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APRIL 2025 I PCB007 MAGAZINE 79 typically used to create base holes for align- ment during drilling. While some CCD-based methods exist, they require cutting through the top copper layer until the CCD can recognize internal fiducials—more common in prototyping or small production batches. Why is X-ray Critical? X-ray is the major control point in PCB production where we can analyze how layers behave aer lamination. Misalignment can occur due to: • PCB design factors (symmetrical or not, copper density, stack) • Material properties (type of the material, new or stored) • Lamination process settings • Equipment used for inner-layer alignment and for lamination • Pre-treatment steps (oxidation, thermal stabilization, etc.) Fiducial Types in X-ray Registration Two primary fiducial methods exist: 1. Pad-stack fiducials: Fiducials on all layers are stacked on each other. is is faster but doesn't provide individual layer data (Figure 2). 2. Layer-to-layer fiducials: Each layer is eval- uated independently, offering better accu- racy and process control, especially for complex boards (Figure 3). Optimizing X-ray Detection If you've ever had an X-ray at a hospital, you know that while the image is essential for diag- nosis, interpreting it requires a trained special- ist. Most of the time, it's difficult for an untrained eye to recognize details in the scan. However, in PCB manufacturing, an X-ray machine must be capable of quickly and accurately detecting fiducials without human intervention. For the soware to recognize fiducials effi- ciently, achieving high contrast and clear image quality is critical. Poor contrast can lead to misalignment issues, affecting the entire registration process. Several machine- specific factors play a role in optimizing fidu- cial detection, including: • Machine construction to ensure stable and repeatable measurements • X-ray source settings to enhance clarity and contrast • Image processing algorithms for precise fiducial recognition Figure 4a shows an example of fiducial detection on a 74-layer PCB with a 10 mm thickness, using layer-to-layer fiducials; an example of HDI board with pad-stack fidu- cial is shown in Figure 4b. Another key function in X-ray registration is the ability to align to specific internal lay- ers when needed. Ensuring the X-ray system supports this feature is critical for certain applications. Figure 2: Pad stack fiducial type. Figure 3: Layer-to-layer fiducial type.