Non-Destructive Evaluation of Solder Bump Quality Under Mechanical Bending Using Laser Ultrasonic Technique
Authors: Jie Gong, Charles Ume, Ph.D., Kola Akinade, Ph.D., Bryan Rogers, Cherif Guirguis, David Chan, Cesar Escobar Company: Georgia Institute of Technology and Cisco Systems Inc. Date Published: 9/28/2014
Abstract: Solder bumps are now widely used as interconnection between silicon chip and substrate/printed wiring board (PWB) in chip packages like flip chip package and ball grid array (BGA) package. This increases input/output density substantially; meanwhile it largely improves device reliability. However, this introduces a new problem that the solder bumps become no longer visible for usual optical inspection. Therefore, there is a need for a non-destructive method of assessing the BGA solder joint quality. A non-destructive technique, Laser Ultrasonic Inspection(LUI) tool has been developed to overcome this new challenge. In this paper, the use of LUI to evaluate solder bump quality from flip chip BGA (FCBGA) package test vehicles that have undergone four point mechanical bend test to create failure modes such as partial cracks or pad cratering will be discussed. In the laser-ultrasound test, a pulsed laser is incident on the chip surface, which will induce ultrasound in thermoelastic regime. Then the transient out-of-plane displacement response is measured by a laser interferometer. The quality of chip package is assessed by analyzing the response. Besides LUI, the samples also went through 3D X-Ray test and X-Ray analysis of the BGA packages did not show these cracks. The results from laser-ultrasound test demonstrate that laser-ultrasound technique is capable of separating good and bad chips; meanwhile it can also reveal the local defect severity for most test cases. Finally, the test samples are cross-sectioned, and the cross-sectioning results correlate well with the results from laser-ultrasound test.
Laser ultrasound, pad cratering, mechanical bending