This invention relates to a high density large scale integrated (LSI) package for logic circuits for use in data processing and communications systems.
LSI packages for logic circuits are indispensible to realize faster and more compact systems. As they become more compact, packages having active elements such as transistors require a structure to diffuse the heat generated by electric power consumption. A multi-chip package having such a heat diffusing structure is proposed in a paper by Hajime Nakamura et al. entitled "Manufacturing Technology of High Circuit Density Multi-Layer Substrates", IEEE, TRANSACTIONS ON COMPUTERS, HYBRIDS, AND MANUFACTURING TECHNOLOGY, VOL. CHMT-4, No. 2, June 1981 issue, pp. 333-337.
The substrate of the package illustrated in FIG. 10 page 337 of this paper is composed of an alumina ceramic. Multiple signal wiring layers and a power bus layer are formed on the surface of the substrate. An IC chip is mounted over the substrate is connected to these wiring layers. To the undersurface of the substrate a heat sink is attached for diffusing the heat from the IC chip.
The multi-layer wiring substrate of this structure has the following disadvantages.
First, for each of the insulating layers among the multiple wiring layers, printed and baked inorganic dielectric paste of glass ceramic type is used. The relative dielectric constant .epsilon.r of inorganic dielectric paste usually ranges between 8 and 9. Accordingly, the signal propagation delay time td per unit length of the signal wiring formed on this insulator is calculated from ##EQU1## (where C is the velocity of light, which is 3.0.times.10.sup.10 cm/sec). Using this expression, td is found to be 9 nanoseconds (ns)/m. Thus, the signal propagation delay time of this wiring is more than double that of usual coaxial cables, which is 4 ns/m.
Second, the conductor wiring layers are formed, for instance, of gold paste, by screen printing. As a result, the wiring width and wiring pitch can be no smaller than 50 microns and 100 microns, respectively.
Third, the heat generated by the IC chip is diffused to the heat sink on the other side through the alumina ceramic substrate. The alumina ceramic substrate usual is about 2 mm thick so as to be strong enough to endure the fabricating stress given for the conductive and dielectric layers due to the temperature change in the process of manufacturing a whole substrate. The heat from the IC chip is prevented from being efficiently diffused to the heat sink by the thick alumina substrate.
Fourth, a heat sink is attached to the entire under surface of the substrate so that it is impossible to form input and output terminals on that under surface when input and output terminals are needed for the multi-chip package. Therefore, the input and output terminals must be formed on the upper surface of the alumina ceramic substrate or the surface on which the wiring and IC chip are mounted. However, since the wiring and IC chip are mounted on that upper surface, terminals cannot be provided on the whole surface of the substrate. Accordingly, this multi-chip package has the disadvantage of being unable to allow the formation of many input and output terminals.
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