David E. Jennings

Failure-mechanism models for conductive-filament formation

This tutorial illustrates design and qualification situations where conductive filament formation, a failure mechanism that can occur in laminates, can affect product performance. Empirical models to design against this failure mechanism are presented in this paper. Examples illustrate the use of these models for design and qualification tests in electronic packaging. >

Assessing time-to-failure due to conductive filament formation in multi-layer organic laminates

Multi-layer organic laminates used in printed wiring boards and laminated multichip modules (MCM-L) can develop a loss of insulation-resistance between two traces, between a trace and a via, and between two vias, due to the growth of conductive filaments along the epoxy resin/glass interface. The growth of the filaments is a function of temperature, humidity, voltage, laminate material, manufacturing processes and the geometry and spacing of the conductors. In order to develop a model which can be used to establish both design guidelines for the prevention of conductive filament formation, and tests for product qualification, a design of experiments study was conducted. Temperature, humidity, and voltage were the stress parameters, and conductor spacing, conductor geometry, laminate material (FR-4, BT and CE) and surface coating (presence and absence of solder mask, solder plate, and post coat) were the laminate parameters. The experimental approach, the analyses of results, and a model for time-to-failure due to conductive filament formation which unifies this study with previous studies, are presented in this paper. >

Moisture ingress into organic laminates

The electronic industrys most common laminates (FR-4, HTFR-4, PI, CE, and BT) were investigated for their equilibrium levels of moisture and diffusion rates and monitored for their moisture content as a function of electrical capacitance. Isothermal sorption tests were performed on the laminates to find the equilibrium moisture content in each laminate at various environmental conditions. The moisture content of each laminate was evaluated as a function of electrical capacitance via a capacitance monitoring plate attached to each laminate. A model was developed to assess the equilibrium content as a function of both temperature and humidity. Data from the sorption-time experiments was used to evaluate a diffusion coefficient for each laminate. The Fickian model was used to facilitate the evaluation. The results of the laminate moisture content analyses were subsequently applied to a printed wiring board (PWB) with a capacitor plate on each of its laminates. The capacitance method of moisture measurement in PWB shows slower moisture ingress than the theoretical moisture diffusion based on laminate experiments.

Quantifying the Impact of Woodpecker Predation on Population Dynamics of the Emerald Ash Borer (Agrilus planipennis)

The emerald ash borer (EAB), Agrilus planipennis, is an invasive beetle that has killed millions of ash trees (Fraxinus spp.) since it was accidentally introduced to North America in the 1990s. Understanding how predators such as woodpeckers (Picidae) affect the population dynamics of EAB should enable us to more effectively manage the spread of this beetle, and toward this end we combined two experimental approaches to elucidate the relative importance of woodpecker predation on EAB populations. First, we examined wild populations of EAB in ash trees in New York, with each tree having a section screened to exclude woodpeckers. Second, we established experimental cohorts of EAB in ash trees in Maryland, and the cohorts on half of these trees were caged to exclude woodpeckers. The following spring these trees were debarked and the fates of the EAB larvae were determined. We found that trees from which woodpeckers were excluded consistently had significantly lower levels of predation, and that woodpecker predation comprised a greater source of mortality at sites with a more established wild infestation of EAB. Additionally, there was a considerable difference between New York and Maryland in the effect that woodpecker predation had on EAB population growth, suggesting that predation alone may not be a substantial factor in controlling EAB. In our experimental cohorts we also observed that trees from which woodpeckers were excluded had a significantly higher level of parasitism. The lower level of parasitism on EAB larvae found when exposed to woodpeckers has implications for EAB biological control, suggesting that it might be prudent to exclude woodpeckers from trees when attempting to establish parasitoid populations. Future studies may include utilizing EAB larval cohorts with a range of densities to explore the functional response of woodpeckers.

Conductive filament formation: a potential reliability issue in laminated printed circuit cards with hollow fibers

E-glass fibers are used as a reinforcement material in the manufacture of laminates used in printed circuit cards and multichip module laminated substrates (MCM-Ls). The principal advantages of using E-glass fibers include high strength, high chemical resistance, and excellent insulating properties at reasonable cost. Although most fibers are solid, hollow fibers can be produced if there is insufficient process control during the manufacture of E-glass fibers. This creates a potential reliability problem in laminates since hollow fibers provide a path for conductive filament formation (CFF) between two differently biased points, which can result in short circuit failure modes. The probability of CFF is a function of temperature, moisture content, the voltage bias and other environmental conditions and physical factors. This paper presents relevant information on E-glass laminate manufacture and the causes of hollow fibers, details of experiments performed to observe hollow fibers, the reliability issues in terms of the CFF mechanism, and analysis as to the opportunity for failure and recommendations for improvement.

