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Micro-solder balls are commonly used in wafer-level ball grid array (BGA) packages as an interconnection medium. To measure the height of a micro-solder ball on a metal pad, we propose a white light projection method to avoid interference from pads in the shadow of the ball. The optical projection of a solder ball under the illumination of a parallel white light beam is studied, and the relationships between the ball height, ball radius, and shadow length are deduced. An experimental platform with a simple optical system and white light emitting diodes lighting source is constructed to obtain ball and shadow images, and a program developed to process these images and calculate the ball height. The heights of the balls on a BGA chip are measured using this new method, and the results verified using a commercial optical profiler. This method is not sensitive to the patterns on the substrate surface and has great potential for future application.
Components, Packaging and Manufacturing Technology, IEEE Transactions on (Volume:2 , Issue: 9 )
Date of Publication: Sept. 2012