IMPACT OF DUST ON THE RELIABILITY OF PRINTED CIRCUIT ASSEMBLIES

Dust is a ubiquitous component of the environments in which we live and work. It can deposit on printed circuit assembly to act as a source of ionic contamination. Two common consequences of dust contaminations in the printed circuit boards are loss of impedance (i.e., loss of surface insulation resistance) and electrochemical migration between traces and component leads. Both failure mechanisms involve the contamination forming a current leakage path on a printed circuit board. Based on studies on ionic contaminations, researchers have argued that the impact of dust in these two failure mechanisms is dependent on its pH, its hygroscopic compositions, and the critical relative humidity of the salts in it. However, due to the lack of experimental results and the complexity of dust compositions, the argument is not substantiated. Very few papers concerning the impact of different natural dusts on these two failure mechanisms can be found in the literature. In practice, mixtures of Arizona dust and salts are used as a substitute for dust in experiments. In this research, natural dusts were collected from four locations: natural outdoor and indoor dust samples from Massachusetts, U.S., natural outdoor dust from Tianjin, China, and the ISO standard test dust (Arizona test dust). Loss of impedance in dust contaminated printed circuit boards was investigated under controlled temperature (20ºC to 60ºC) and relative humidity (50% to 95%) ranges. The impact of dust on electrochemical migration and corrosion was evaluated under temperature-humidity-bias tests (50ºC, 90% RH, and 10 VDC). In addition to the conventional DC measurement where only resistive data can be obtained, electrochemical impedance spectroscopy were adopted to obtain nonlinear equivalent circuit models of the electrochemical process, which helps to understand the underlying physics-of-failure. The variation of impedance with relative humidity exhibited a transition range. Below the range, the impedance was constant, and above it, the impedance degraded by orders of magnitude. The value of the transition range decreased with an increase of dust deposition density. The equivalent circuit modeling showed that the dominant resistive path gradually shifted from the bulk to the interfacial with the increase of temperature from 20 ºC to 60 ºC. There were big variations among different dusts, which were quantified using the degradation factor introduced in the research, the critical transition range, and time-to-failure. This result demonstrated that a single salt or a mixture of compounds can not be representative of all dusts. It also indicated that using the ISO standard test dust in place of natural dust samples for reliability evaluation could lead to inaccurate results. Dust should be collected from the field in order to evaluate its impact. It is showed in this thesis that some critical characteristics of dust can be used to classify different dusts for the failure mechanisms of interest. Moisture sorption capability of dust can be used to classify different dusts regarding the loss of impedance failure. The dust with the highest moisture sorption capability had the highest degradation factor. Ion species/concentration or conductivity of dust aqueous solution can be used to classify dust regarding the electrochemical migration related failures. Dust with the highest ion concentration and conductivity had the lowest time-to-failure. The underlying principals behind those critical characteristics were described and discussed based on the physics-of-failure

Study of ionic contamination in electronic product assembly

Dissertação de mestrado em Chemical Analysis and Characterization TechniquesThe ever-growing demand for better PCB (printed circuit board) performance as well as component miniaturization and increasing density, are all challenges the automotive industry is facing nowadays. However, these alterations in the PCB layout alongside changes in production materials and techniques may jeopardize product reliability. One of the factors that has the ability to threaten PCB functioning is the introduction of ionic contaminants that increase the probability of electrochemical reactions and premature circuit failures. Methods have been created along the years in order to evaluate the presence of these contaminants on printed boards; one of them is the ROSE (Resistivity of Solvent Extract) test. This thesis aimed to study the ionic contamination in electronic product assembly using the CM22 contaminometer, a ROSE test machine. Essentially, this project was divided in two studies. The first one was the study of ROSE machine response and establishment of parameters to test its performance at Bosch Car Multimedia Portugal S.A. The results revealed that although the relation between the machine response and the dissolved contamination was linear, there was some disparity in values of PCBs from the same product family. The temperature and regeneration time of the extraction bath were identified as possible influencers of this discrepancy. Further tests showed temperature is not a significant factor for this behaviour and, at lower contamination levels, the regeneration time has little impact. The second study intended to evaluate the ionic contamination levels of printed circuit boards in 5 different stages of their production run. This research revealed that the ROSE levels of the PCBs obtained from the supplier decrease during the first steps of the process. Solder paste application, component insertion and reflow steps do not introduce much ionic residues on the board surface. Yet, the most important conclusion to this study is that the flux used for rework and selective soldering greatly increases surface contamination presence. In conclusion, the CM22 ROSE contaminometer is suitable to monitor processes in assembly plants. It can detect contamination trends at a process line and can be used as process control tool especially in a no-clean soldering process like the one used in Bosch. For this reason, the evaluation of ionic content should be carried out in different stages of PCBA production since it can be very deceptive to do them only after reflow process as it normally done.A crescente demanda por melhor desempenho de PCBs (placas de circuito impresso), bem como a miniaturização de componentes e o aumento da sua densidade, são desafios que a indústria automóvel enfrenta atualmente. No entanto, as alterações no seu design juntamente com as mudanças nos materiais e técnicas de produção podem comprometer a fiabilidade destas placas. Um dos fatores que tem a capacidade de ameaçar o funcionamento de um PCB é a introdução de contaminantes iónicos capazes de criar falhas elétricas no circuito. Vários métodos foram criados para avaliar a presença destes contaminantes em placas impressas; um deles é o teste ROSE (Resistividade do Extrato Solvente). Esta tese teve como objetivo o estudo da contaminação iónica na montagem de produtos eletrónicos utilizando o contaminometer CM22, um dispositivo de teste ROSE. Para isto, primeiro estudou-se a resposta da máquina ROSE e estabeleceu-se parâmetros para testar o seu desempenho na Bosch Car Multimedia Portugal S.A. Os resultados revelaram que embora a relação entre a resposta do aparelho e a contaminação dissolvida seja linear, existiu alguma disparidade nos valores de placas da mesma família. A temperatura e o tempo de regeneração do banho foram identificados como possíveis influenciadores desta discrepância. Testes adicionais mostraram que a temperatura não influencia este comportamento e, em níveis de contaminação mais baixos, o tempo de regeneração tem pouco impacto. O segundo estudo pretendia avaliar os níveis de contaminação iónica de placas de circuito impresso em 5 fases diferentes da sua montagem. Este estudo permitiu verificar que o PCB obtido do fornecedor possui um nível de ROSE que diminui ao longo dos primeiros passos do processo. As etapas de aplicação da pasta de solda, inserção dos componentes e refluxo não introduzem muitos resíduos à superfície da placa. No entanto, a conclusão mais importante deste estudo é que o fluxo usado para retrabalho e soldadura seletiva aumentam muito a contaminação na superfície das placas. Em conclusão, o contaminometer CM22 é adequado para monitorizar os processos de fabrico. Ele pode detetar tendências de contaminação na linha de produção e pode ser usado como ferramenta de controlo de processo, especialmente num processo sem limpeza como o da Bosch. Por esse motivo, a avaliação do conteúdo iónico deve ser realizada em diferentes etapas da produção de PCBAs, dado que pode ser enganoso fazê-lo somente após o refluxo, como normalmente é

