As with plastic packaging technologies, Japan leads the world in the production of ceramic packages. Over 80% of all ceramic packages used in noncaptive markets are supplied by three large Japanese companies: Kyocera, NTK, and Sumitomo Metals and Alloys. In addition, companies supplying their own in-house needs for ceramic packages include Shinko, Ibiden, NEC, Hitachi, Matsushita, Oki, Toshiba, and others. The primary ceramic package applications today fall into two categories: single-chip and multichip packages. Single-chip PGAs are the dominant ceramic package application, with a market estimated at $1.2 billion in 1993 and projected to grow to $2.0 billion by 1997. The highest-volume applications for PGA packages include computer and telecommunications products. The market for multichip packages was about $670 million in 1993, half the size of the PGA market. But multichip packages are expected to surpass the PGA market by 1997 with sales of over $3.0 billion. The growth in multichip packaging is driven by the increased demands of advanced personal computers, workstations, mainframes, supercomputers, and evolving HDTV applications.
Like plastic packaging development, ceramics have also had improvements in materials, design, and process technologies. The state-of-the-art in ceramic packaging, plotted in Figure 4.7, shows the current wiring dimension to be about 50 micron vias and lines, spaced about 100 micron apart. The number of layers used in ceramic packages is typically 20; however, 61 layers have been demonstrated. IBM uses a 63-layer multichip module (MCM) in its R6000 workstation.
Figure 4.7. Ceramic packaging trends.
Table 4.7 shows the variety of materials being used with ceramic substrates. These materials include Al(sub)2O(sub)3, AlN, mullite, and a variety of glass-ceramics that include both glass added to alumina and crystalizable glasses. Whereas most of these low-temperature ceramics are metalized with Ag, Ag/Pd, or Au as fired in air, a few firms are beginning to co-fire with copper using special binders, or special atmosphere cycles, to remove organics from greensheets. Table 4.7 illustrates the properties of some of the glass-ceramics being pursued by Kyocera, Panasonic, Oki, Fujitsu, NEC, and NTK.
Japanese Ceramic Substrate Materials
Of particular interest to the consumer electronics industry is the aspect of ceramic technology often referred to as LTCC (low-temperature ceramic carrier), currently metalized with Ag/Pd and co-fired with capacitors and resistors. The LTCC illustrated in Figure 4.8 was designed and developed for use in VCRs and other consumer video products by Panasonic at its plant in Saijo.
Figure 4.8. Consumer ceramic substrate (Panasonic).
Panasonic also designed and developed a low-cost process technology for co-firing with copper that is illustrated in Figure 4.9. This process involves forming thick films with CuO and co-firing in air to initially remove organics from greensheets and paste, and then reducing the oxide in forming gas, and finally forming a bond between glass that flows from ceramic walls and slightly oxidized copper in the via in Nitrogen atmosphere.
Figure 4.9. Low-cost ceramic co-firing process with copper (Panasonic).
Panasonic package designers expect to integrate capacitors and resistors into the substrate in the near future. Figure 4.10 illustrates glass ceramic/copper with surface resistors currently practiced by Panasonic.
Figure 4.10. Consumer ceramic substrate with Cu (Panasonic).
The large companies in Japan like NEC, Hitachi, Toshiba, and Fujitsu understand the importance of ceramic packaging in relation to other technologies, since they utilize the full compass of packaging technologies for products that range from camcorders to supercomputers. Figure 4.11 compares Kyocera's design rules for PWB and ceramic technologies. The leading companies perceive laminated MCMs based on PWB technology to be low cost, but it makes the high wiring density needed for high performance difficult to achieve. Japanese firms believe they can realize high density, but only at a higher cost, as with ceramics.
Figure 4.11. Multilayer ceramic (MLC) and printed wiring
board (PWB) compared (Kyocera).
Thin-film technology has been demonstrated and used by NEC in its mainframe and supercomputers, but only at very high cost. NEC believes ceramic MCM technology has the best potential; where it lags, NEC plans to supplement with surface thin-film layer or layers to achieve the required redistribution and wiring. NEC understands the cost implications of the technologies but has only recently been able to develop and deliver the ceramic greensheets. A material that NEC recently developed, G100, has a dielectric constant of 4.4 and is co-firable with gold, having a thermal expansion coefficient of 4.5 x 10 (to power)-6 per degree C. The material used in accomplishing this 4.4 dielectric constant consists of 15% quartz glass plus 20% cordierite plus 65% borosilicate glass. Fired in air at 900 degrees C, the material has a mechanical strength of 1600 kg/sq. cm. The ceramic packaging technology roadmap followed by NEC during the last fifteen years is illustrated in Table 4.8.
NEC's Ceramic Roadmap
NEC is exploring ways to achieve an even lower effective dielectric constant. One approach is to develop hollow space between ground and signal planes (Fig. 4.12). Plotting dielectric constant and signal propagation delay for various NEC materials, including the new material with and without the above hollow structure (Fig. 4.13), shows that 4.4 material behaves normally with signal propagation delay of 7.6 nsec/m, but the hollow structure behaves as if it has dielectric planes. The transmission properties are greatly influenced by the configuration of ground plane and wiring plane open area and signal line channel.
Figure 4.12. Hollow structure in ceramic
for improved dielectric constant.
Figure 4.13. Propagation delay versus dielectric constant (NEC).
As illustrated in Figure 4.14, the rectangular open area seems to give constant overlap area between ground pattern and signal line, thus minimizing the capacitance variation that is typical of conventional square mesh patterns. NEC has applied this new material for a medium-cost, 156 Mb/s optical inter-connection module. In this application this technology has reduced (1) the number of modules to one from two from previous SMT/PWB technology, (2) the volume size by one-seventh, and (3) the power consumption by one-third. The total size of the final unit is 10 cc. NEC expects to use this new ceramic package in its personal computers as an interface to the CRT.
Figure 4.14. Package design improvement.
Use of aluminum nitride is much more emphasized in Japan than in the United States. Ibiden, Kyocera, NTK, NEC, Toshiba, and Sumitomo Electric are some of the firms that have invested very heavily in developing and producing AlN pin grid arrays, as well as high-thermal-conductivity heat sinks. Ibiden, for example, makes a large substrate by hot pressing to achieve a thermal conductivity of 180 W/mk. NEC claims to have achieved as high as 200 W/mk and has transferred the technology to its subsidiary. The Japanese market for AlN is currently estimated at $100 million per year, much less than previously projected. The primary reason for this slow growth has been attributed to the twenty to thirty times higher raw material cost over standard alumina and the one-half to two times higher substrate cost. NEC expects to use AlN for high-performance applications requiring higher thermal conductivity, where the additional cost can be justified.
A new ceramic substrate approach being pursued by Ibiden involves bonding epoxy-glass FR-4 layers to porous ceramic. Such a structure, illustrated in Figure 4.15, improves TCE, dielectric constant, and mechanical strength, and permits direct flip chip bonding.
Figure 4.15. Ceracom substrate with low TCE and low dielectric constant (Ibiden).