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.
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Wang et al. (2012) studied this question.
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