Automated coplanarity inspection of BGA solder balls by structured light

Abstract

The coplanarity of BGA (Ball Grid Array) solder balls is critical for reliable connectivity in semi-conductor units. However, existing ball height inspection techniques require high-cost equipment and skilled operators, which are not feasible for using in real-time manufacturing process. In this paper, a fast measurement method of coplanarity inspection of solder balls is developed based on structured light technique. The hardware of the system includes a camera and a projector. The digital-light-processing (DLP) projector emits programmed image patterns and the digital complementary-metal-oxide-semiconductor (CMOS) camera captures distorted image patterns on solder balls. The image processing software is developed to compare the original and distorted image patterns to reconstruct the three-dimensional (3D) solder balls. The method has been calibrated extensively and exhibits accuracy within 5 μm mean squared error compared to ground truth values determined by X-ray computed tomography. The proposed method achieves reliable, in-line ball height measurement and could be potentially used for real-time in-line coplanarity inspection of BGA chip during manufacturing.