Thin-film multichip modules fabricated by chip-on-wafer (COW) technology have been developed for high performance systems. Copper/polyimide thin-film wiring layers are fabricated on a silicon substrate. LSI chips with copper-cored solder bumps (CCSBs) are flip-chip bonded to the silicon substrate. The substrate is housed in a co-fired aluminum nitride (AlN) ceramic package to enhance thermal reliability. The electrical properties, such as the characteristic impedance and crosstalk noise, of the copper/polyimide wiring substrate were examined. Experimental results have shown that the substrate can propagate high-speed signals exceeding 100 MHz. Next, this combination of a large silicon substrate and an AlN package was investigated thermally and mechanically. The results of warping tests and thermal cycling tests show that AlN is an excellent packaging material for silicon-based multichip modules. A digital signal processing module has been developed as an example of a high-performance multichip module.