Conductive Filament Formation In Printed Wiring Boards

In this paper the preliminary results of a study to characterize the conductive filament formation (often called metal migration) of woven fabric printed wiring boards with respect to resin material, board surface coating, applied bias, geometry of the electrodes and spacing between the electrodes are presented.The experiments were accelerated life tests at controlled temperature-humidity-voltage conditions.

Biotic mortality factors affecting emerald ash borer (Agrilus planipennis) are highly dependent on life stage and host tree crown condition.

Emerald ash borer (EAB), Agrilus planipennis, is a serious invasive forest pest in North America responsible for killing tens to hundreds of millions of ash trees since it was accidentally introduced in the 1990 s. Although host-plant resistance and natural enemies are known to be important sources of mortality for EAB in Asia, less is known about the importance of different sources of mortality at recently colonized sites in the invaded range of EAB, and how these relate to host tree crown condition. To further our understanding of EAB population dynamics, we used a large-scale field experiment and life-table analyses to quantify the fates of EAB larvae and the relative importance of different biotic mortality factors at 12 recently colonized sites in Maryland. We found that the fates of larvae were highly dependent on EAB life stage and host tree crown condition. In relatively healthy trees (i.e., with a low EAB infestation) and for early instars, host tree resistance was the most important mortality factor. Conversely, in more unhealthy trees (i.e., with a moderate to high EAB infestation) and for later instars, parasitism and predation were the major sources of mortality. Life-table analyses also indicated how the lack of sufficient levels of host tree resistance and natural enemies contribute to rapid population growth of EAB at recently colonized sites. Our findings provide further evidence of the mechanisms by which EAB has been able to successfully establish and spread in North America.

Evaluating a New Method for Monitoring the Field Establishment and Parasitism of Oobius agrili (Hymenoptera: Encyrtidae), an Egg Parasitoid of Emerald Ash Borer (Coleoptera: Buprestidae)

Summary We tested 10 cm × 10 cm mesh egg-sentinel envelopes (ESEs) containing emerald ash borer eggs on filter paper, and assessed how 2 different mesh sizes (0.5 mm and 1 mm) affected parasitism by Oobius agrili in the laboratory, and parasitism and predation in the field. Mesh size significantly affected parasitism in the laboratory, with the 0.5 mm mesh reducing parasitism by approximately 20% relative to the 1 mm mesh. Parasitism was much lower in the field with no significant difference among treatments, but the 0.5 mm mesh did significantly reduce predation by almost 50% in comparison to the 1 mm mesh. To reduce egg predation while enabling detectable levels of parasitism by O. agrili, we therefore recommend using mesh screen 0.5 mm wide to create ESEs for field deployment.

Effects of the emerald ash borer invasion on the community composition of arthropods associated with ash tree boles in Maryland, U.S.A.

Emerald ash borer (EAB) Agrilus planipennis Fairmaire is an invasive non‐native woodboring beetle that has killed hundreds of millions of ash trees (Fraxinus spp.) in North America. Identifying the arthropod community associated with ash trees has been highlighted as an important research requirement in understanding the wider effects of EAB. We harvested live ash trees infested with EAB at 37 sites in Maryland from 2011 to 2014 and collected a total of 2031 arthropods. All arthropods were identified to order and 94.6% were identified to family or below. The community comprised 13 orders, 60 families and 41 genera, with 28 arthropod species identified. Coleoptera, Diptera, Hymenoptera and Lepidoptera accounted for 98.3% of arthropods collected. Data on changes in richness and diversity over time were fitted to second‐order polynomial models, corresponding with a transition in the dominant taxa from woodboring beetles (Cerambycidae) to parasitoids (Braconidae and Eulophidae). This resulted in changes to community composition as the EAB infestation intensified. The findings of the present study provide further evidence of the diversity of arthropods at risk from EAB. Given the number of invasive non‐native insects threatening North American forests, establishing what taxa are present is important for predicting the likely broader impacts of these invasions.

Hollow Fibres in PCB, MCM‐L and PBGA Laminates May Induce Reliability Degradation

The presence of hollow fibres in laminates used in printed circuit boards, multichip module laminated substrates, and plastic ball grid arrays has a potentially significant impact on reliability. Hollow fibres are vacuous glass filaments in E‐glass laminate reinforcements and are seen as a reliability problem since they can provide paths for failure mechanisms such as conductive filament formation. This paper discusses the manufacture of woven fabric laminates and the cause of hollow fibres. Experiments conducted to assess the occurrence of hollow fibres in laminates are then presented along with the key reliability issues.