Diagnosing faults in modern mixed-signal system-on-a-chip (SoC) devices is a complex task. Modern chips integrate digital processors, analog signal chains, RF communication blocks, and power-management circuits on the same silicon die. Many such devices include proprietary architectures or security-sensitive intellectual property. During debugging, internal access to signals or subsystems is frequently required. Unless carefully controlled, these interfaces may unintentionally expose confidential design details. The conflict between highly accessible debugging and the need to protect intellectual property has become a challenge in modern SoC failure analysis and is frequently discussed in research on secure IP debugging and fault isolation methods. Why Mixed-Signal Debug Is Different Why Mixed-Signal Debug Is Different Digital logic debugging is a well-established process developed over several decades. Scan chains, boundary scan, and logic analyzers are tools that enable engineers to trace internal states and logic faults with moderate efficiency. However, different rules apply to mixed-signal devices. All these subsystems are integrated on the same silicon substrate and usually operate concurrently. Minor variations in device parameters may shift the operating point of an analog circuit enough to reduce its accuracy and cause a failure. When issues manifest in silicon, the observed symptoms rarely indicate the root cause and instead often reflect secondary effects of the underlying problem, as reported in many studies on failure analysis of complex SoCs. The Many Faces of Electrical Failure The Many Faces of Electrical Failure On the digital side, failures are easier to categorize. A logic node can become stuck at a fixed value due to oxide degradation or a faulty interconnect. Elsewhere, shorting may occur between two adjacent metal lines, creating a bridging fault that leads to unpredictable logic behavior. Analog circuits are more prone to failures that are less immediately noticeable. Such failures tend to be frustrating, since the system appears to be operating, but not as well or as reliably as it could, affecting during/ end of life performance-an effect that has been clearly noted in semiconductor reliability and failure analysis literature. Where Traditional Debugging Falls Short Where Traditional Debugging Falls Short Most of the tools that engineers have for digital debugging do not provide much insight into analog behavior. Light Emission and Optical Beam induced Resistance Change may sometimes offer useful information. Internal nodes can be accessed through direct contact using micro-needles or specialized probes to measure voltages or currents. However, there are disadvantages to this approach. Microprobing and dropping optical probe pads may damage the chip, and may expose the state of internal circuitry that constitutes company secrets. As SoCs have become more advanced, with enhanced security, the industry has begun to re-evaluate how debugging should be conducted, particularly in the context of secure hardware analysis. Debug Interfaces as a Security Risk Debug Interfaces as a Security Risk In the development process, debug infrastructure is required, although it can also become a side channel vulnerability when exposed in operational systems. Interfaces like JTAG or internal trace buffers can be used to observe system behavior in depth. Optical tools may also act as side-channel threats in certain situations, as explored in research on the convergence of failure analysis and hardware security. Though such information can be invaluable to engineers, it can also be exploited by those attempting to reverse-engineer a device or steal valuable information. For example, an attacker can intercept power-consumption patterns or electromagnetic radiation to gain insight into internal processes. Side-channel measurements in systems with cryptographic engines can reveal secret keys or algorithm specifications. These risks have led modern SoCs to adopt increasingly secure debugging models, restricting access to and limiting the duration of diagnostic operations. Tools for a More Secure Debug Flow Tools for a More Secure Debug Flow Engineers are considering more sophisticated failure analysis methods to address both complexity and security issues. Electromagnetic emission analysis is one such approach. As it does not require physical interaction, measurements can often be taken while units are operating in a typical environment-a technique that aligns with established emission-based diagnostic methods used in semiconductor testing. Thermography and Time-domain reflectometry are methods that have been reported to be useful in defect localization in interconnect structures. In this method, engineers analyze reflections by injecting a short electrical pulse into a conductor. Any discontinuity, such as an open circuit or short circuit, leaves an imprint on the reflected waveform. Scanning electron microscope voltage-dependent contrast (SEM-VC) is used to identify floating nodes or faulty interconnects. The device layout is analyzed while sensitive areas are masked to prevent exposure of proprietary circuitry. Meanwhile, most contemporary chips have built-in self-test circuits that internally gather diagnostic information. These features, along with secure trace buffers and authentication mechanisms, enable engineers to monitor system activity without exposing the system architecture, an approach that is gaining increased support in secure debugging methodologies. Debugging in a Security-First World Debugging in a Security-First World The higher the level and value of semiconductor devices manufactured, the greater the demand for secure debugging. Engineers need to discover defects more quickly and accurately, but at the same time, the tools they use in the failure analysis process should not compromise intellectual property. Techniques such as electromagnetic emission analysis, reflectometry, and laser-based probing are frequently referenced in modern fault isolation literature and could be viewed as optical side channel attack. In practice, the best approach is to incorporate encrypted security and balance the flow for debugging architecture by using advanced DFT/AI features. System designers can ensure both high reliability and strong protection of their designs through controlled access mechanisms and advanced diagnostic tools. This balance is crucial for mixed-signal SoCs. Conclusion Conclusion The outcome of combining complex fault isolation strategies with effective security encryption allows engineers to debug systems without compromising proprietary designs. Resilience, flexibility, and security-by-design represent the future of SoC debugging. Whether applied in the development of next-generation automotive ECUs or safety-critical medical sensors, these approaches can help create more advanced and secure debugging systems. **References**1. Amrutha Sampath et al., “Evaluation of the Analyzability of Complex Secure Intellectual Property Using Fault Isolation Techniques versus the Hardware Security Threat They Pose,” Proceedings of ISTFA, 2024. 2. Amrutha Sampath et al., “Effectiveness of Photon Emission Microscopy in Identifying Reliability Issues and Aiding Design Debug,” Proceedings of ISTFA, 2025. 3. Amrutha Sampath et al., “Electrical Fault Isolation of Stuck-at-Reset Hard Failures,” Proceedings of ISTFA, 2023.
