Feasibility Studies and Advantages of Using Dual Fiber Array in Laser Ultrasonic Inspection of Electronic Chip Packages

Fabrication and inspection techniques of electronic packages are two key factors influencing a chip's success in post-Moore's law era. As the electronic packaging industry rapidly evolves to cope with modern demands, a versatile inspection method for present and future packages is required. One approach is the development of a fiber array laser ultrasonic inspection system as presented in this paper. The new system uses two laser excitation points, allowing higher total energies to be delivered to the chip's surface without exceeding the thermoelastic regime and causing damage. These beams induce strong ultrasonic waves, which can penetrate deeper and farther into and across a package. As a result, larger and thicker packages can be nondestructively inspected than was previously possible. Flip-chip ball grid array packages of size 52.5 mm × 52.5 mm from Cisco were evaluated using this system and compared against a single-beam system. Benefits of the new system include an increase in vibration amplitude due to high laser power, improved signal-to-noise ratio (SNR), an increase in resolution/sensitivity to measure microcracks, and high speed. The new dual-fiber-array system can administer 400-mW power safely, and as a result, the peak-to-peak amplitude of the generated ultrasound is three times that of ultrasound generated from a single laser beam. The SNR of 8:1 is achieved on the current Cisco package with double excitation point sources compared to the SNR of 1:1 that was obtained with a single excitation point source.