Reliability analysis of electronic boards: analyzing of field return data and the impact of varistor degradation

The rapid developments in electronics, especially in the last decade, have initiated the inception of electronics reliability. Manufacturers such as public utilities like white appliances, the airline and the military are conscious of the costs of unreliability. They usually suffer high costs of failure under warranty. Field return data or warranty data can be used for variety of purposes that include the prediction of future claims, the estimation of field reliability, and the identification of opportunities for quality and reliability improvement. Filed return data sometimes gives us valued information about most failed components during their usage in the field. By examining the cause of failure for a specific components reported on the filed return data, designers and reliability engineers can have a superior vision about reliability of that component and whole system. However, the problem is usually these data are not well recorded. In reality, field return data contains errors corresponding to improper, correlated, incomplete, and poorly collected data that result in misleading reliability predictions. Very few literatures concerning the impact of improper filed return data analyzing and consequently prediction of electronic cards reliability. The first part of this study presents filtering and modeling based on probabilistic distribution function. We propose a new technique to eliminate improper data, correlated to incomplete and poorly collected data, from the whole field return data. In the second step of modeling, we use the filtered data to develop our reliability model. Rather than conventionally using a single distribution for all service times that does not accurately model the substantial changes of the electronic boards reliability performance over time, we use different distributions for different service time intervals. In order to determine the distributions we propose a technique that deals with forward and backward time analysis of the data. One of the most field reported data was for varistior and related failures. ZnO varistors have been widely used in electrical and electronics systems against overvoltage surges for their high nonlinear electrical properties (non-Ohmic current-voltage characteristics) and excellent energy handling capabilities. Choosing an appropriate varistor plays an important role in system safety and reliability. If ZnO varistors are used within their well-defined specifications, degradation due to the environment is not likely. ZnO varistors in-use are usually subjected to a long-term AC or DC voltage stress and surge which may lead to degradation of the varistors with an increase of the varistor clamping voltage Vv. This is called ageing phenomenon and will eventually make the varistors degraded and even thermally broken down or destructed. Degraded varistors affect system reliability and safety. Based on studies on varistor degradation, researchers have argued that the impact of varistor degradation in reliability of electronics is following one-failure mechanisms, which is dependent on increasing of lekage current or decreasing in Vv parameter. However, due to the lack of experimental results and the complexity of varistor structure, the argument decreasing leakage current that results in an increase of Vv, is totally disregarded. Very few papers concerning the impact of different degradation of varistor can be found in the literature. Further, most of these studies perform a specific test on the varistor degradation without considering its relationship with the system where the varistor is placed. In practice, commercial varistor are used as a sample for degradation in experiments. In this research, varistor samples were as same as the real field used application of white appliances. Accelerated degradation tests are applied for different AC current levels; then voltage values are measured. Different from the common practice in the literature that considers a degradation with only decreasing Vv values, we demonstrate either an increasing or a decreasing trend in the Vv parameter. The tests show us a decreasing trend in Vv for current levels above a certain threshold and an increasing trend for current levels below this threshold. We find a threshold value to determine the border of moderate and heavy overvoltages. We simulated the related circuit in SPICE software for different analysis and different overvoltage types. As a conclusion from simulation results, we observed that as a ZnO varistor degrading (Vv parameter increasing), the related system capability to handle the overvoltage is decreasing. It means that, if we consider a typical overvoltage type for a specific environment, the designed circuit maybe stand against this overvoltage, but in case of the degradation the failing is possible. Considering both of these degradation mechanisms, we present a mathematical degradation model. The proposed model exploits physics of the degradations for a single grain boundary that is the core structure of ZnO varistors. To validate the proposed model, we perform Monte Carlo simulations and the results are compared with those obtained from accelerated AC tests. At the end as a summary of this study, we introduce a conceptual accelerated AC test methodology to analyze the reliability of a new ZnO varistor.