Ball grid array (BGA) substrate conduct paths inspection using two-dimensional wavelet transform

The objective of this paper is to implement two-dimensional wavelet transform (2-D WT) in the detection of mousebite, spur, open, and short defect candidates on ball grid array (BGA) substrate conduct paths. Once the defect candidates are located, traditional BGA substrate inspection algorithms can further detect true defects among these suspicious defects, Therefore, the scope and effort during the inspection stage can be significantly reduced. The binary BGA substrate image is processed that shows only conduct path boundaries, which are decomposed directly by 2-D WT. Then, the inter-scale ratio from the wavelet transform modulus sum (WTMS) across adjacent decomposition levels for the edge pixels on BGA substrate conduct path boundaries is calculated. Since irregular edges in a small domain can preserve much more wavelet energy, an edge pixel is considered as an abnormal one or a defect candidate if its inter-scale ratio is less than a pre-defined threshold. The proposed approach is template-free and easy to implement, so it is suitable for small batch production. Real BGA substrates with synthetic boundary defects are used as testing samples to evaluate the performance of the proposed approach. Experimental results show that the proposed method is able to capture all the true mousebite, spur, open, and short defects without any missing errors by appropriate wavelet basis, decomposition level, and image resolution.