(Dr. I. Charles Ume, advisor)
"Design of an Advanced System for Inspection of Microelectronic Devices and Their Solder Connections Using Laser-Induced Vibration Techniques"
Inspection of solder bump interconnections between integrated circuit packages and printed wiring boards is more difficult than conventional lead-frame solder connections because the solder joints are hidden from view. Current methods, such as automated optical inspection, automated X-ray inspection and acoustic micro imaging have limited capabilities for inspecting the mechanical integrity of solder joints. A new noncontact, nondestructive inspection technique developed at Georgia Tech is used to evaluate the chip-to-substrate mechanical integrity by detecting missing solder balls; nonwetted, disbonded or cracked solder joints; and misaligned or cracked IC packages. In addition, this new technique may provide a nondestructive means to detect residual stress in the chip-substrate bond due to warpage or coefficient of thermal expansion mismatch.
Pulsed laser energy excites an IC package into vibration, and a laser vibrometer measures its out-of-plane surface displacement. Defects in the solder joints between the package and substrate or in the package itself cause measurable changes in its vibration response. Signal processing techniques are used to identify defects by comparing vibration signatures of tested packages to a reference, defect-free package.
The long-term goal of this research is to develop a low-cost, highly sensitive, accurate, fast, highly automated prototype system to demonstrate the use of this technique for online inspection, process development and failure analysis. To help reach this goal, significant improvements to the original inspection system were made as part of this thesis work. These improvements increased the sensitivity to defects, created a more versatile experimental system, and extended the range of testing to more complex specimens. The specific contributions of this thesis include: replacement of the original motion stages with a high accuracy, high precision X/Y table; the addition of a vision system to precisely locate specimens; increased vibration measurement signal strength; more versatile fixtures to hold various test substrates; improvements to the delivery of pulsed laser excitation energy; and improved safety and protection of delicate equipment with a system enclosure. These improvements enabled tests of a chip-scale package on an organic substrate with 98 hidden solder connections. Defects such as misaligned chips and missing solder balls were detected in this CSP.