# Development of a Non-Invasive On-Chip Interconnect Health Sensing Method Based on Bit Error Rates
## 2 Digital Signal Characteristics  
Electronic devices primarily operate based on digital signals, which are a series of discrete pulses, representing logical ones and zeroes. These digital signals have high network capacities owing to multiplexing, enabling multiple communication lines over a single communications channel. Thus, digital signals are transmitted through channels and interconnects in electronics. Figure 1a depicts a digital signal transmission system, which consists of a transmitter, receiver, and channel. Although digital signals are a sequence of discrete pulses, noise can affect ideal digital signals, as shown in Figure 1b. The deviation of a noisy signal from an ideal signal can be considered from two different aspects: timing deviation (\(\Delta t\)) and amplitude deviation (\(\Delta V\)) [11]. Timing deviation is defined as timing jitter, whereas amplitude deviation is defined as amplitude noise. In particular, timing jitters affect the performance of a transmission system at the edges of signal transitions, whereas amplitude noise affects the performance during the signal transitions themselves. Signal integrity (SI) is a set of digital signal characteristics affected by timing jitter and amplitude noise. SI analysis involves evaluating whether the signal reaches its receiver as scheduled without a loss in logic levels. Thus, a digital signal with good SI has fast transitions, valid logic levels, and accurate time placement [12]. SI analysis is widely used for electronic packages and assemblies to evaluate the electrical performances of their internal connections in the case of integrated circuits (ICs), printed circuit boards (PCBs), backplanes, and inter-system connections.  
Thus, digital signals can serve as a means for detecting physical degradation of interconnect surfaces based on the skin effect [13; 14], which is a phenomenon wherein high-frequency signalsaccumulate near the surface of a conductor. [PERSON] et al. [13] presented a diagnosis method for solder joints by monitoring eye parameters, which are sets of parameters obtained from SI analysis. Eye parameters were collected during solder joint degradation testing, and the observed jitters showed significant deviation with the degradation of the solder joints subjected to thermo-mechanical stress conditions. Furthermore, [PERSON] et al. [14] developed a health monitoring system based on eye parameter measurement; in their study, the eye height extracted from the eye diagram showed gradual degradation under an accelerated life test of solder joints, and it was used for anomaly detection of the solder joints.  
BER, which can also be defined as the number of error bits compared to the total number of bits received over a data communication channel, is affected by the physical degradation of a transmission channel. In digital transmission, error bits are generated when the incoming digital signals do not satisfy the bit decision criteria of a receiver owing to noise or damage to the transmission channel. BER is widely used to describe the overall performance of a communication channel based on the rate of error bits. Thus, in this study, we developed an on-chip monitoring method for non-destructive and non-invasive interconnect health sensing based on BER.