Daeil Kwon

Early Detection of Interconnect Degradation by Continuous Monitoring of RF Impedance

Traditional methods used to monitor interconnect reliability are based on measurement of dc resistance. DC resistance is well suited for characterizing electrical continuity, such as identifying an open circuit, but is not useful for detecting a partially degraded interconnect. Degradation of interconnects, such as cracking of solder joints due to fatigue or shock loading, usually initiates at an exterior surface and propagates toward the interior. At frequencies above several hundred megahertz, signal propagation is concentrated at the surface of interconnects, a phenomenon known as the skin effect. Due to the skin effect, RF impedance monitoring offers a more sensitive and reproducible means of sensing interconnect degradation than dc resistance. Since the operation of many types of electronic product requires transmission of signals with significant frequency components in the gigahertz range, this has the further implication that even a small crack at the surface of an interconnect may adversely affect the performance of current and future electronics. This paper demonstrates the value of RF impedance measurements as an early indicator of physical degradation of solder joints as compared to dc-resistance measurements. Mechanical fatigue tests have been conducted with an impedance-controlled circuit board on which a surface mount component was soldered. Simultaneous measurements were performed of dc resistance and time domain reflection coefficient as a measure of RF impedance while the solder joints were stressed. The RF impedance was observed to increase in response to the early stages of cracking of the solder joint while the dc resistance remained constant. Failure analysis revealed that the RF impedance increase resulted from a physical crack, which initiated at the surface of the solder joint and propagated only partway across the solder joint. A comparison between RF impedance and event detectors was made to compare their respective sensitivities in detecting interconnect degradation. These test results indicate that RF impedance can serve as a nondestructive early indicator of solder joint degradation and as an improved means for assessing reliability of high-speed electronics.

Early detection of interconnect degradation using RF impedance and SPRT

Many types of electronic products are now operating at higher frequencies or digital bit rates. At high frequencies, signal propagation is concentrated at the surface of interconnects, a phenomenon known as the skin effect. Degradation of interconnects, such as cracking of the solder joints due to fatigue or shock loading, also usually initiates at the surface and propagates inward. Therefore, even a small crack at the surface of an interconnect may change RF impedance and adversely affect the performance of high speed electronic circuits. Traditional methods used to monitor interconnect reliability are based on a measurement of DC resistance. But, more accurate and sensitive alternatives are required for monitoring the reliability of current and future electronic products. RF impedance analysis offers an improved means of sensing interconnect degradation. This study demonstrates the value of RF impedance measurements as an early indicator of physical degradation of solder joints compared to DC resistance measurements. Mechanical fatigue tests have been conducted with an impedance-controlled circuit board on which a surface mount component was soldered. Simultaneous measurements were performed of DC resistance and the time domain reflection coefficient as a measure of RF impedance while the solder joints were stressed. During the test, the RF impedance provided detectable failure precursors by the sequential probability ratio test (SPRT), while the DC resistance remained constant with no precursors. Failure analysis revealed that the change in RF impedance resulted from a physical crack that initiated at the surface of the solder joint and propagated only part of the way across the solder joint. These test results indicate that the combination of RF impedance and the SPRT can serve as a non-destructive early indicator of solder joint degradation.

IoT-Based Prognostics and Systems Health Management for Industrial Applications

Prognostics and systems health management (PHM) is an enabling discipline that uses sensors to assess the health of systems, diagnoses anomalous behavior, and predicts the remaining useful performance over the life of the asset. The advent of the Internet of Things (IoT) enables PHM to be applied to all types of assets across all sectors, thereby creating a paradigm shift that is opening up significant new business opportunities. This paper introduces the concepts of PHM and discusses the opportunities provided by the IoT. Developments are illustrated with examples of innovations from manufacturing, consumer products, and infrastructure. From this review, a number of challenges that result from the rapid adoption of IoT-based PHM are identified. These include appropriate analytics, security, IoT platforms, sensor energy harvesting, IoT business models, and licensing approaches.

Detection of solder joint degradation using RF impedance analysis

The trend for many types of electronic products is toward higher operating frequencies or digital bit rates. At high frequencies, signal propagation is concentrated at the surface of interconnects, a phenomenon known as the skin effect. Degradation of interconnects, such as cracking of solder joints due to fatigue or shock loading, also usually initiates at the surface and propagates inward. Therefore, even a small crack at the surface of a solder joint may affect the performance of high speed electronic assemblies. Traditional DC resistance measurements are not appropriate for detecting such a small fault. More accurate and sensitive alternatives are required for monitoring the reliability of current and future electronic products. RF impedance analysis offers an improved means of sensing interconnect degradation. This study demonstrates the use of RF impedance changes as an early indicator of physical degradation of solder joints, due to the skin effect, and compares this to DC resistance measurements. Mechanical shear tests at an elevated temperature have been conducted with an impedance- controlled circuit board on which a surface mount component was soldered. Simultaneous measurements were performed of DC resistance and the time domain reflection coefficient, as a measure of RF impedance, while the solder joints were stressed. The RF impedance was observed to increase in response to cracking of the solder joint earlier than the DC resistance. These results were qualitatively repeatable over multiple trials.

