The Micro Solder Ball Height Inspection System Based On Machine Vision

The Ball Grid Array (BGA) package is a kind of high density, area array advanced packaging technology. Solder joint reliability is highly dependent on solder ball height uniformity/coplanarity, so it is very important to measure the height of solder balls on chips. At present, packaging enterprises need a kind of micro solder ball height inspection equipment with fast speed, moderate accuracy, low cost. In this paper, a novel projection method for solder ball height measurement was proposed, and a machine vision system to realize this method was designed. The main research contents were listed as follows:1) According to the SEM image of micro bump, the ideal spherical cap assumption of micro BGA solder ball was verified. Based on this assumption, a new projection method for measuring solder ball height was proposed under the illumination of a parallel light beam. The influence factors on height mearurement results were analyzed, such as light incident angle, ball radius, and length of shadow. A imaging method using a low-angle LED ring light source and a white LED dot light source was designed.2) For the solder ball image and shadow image, the contour of solder ball and shadow were extracted through filtering, automatic segmentation using Otsu threshold method, hole filling, contour extraction. The solder ball and shadow contour was proposed based on circle, ellipse fitting, respectively. Experimental research show the image processing algorithm is efficient and stable.3) An automatic micro bump height inspection system based on machine vision was constructed with a self-designed machine structure, control circuit and control software developed by OpenCV&Visual C++. The micro solder ball with height of about hundreds of micros was measured automaticly.4) The effect of chip translation, rotation, substrate material, and the warpage of substrate on micro solder ball height measurement results were studied. Experiments and theoretical research show that the substrate X direction warpage is the main aspect to influence height results, as the X direction warpage changed the angle between the substrate and the incident light. According to the height distribution characteristics of height results, an automatic compensation algorithm was presented.