Ph.D. Dissertation Defense by Dathan S. Erdahl
Friday, April 8, 2005
(Dr. I. Charles Ume, Chair)
"Microelectronics Device Inspection System Implementation and Modeling for Flip Chips and Multi-Layer Chip Capacitors"
Increased consumer demand for smaller electronics with more capabilities is driving the electronic packaging industry to develop smaller, more efficient component level packages. Because of this need, surface mounted components such as flip chips, ball grid arrays (BGAs), and chip-scale packages (CSPs) are being developed for use in high-volume production. Compared with traditional wire bonding packages, these packages have advantages such as a lower profile, a smaller footprint and higher solder joint density, however, these advantages come with a price. All of these technologies use solder bumps to attach the active silicon to the substrate instead of traditional wire lead frames. Therefore, it is difficult to search for defective connections using traditional methods such as machine vision, acoustic microscopy or x-ray inspection.
To meet this need, a novel, non-contact, non-destructive, on-line approach for inspecting solder joint quality in these packages has been developed and tested. The system consists of an Nd:YAG laser that delivers pulses of infrared energy to the surface of the chip, a laser interferometer to record surface vibrations, and a high-speed data acquisition system to record the signals. In this technique, the pulsed laser generates ultrasound on the chip's surface, exciting the whole chip into a vibration motion, and the interferometer measures the vibration displacement of the chip's surface at several points. Changes in the quality of the device or its attachment to the board produce changes in the free vibration response. Using and adapting signal-processing algorithms developed for this system allows characterization of the differences between good devices and devices with defects, both in time domain and frequency domain.
Previously, this system and inspection methods have been used to inspect flip chips and chip scale packages for missing and misaligned solder balls. To characterize the resolution of the system for open solder joints, a careful study of the vibration modes excited by the laser source in a flip chip was performed on specimens with intentionally created defects. Experimental measurements of excited modes were compared with a modal analysis model created in ANSYS, and defects were detected as small changes in the mode shape on the surface of the chips.
Defects with solder bumped devices are not the only manufacturing defects that are difficult to detect during printed circuit board (PCB) assembly. Current inspection methods have been inadequate in inspecting multi-layer ceramic capacitors (MLCCs). Flex cracks in the MLCCs often cause the capacitors to fail after being cracked through flexure of the circuit board during assembly. Samples that have been cracked intentionally were compared with reference samples to determine the feasibility of using this technique to monitor the condition of MLCCs on an assembly line. Currently, there is no on-line inspection method for controlling this problem, but this technique was able to differentiate between good and damaged capacitors.
Because of the flexibility, accuracy and speed of this system, it is expected to have a great impact on the electronic packaging industry, allowing both on-line inspection, and off-line process optimization. The development of inspection techniques to identify defects in a wider range of microelectronics devices, such as more complex flip chips and MLCCs, adds to the flexibility and adaptability of the inspection system. Finite element modeling with parameterized models will aid in adapting these inspection techniques to new packages, allowing more rapid identification of modal properties that aid in inspection setup.