Research On Key Technology For Flip Chip Fault Diagnosis Using Ultrasonic Excitation And Vibration Analysis

Flip chip is a favourable interconnection technology for microelectronic packaging. With the advantage of preferable alignment accuracy, short interconnection lines and high I/O density, flip chip technology has been widely used to promote signal transmission speed and decrease package volume. As flip chip technology trends to high density and fine pitch, scale effects and surface effects will be more noticeable and thermal/stress mismatch between materials will be more deteriorating, which can cause stress concentration and damage accumulation, inducing the solder joint defects. In addition, the flip chip solder joints are hidden between the chip and substrate. It makes the solder joint fault diagnosis to be more difficult. To overcome this difficulty, in this dissertation the key technologies of fault diagnosis are researched for the typical defects of flip chip solder joints, such as missing solder joint, open solder joint and crack. The main research contents are described as follows.To diagnose the missing solder joint defect, a sheet-springs theoretical model was established. By calculating this model, it finds that the resonance frequencies of chips with missing solder joints are smaller than the good chips, and the chip resonance frequencies decrease with the increase of missing solder joints. The flip chip finite element model was constructed by the COMSOL software. The finite element simulation analysis validates that missing solder joint induce the chip resonance frequency to decrease, and reveals the vibration velocity of missing solder joint location is large than the vibration velocity of non-defective location. This difference on vibration velocity can be represented by the feature coefficient a. Based on the air-coupled ultrasonic excitation and laser Doppler vibration measurement, the flip chip defect inspection system was constructed. The missing solder joint defects were introduced to test chips by removing solder joints before bonding. These chips were inspected applying the flip chip defect inspection system. The inspection results demonstrate that the missing solder joint defect can be recognized by the chip resonance frequency, and the defect location can be located through the chip vibration velocity.For the typical defect of open solder joint, the simulation model of flip chip with open solder joint was established. The chip modal features were studied by simulation analysis, revealing the differences on modal shapes and resonance frequencies between the defective chip and good chip. In the experimental investigation, the chip transient vibration under ultrasonic excitation was measured by the defect inspection system. The experiment results prove that the occurrence of open solder joint in the flip chip is the source of the modal shape variation and resonance frequency decrease. Furthermore, the velocity spectrum of open solder joint site and non-defective site were investigated. It is found that the open solder joints cause the distribution of chip vibration energy to change, which can be represented by the root mean square frequency(RMSF) of spectrum. Therefore, the open solder joint defect can be distinguished through the chip modal features, such as modal shapes and resonance frequencies, and the defect site can be located by the chip vibration spectrum with its RMSF.Cracks were introduced to flip chip solder joints by virtue of thermal cycling and adding pressure with heating. The flip chips with crack defects were investigated through modal analysis, revealing the modal shape difference between cracks chip and good chip, which can be represented by the features of modal shape image, such as aspect ratio and characteristic angle. At the same time, the chip resonance frequencies were extracted for analysis, finding that the cracks cause the chip resonance frequencies to decrease. It demonstrates that the cracks of solder joints can be recognized by the chip modal features, such as modal shape and resonance frequency. Then the vibration signals of crack solder joints were analyzed applying LMD method. The analysis results prove that the average envelope spectrum differences of PF components between crack location and non-defective location can be expressed by Shape Factor to determinate the crack solder joint location.This thesis combines ultrasonic excitation and vibration analysis to the key technology research of flip chip solder joint fault diagnosis, achieving the recognition and locating of typical defects. In the future work, the researches on the improvement of experiment system, the recognition and classification of mixed defects, and the analysis of complex vibration response signal are required to optimize the diagnosis method and achieve the development of flip chip online inspection system.