In order to improve the performance of Automatic Optical Inspection(AOI) system,an illuminator for leadless tin solder joint inspection was studied.The structure of the illuminator was introduced,which is composed of the LED arrays with three colors and different radiation angles.Then,the radiance model of the illuminator and the irradiance model of the solder joint were developed.Finally,the dimensions of the illuminator were optimized based on the irradiance model.The simulation and experimental results verify the validity of proposed illuminator for leadless solder joint inspection,and show it can enlarge the feature distances from different classes.The maximum feature distance of different solder joints illuminated by the proposed illuminator is 11.88 times of that illuminated by the hemisphere illuminator.The simulation and experimental results verify the validity of the design.