Nondestructive Sensing of Interconnect Failure Mechanisms Using Time-Domain Reflectometry

This paper presents time-domain reflectometry (TDR) as a nondestructive sensing method for interconnect failure mechanisms. Two competing interconnect failure mechanisms of electronics were considered: solder joint cracking and solder pad cratering. A simple theoretical analysis is presented to explain the effect of each failure mechanism on the TDR reflection coefficient. Mechanical fatigue tests have been conducted to confirm the theoretical analysis. The test results consistently demonstrated that the TDR reflection coefficient gradually decreased as the solder pad separated from the circuit board, whereas it increased during solder joint cracking. Traditional test methods based on electrical resistance monitoring cannot distinguish between failure mechanisms and do not detect degradation until an open circuit has been created. In contrast, the TDR reflection coefficient can be used as a sensing method for the determination of interconnect failure mechanisms as well as for early detection of the degradation associated with those mechanisms.

Identification of interconnect failure mechanisms using RF impedance analysis

This paper presents RF impedance analysis as a non-destructive indicator of interconnect failure mechanisms. Physical changes on a printed circuit board can be detected using the TDR reflection coefficient as a measure of RF impedance. The test circuit in this study consisted of an impedance-controlled circuit board, a surface mount low-pass filter, and two solder joints providing both mechanical and electrical connections between them. A cyclic mechanical load was applied to generate either solder pad separation failures or cracking of the solder joints. In the case of pad separation, the test results showed that the TDR reflection coefficient gradually decreased as the pad lifted off the circuit board, which was distinguishable from the TDR response due to solder joint cracking. These results are explained using a simple theoretical analysis of the effect of each failure mechanism on impedance. This demonstrates that RF impedance analysis provides for the determination of failure mechanisms as well as early detection of both pad separation and solder joint degradation. The detection sensitivity for solder joint cracking using this technique is also discussed.

Effect of Solder Joint Degradation on RF Impedance

This study demonstrates the value of RF impedance measurements as an early indicator of physical degradation of solder joints compared to DC resistance measurements. Mechanical fatigue tests have been conducted with an impedance-controlled circuit board on which a surface mount component was soldered. Simultaneous measurements were performed of DC resistance and the time domain reflection coefficient, as a measure of RF impedance, while the solder joints were stressed. The RF impedance was observed to increase in response to cracking of the solder joint while DC resistance remained constant. Failure analysis revealed that the RF impedance increase resulted from a physical crack initiated at the surface of the solder joint, which had propagated only partway across the solder joint.

Remaining-Life Prediction of Solder Joints Using RF Impedance Analysis and Gaussian Process Regression

Solder joints are among the most common failure sites in electronic assemblies. This paper presents a prognostic approach that allows for the remaining useful life prediction of solder joints using an RF impedance analysis and the Gaussian process (GP) regression. While the solder joints were exposed to a mechanical stress condition to generate fatigue failures, the RF impedance of the solder joint was continuously monitored. The RF impedance provided an early indication of the impending solder-joint failure in the form of a gradual increase prior to the end of life. A GP model was applied to the RF impedance obtained from the fatigue tests in order to estimate the remaining life of the solder joint in real time. It was demonstrated that the GP model successfully predicted the time to failure of the solder joint with high accuracy prior to failure. The prediction performance was also evaluated using prognostic metrics.

Define electrical packing temperature cycling requirement with field measured user behavior data

In this paper, a user behavior based solder joint reliability modeling approach has been proposed to estimate the design and test requirements for the second level interconnect (SLI) reliability prediction. This approach uses a numerical tool to integrate solder joint creep damage during the actual use condition that was collected from a large user sample size. The resultant damage per time period was then input to the solder joint fatigue model to estimate equivalent damage to testing duration. The is a physics based approach and is expected to provide more accurate product life prediction and reliability performance demand for BGA package designs.

A prognostic method of assessing solder joint reliability based on digital signal characterization

Electronics are subjected to thermal, mechanical or chemical stress conditions during their operations. These stress conditions accumulate damages to the electronic system, adversely affecting its components and interconnects including solder joints. Solder joint degradation can cause system malfunctioning, which eventually results in catastrophic failures. In order to prevent system failures, it is required to manage the system health continuously by monitoring damages such as solder joint degradation. Regardless of stress conditions, solder joints often start to degrade from their surface where high speed signals in an electric circuit are concentrated due to the phenomenon referred to as the skin effect. Thus, high speed signals such as digital signals can serve as a sensitive means of detecting solder joint degradation at early stages. This paper presents a diagnosing method of assessing solder joint reliability based on digital signal characterization. Accelerated life tests are conducted to generate solder joint failure. The test setup consists of a circuit board with solder joints, a digital signal transceiver, an environmental test chamber and a stress application fixture. Constant mechanical shear stress is applied to the solder joints of a circuit board at an elevated temperature. A digital signal transceiver generates high speed signals travelling through the solder joints, and continuously monitors the signal characteristics which indicate signal integrity. The test results confirm that the signal characteristics show statistically significant variations between intact and cracked state of the solder joint, and the parameter variations indicate the deterioration of the signal integrity. These results suggest that digital communication signals can be used as a non-destructive diagnostic tool of physical degradation. It can be improved as a tool which provides early warning of impending system failures without installing additional fault sensing instruments. This diagnostic approach may serve as a proactive prognostic module that allows for real-time health management of electronic products and systems.