Inspection of Flip Chip and Chip Scale Package Interconnects Using Laser Ultrasound and Interferometric Techniques

摘要

Inspection of solder bump interconnections is more difficult than conventional lead-frame solder connections because the solder joints are hidden from view. Current inspection methods, such as automated optical inspection (AOI), automated x-ray inspection (AXI) and acoustic micro imaging (AMI) have limited capabilities for inspecting the mechanical integrity of solder joints. A new noncontact, nondestructive inspection technique developed at Georgia Tech can evaluate the chip-to-substrate mechanical integrity by detecting missing solder balls; nonwetted (open), disbonded or cracked solder joints; and misaligned or cracked 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 a microelectronic component into vibration, and an interferometer measures its out-of-plane surface displacement. Defects in the solder joints or silicon chip itself cause measurable changes in the vibration response. Signal processing techniques are used to identify defects by comparing vibration signatures of tested devices to a reference, defect-free device. The long-term goal of this research is to develop a low-cost, high-sensitivity, accurate, fast, highly automated prototype system to demonstrate the use of this technique for online inspection, process development and failure analysis. This paper discusses the design and performance of the current inspection system, presenting specific results for a wafer-level chip scale package (CSP) and a flip chip. These results aid the discussion of system performance and limitations. Novel applications of this inspection technique and the expected impacts in microelectronics manufacturing and development are also discussed.