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Solder ball height measurement by projection method

The height uniformity of solder ball is an important aspect of solder joint reliability for BGA package. A novel optical method for solder ball height inspection was proposed. A parallel beam was used to illuminate the solder ball, and the shadow of ball was used to measure its height. A machine vision system has been designed to realize this method. Solder ball image and shadow image are obtained by two time lightning and photograph. Digital image process algorithms are developed to evaluate the center and radius of solder balls from the solder ball image, and the projection length from the shadow image. With those information form images, the height of a solder ball and coplanarity is calculated. The experimental results obtained from this new method show good agreement with that obtained by SEM and Veeco.