Automatic inspection of solder joints using layered illumination

Efficient techniques for solder joint inspection are described. Using three layers of ring shaped LEDs with different illumination angles, three frames of images are sequentially obtained. From these images the regions of interest (soldered regions) are segmented, and their characteristic features including the average gray level and the number of highlights-referred to as 2D features-are extracted. Based on the minimum distance classification rule, each solder joint is classified into one of the pre-defined types. If the classification is ambiguous, the distribution of tilt angles-referred to as 3D features-is calculated, which requires more computation while providing more information and better performance. The proposed inspection system has been implemented and tested with various types of solder joints in SMDs. The experimental results have verified the validity of this scheme in terms of speed and recognition rate.