnect as many as 133 chips. Their fabrication involves hundreds of complex chemical processes that must be precisely controlled. 

Extraction Replication (Crystals In Fired Moly). The multilayer ceramic package that Is used by IBM has a sintering operation In which the alumlna/glass body undergoes slnterlng-wlth-a-llquld phase simultaneously with the firing of the molybdenum lands. 

Yonezawa, M., Low-firing multilayer capacitor materials, Am. Ceram. Soc. Bull., 62, 1375 (1983). 

Figure 9.5 shows an example of a large industrial continuous furnace. The classic use is for firing bricks, pottery, tiles, and whitewares. Similar furnaces are used in the production of advanced ceramics such as multilayer ceramic chip capacitors. 

Nishigaki, S., et al., A new low temperature fireable Ag multilayer ceramic substrate having post fired Cu conductor (LFC-II), ISHM 1986 Proceedings, pp. 429 37. 

A description of the application of ceramic and photopolymer technologies to achieve high-resolution electronic patterns follows. The first section discusses ceramic dielectric vias, and the second, conductive circuitry. Improved photosensitive ceramic coating compositions and more particularly, compositions that function as precursors to fired dielectric ceramics, are mainly useful in preparation of multilayer thick-film substrates. 

Ceramic and ferrite components such as multilayer ceramic capacitors, chip resistors, and chip inductors are generally terminated with a fired-on silver or silver palladium paste. Because silver dissolves easily into molten Sn-Pb solder, a Ni/Sn or Ni/Au overplate is recommended. 

The basic manufacturing process for multilayer ceramic substrates is shown in Figure 1.3 [16]. First, the ceramic powder and organic binder are mixed to make a milky slurry. The slurry is cast into tape using the doctor blade method, to obtain a raw ceramic sheet (green sheet) that before firing, is flexible like paper. Vias for conduction between layers and wiring patterns... 

To improve migration resistance, in Ag-Pd with Pd added to Ag, the palladium oxidizes in the atmosphere from 450°C to 800°C, and at high temperature it deoxidizes, transforming to metal palladium. The oxidation reaction at this low temperature causes volume expansion of the conductor element, and during the firing process, it is the cause of delamination of the multilayer ceramic [37]. In the same Ag-Pd composition, the degree of... 

S. Nishigaki et al., ANew Low Temperature Fireable Ag Multilayer Ceramic Having Post-Fired Cu Conductor (LFC-2), Proc. 1986 Int. Symp. on Microelectronics, 1986, pp. 429-449. 

Two types of sonar projectors are shown in Fig. 3.43. The Tompilz projector uses a stack of longitudinally poled piezoelectric slabs. The slabs are doughnut shaped so that a center bolt can be used to connect the head mass to the tail mass and prestress the multilayer ceramic stack. The slabs typically are metalhzed with thick-film fired-on metals, with thicker metal shims separating slabs for better current-carrying capability. The multilayer stack is electrically connected in parallel. 

The multilayer sensor structure consists of cermet and polymer based layers sequentially deposited on a 96% alumina ceramic substrate using a thick film screen printing process. The cermet layers are of ceramic-metal composition which require firing at a temperature of 850°C and the polymer layers are cured at temperatures below 100°C. Layout of this multilayer sensor structure is shown in Figure 1. 

TFML interconnections can be fabricated on a variety of substrates, including ceramics, metals, or silicon wafers. An approach proposed by Honeywell (8), which uses a multilayer co-fired ceramic substrate, is illustrated in Figure 1. The co-fired ceramic substrate is 50-100 mm square, with internal metal layers for power and ground distribution and pins brazed to the bottom for connection to a PWB. Metallized strips on the bottom of the substrate contact the PWB to conduct heat away from the package. A metal seal ring around the perimeter of the substrate permits hermetic sealing to provide mechanical and environmental protection for the chips and interconnections. 

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