Gold-aluminum wirebond interface testing using laser-induced ultrasonic energy

A new, non-destructive method of testing wirebonded interfaces is under development. The method uses thermoelastically-generated pulse of ultrasonic waves as a probe. This ultrasonic wave is generated in a gold ball's surface by rapid heating with a laser pulse; it then travels through the bonded interface, and is detected on the surface of the die by an interferometer. Numerical reduction of the time vs. surface displacement data to frequency spectra is used to test far correlation to strength. Frequency shifts have been measured between aged and unaged bonds; transmitted power differences are seen in bonds made with off-optimum bonding parameters. The technique has been applied to ball bonds made on several common bond pad metals, and to a lesser degree to gold wedge bonds. A number of tentative models for the observed frequency and power shifts are given, although further experimentation is needed to substantiate the appropriate models.