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High-density microelectronic circuity requires interconnection wire diameters of less than 0.005 inch and pad sizes less than 25 X 10-6square inch. Most ohmic contact pads on chips are aluminum as the material with the least of possible evils. Metallization of substrates to interconnect chips commonly consists of chromium for adhesion and gold or aluminum for conductivity. Substrate materials are chosen on the basis of dieleetrie strength, dielectric constant, dissipation factor, thermal conductivity, thermal expansion, and surface finish. Ultrasonic bonding avoids the troubles caused by the high temperatures employed for thermocompression and 'wedge bounding. Parallel-gap welding can be used on very small pads. Laser and electron beam welding techniques are still in the development stage. Difficulties associated with expansion coefficients of different materials in the evaporated leads bonding process can be avoided by careful choice of materials. Flip-chip bonding has the ad.vantages of reducing the number of interconneetion bonds and the package size, but it reduces accessibility for inspection and testing, sometimes requires multilayer cireuitry, and increases thermal resistance and turnaround time for design changes. Ultimately the choice of bond depends on such factors as cost, reliability, ar.d maintainability. When multilayer circuitry is required, choice of a dielectric to insulate crossover points becomes a problem. A knowledge of the advantages and disadvantages of all interconnection rnethods will enable the designer to preplan his circuitry so as to employ a method that is best for his particular application.
Parts, Materials and Packaging, IEEE Transactions on (Volume:2 , Issue: 4 )
Date of Publication: Dec 1966