Stereo Vision Based Automated Solder Ball Height and Substrate Coplanarity Inspection

Solder ball height and substrate coplanarity inspection is essential to the detection of potential connectivity issues in semi-conductor units. Current ball height and substrate coplanarity inspection tools such as laser profiling, fringe projection, and confocal microscopy are expensive, require complicated setup and are slow, which makes them difficult to use in a real-time manufacturing setting. Therefore, a reliable, in-line ball height and substrate coplanarity measurement method is needed for inspecting units undergoing assembly. Existing stereo vision measurement techniques determine the height and depth of objects by detecting corresponding feature points in two views of the same scene taken from different viewpoints. After detecting the matching feature points, triangulation methods are used to determine the height and depth of an object. The issue with existing techniques is that they rely on the presence of edges, corners, and surface texture for the detection of feature points. Therefore, these techniques cannot be directly applied to the measurement of solder ball height or coplanarity due to the textureless, edgeless, smooth surfaces of solder balls. In this paper, an automatic, stereo vision based, in-line ball height and coplanarity inspection method is presented. The proposed method includes an imaging setup together with a computer vision algorithm for reliable, in-line ball height measurement. The imaging set up consists of two different cameras mounted at two opposing angles with ring lighting around each camera lens which allows the capture of two images of a semiconductor package in parallel. The lighting provides a means to generate features on the balls which are then used to determine height. Determining and grouping points with the same intensity on the ball surface allows the formation of curves, also known as iso-contours, which are then matched between the two views. Finally, an optimized triangulation is performed to determine ball height. Furthermore, the coplanarity of ball grid array (BGA) package is derived from calculated substrate depth results without requiring any additional imaging setup. The proposed in-line ball height and substrate coplanarity inspection method has been tested on three products through a measurement capability analysis (MCA) procedure and exhibits high accuracy, repeatability and reproducibility compared to the laser-scanning and confocal inspection tool results.