Online-offline laser ultrasonic quality inspection tool for multilayer ceramic capacitors - part II

Abstract

The adoption of surface mount technology has provided many competitive advantages for the electronics industry. Smaller, lighter, and more efficient packages create better products for consumers, but new manufacturing problems are created through the adoption of new technology. Inspection and verification systems are employed to prevent known defective parts from reaching consumers, but not all future reliability problems can be identified through traditional inspection methods. In multilayer ceramic capacitors (MLCCs), flex cracks can be created during manufacturing. However, traditional online and offline inspection methods have been unable to consistently identify these defects. The location and orientation of the cracks, perpendicular to the printed wiring board (PWB) and obscured by the MLCCs end metallization for solder connection, makes detection using immersion ultrasound or x-ray inspection difficult. The metallization occludes the ability of these methods to image the defect. Therefore, a new inspection technique is required to identify these defects. A laser-generated ultrasonic system has been designed and constructed to meet the need of a noncontact, nondestructive, fast, and accurate system for identifying defects in a laboratory environment, as well as on the manufacturing fine. The system consists of a pulsed infrared laser that excites samples of both an 0805, 10 /spl mu/F, 6.3 V, X5R and an 0603, X5R, 6.3 V, 1 /spl mu/F, MLCC, both having square cross sections, into small-amplitude structural vibration after attachment to the PWB through the process of laser-generated ultrasound. An interferometer, sampled with a synchronized data acquisition system, records the structural impulse response of the MLCC. Monitoring the impulse response for changes indicates changes in the structure of the device. Several MLCC samples were prepared and mounted on PWBs, and some of them were intentionally flexed until a loss of insulation resistance was recorded, indicating the presence of a flex crack. Measurements were taken on the as-manufactured and flexed-to-failure samples. The test results contained in part I were obtained with 0805 MLCC samples with rectangular cross sections, which are less stiff than MLCCs with square cross sections. These results indicated potential for success using the method described here to identify flex cracks. These results indicated potential for success using the method described here to identify flex cracks. The current results also indicate that the laser ultrasonic system could be used to detect flex cracks in 0805 MLCCs with a square cross section and in the smaller 0603 package configuration with a square cross section. Classification of the capacitors requires a reduction in the variance of the error ratio values, so an average of the values computed in comparison with each of the reference devices is implemented.