PCB Quality Metrics that Drive Reliability (PD 18)

Risk based technology infusion is a deliberate and systematic process which defines the analysis and communication methodology by which new technology is applied and integrated into existing and new designs, identifies technology development needs based on trends analysis and facilitates the identification of shortfalls against performance objectives. This presentation at IPC Works Asia Aerospace 2019 Events provides the audience a snapshot of quality variations in printed wiring board quality, as assessed, using experiences in processing and risk analysis of PWB structural integrity coupons. The presentation will focus on printed wiring board quality metrics used, the relative type and number of non-conformances observed and trend analysis using statistical methods. Trend analysis shows the top five non-conformances observed across PWB suppliers, the root cause(s) behind these non-conformance and suggestions of mitigation plans. The trends will then be matched with the current state of the PWB supplier base and its challenges and opportunities. The presentation further discusses the risk based SMA approaches and methods being applied at GSFC for evaluating candidate printed wiring board technologies which promote the adoption of higher throughput and faster processing technology for GSFC missions

Electrochemical migration of Sn and Ag in NaCl environment

The impact of chloride ion concentration on electrochemical migration (ECM) of tin and silver was studied by using an in-situ optical and electrical inspection system. It was found, that in both cases, dendrites grow not only in an electrolyte solution at low chloride concentration but also in an electrolyte at medium and high or even saturated chloride concentrations as well. According to the results, the migration susceptibility has decreased at low and medium concentration levels in both cases. However, the ECM susceptibility of Ag has increased, while the migration susceptibility of Sn was decreased at the saturated concentrations

Surface Insulation Resistance Degradation and Electrochemical Migration on Printed Circuit Boards

Widespread adoption of lead-free materials and processing for printed circuit board (PCB) assembly has raised reliability concerns regarding surface insulation resistance (SIR) degradation and electrochemical migration (ECM). As PCB conductor spacings decrease, electronic products become more susceptible to these failures mechanisms, especially in the presence of surface contamination and flux residues which might remain after no-clean processing. Moreover, the probability of failure due to SIR degradation and ECM is affected by the interaction between physical factors (such as temperature, relative humidity, electric field) and chemical factors (such as solder alloy, substrate material, no-clean processing). Current industry standards for assessing SIR reliability are designed to serve as short-term qualification tests, typically lasting 72 to 168 hours, and do not provide a prediction of reliability in long-term applications. The risk of electrochemical migration with lead-free assemblies has not been adequately investigated. Furthermore, the mechanism of electrochemical migration is not completely understood. For example, the role of path formation has not been discussed in previous studies. Another issue is that there are very few studies on development of rapid assessment methodologies for characterizing materials such as solder flux with respect to their potential for promoting ECM. In this dissertation, the following research accomplishments are described: 1). Long-term temp-humidity-bias (THB) testing over 8,000 hours assessing the reliability of printed circuit boards processed with a variety of lead-free solder pastes, solder pad finishes, and substrates. 2). Identification of silver migration from Sn3.5Ag and Sn3.0Ag0.5Cu lead-free solder, which is a completely new finding compared with previous research. 3). Established the role of path formation as a step in the ECM process, and provided clarification of the sequence of individual steps in the mechanism of ECM: path formation, electrodeposition, ion transport, electrodeposition, and filament formation. 4). Developed appropriate accelerated testing conditions for assessing the no-clean processed PCBs' susceptibility to ECM: a). Conductor spacings in test structures should be reduced in order to reflect the trend of higher density electronics and the effect of path formation, independent of electric field, on the time-to-failure. b). THB testing temperatures should be modified according to the material present on the PCB, since testing at 85oC can cause the evaporation of weak organic acids (WOAs) in the flux residues, leading one to underestimate the risk of ECM. 5). Correlated temp-humidity-bias testing with ion chromatography analysis and potentiostat measurement to develop an efficient and effective assessment methodology to characterize the effect of no-clean processing on ECM