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

Health-related aerosol particle studies, respiratory tract deposition and indoor source identification

Aerosol particles have, since Classical Antiquity, been linked to adverse effects on human health. It is estimated that the particles in urban air pollution causes 100 000 deaths in Europe each year, whereof 5 000 in Sweden. These figures do not include the outcomes of indoor sources or smoking, which shortens the lives of millions of people worldwide. Many studies indicate that fine particles (<2.5 μm) are to be more toxic than larger ones. Especially the ultrafine particles (<0.1 μm), typically originating from combustion sources, have been of much concern. Part of the reason could be their high probability to deposit deep into the lung once inhaled. A novel method has been developed for determination of fine and ultrafine particle deposition in the respiratory tract. It is designed to be used on larger groups of human subjects in exposure studies and in typical ambient and indoor environments. The method is demonstrated to have a precision in the determined deposition fraction (DF) of 0.02–0.08 and to be sensitive enough to quantify differences between breathing patterns and between hygroscopic and hydrophobic aerosols. The results for hydrophobic particles are in agreement with the well-established ICRP 66 model. The developed instrument was used to investigate the influence of hygroscopicity (the ability to grow by uptake of water), exercise level, gender and intersubject variability on size-dependent deposition of fine and ultrafine particles (12-320 nm) during spontaneous breathing. DF was measured for 29 healthy adults (20 men, 9 women) in four exposure situations; rest and light exercise with both hydrophobic (Di-Ethyl-Hexyl-Sebacate) and hygroscopic (NaCl) particles. DF was 2-4 times higher for the hydrophobic ultrafine particles than for the hygroscopic. DF of hygroscopic ultrafine particles could be estimated by calculating their equilibrium size at 99.5% relative humidity. The differences in average DF due to exercise level and gender were essentially insignificant, but the minute ventilation was 4-fold higher during exercise and 18%-46% higher for the males. Consequently the deposited dose of particles was 4-fold higher during exercise and considerably increased for the male subjects. Some individuals generally had a high DF in all four sessions. To assist the work for healthy indoor environments, a methodology for identifying sources to particles larger than 0.5 μm was designed and applied in a study of three houses in southern Sweden. The methodology includes (1) visual inspection in order to identify deposited particles and potential sources, (2) measurement of airborne particles at different positions in a building with simultaneous logging of activities and (3) isolation of potential sources in a test chamber for controlled characterizations of the generated particles. The results show that source identification is facilitated by knowledge of concentration variations between different rooms, real-time measurements together with activity reports and information on particle characteristics that are comparable with results from laboratory simulations. Major particle emissions from textile handling, likely due to detergent zeolite residues, were found in the studied houses

Deposition of particle pollution in turbulent forced-air cooling

Rotating fans are the prevalent forced cooling method for heat generating equipment and buildings. As the concentration of atmospheric pollutants has increased, the accumulation of microscale and nanoscale particles on surfaces due to advection-diffusion has led to adverse mechanical, chemical and electrical effects that increase cooling demands and reduce the reliability of electronic equipment. Here, we uncover the mechanisms leading to enhanced deposition of particle matter (PM$_{10}$ and PM$_{2.5}$) on surfaces due to turbulent axial fan flows operating at Reynolds numbers, $Re \sim 10^5$. Qualitative observations of long-term particle deposition from the field were combined with \textit{in situ} particle image velocimetry on a telecommunications base station, revealing the dominant role of impingement velocity and angle. Near-wall momentum transport for $10 < y^+ < 50$ were explored using a quadrant analysis to uncover the contributions of turbulent events that promote particle deposition through turbulent diffusion and eddy impaction. By decomposing these events, the local transport behaviour of fine particles from the bulk flow to the surface has been categorised. The transition from deposition to clean surfaces was accompanied by a decrease in shear velocity, turbulent stresses, and particle sweep motions with lower flux in the wall-normal direction. Finally, using these insights, selective filtering of coarse particles was found to promote the conditions that enhance the deposition of fine particle matter

Indoor acids and bases.

Numerous acids and bases influence indoor air quality. The most abundant of these species are CO2 (acidic) and NH3 (basic), both emitted by building occupants. Other prominent inorganic acids are HNO3 , HONO, SO2 , H2 SO4 , HCl, and HOCl. Prominent organic acids include formic, acetic, and lactic; nicotine is a noteworthy organic base. Sources of N-, S-, and Cl-containing acids can include ventilation from outdoors, indoor combustion, consumer product use, and chemical reactions. Organic acids are commonly more abundant indoors than outdoors, with indoor sources including occupants, wood, and cooking. Beyond NH3 and nicotine, other noteworthy bases include inorganic and organic amines. Acids and bases partition indoors among the gas-phase, airborne particles, bulk water, and surfaces; relevant thermodynamic parameters governing the partitioning are the acid-dissociation constant (Ka ), Henry's law constant (KH ), and the octanol-air partition coefficient (Koa ). Condensed-phase water strongly influences the fate of indoor acids and bases and is also a medium for chemical interactions. Indoor surfaces can be large reservoirs of acids and bases. This extensive review of the state of knowledge establishes a foundation for future inquiry to better understand how acids and bases influence the suitability of indoor environments for occupants, cultural artifacts, and sensitive equipment

Studies of aerosol particle formation from various sources using ion and electron beam analytical techniques.

The thesis presents the results of studies of aerosol particle formation using ion and electron beam analytical techniques. The sources of aerosol particle formation studied are the following: 1. production of primary aerosol particles in the high Arctic region during summers 2. emission of ultrafine aerosol particles from wear on the road-tire interface 3. emission of aerosol particles from district heating units operating on three commonly-used biofuels. A source of primary and nearly hydrophobic aerosol particles within the Arctic pack ice region during summers with a composition similar to that of average crustal rock was identified. Wear on the road-tire interface was found to generate numerous ultrafine aerosol particles of varying morphology. The particle number emission factors per vehicle and kilometer driven are similar in magnitude to the tail-pipe exhaust emissions obtained by use of modern engine technology. Particle emissions from the combustion of biomass were characterized in terms of their elemental composition and particle formation mechanisms. List of papers in this dissertation Paper I: Aerosol particle elemental size distributions during the Arctic Ocean expedition in the summer of 2001. Paper II: Summer high Arctic aerosol particles classified using Transmission Electron Microscopy. Paper III: Traffic-generated emissions of ultra fine particles from the road surface-tire interface. Paper IV: Particle emissions from district heating units operation on three commonly used biofuels. Paper V: Laboratory and field test of a method for high-temperature characterization of fly ash and fly ash precursors

On aerosol particle measurements in urban environments and instrument development

The urban atmosphere is affected by many pollution sources, such as traffic, residential wood combustion and various industrial activities. The spatial accumulation of pollution sources in urban areas results typically in high local concentrations of both gaseous and particle pollution. The meteorological condition prevalent during wintertime are likely to increase the concentrations, due to higher heating needs and lesser atmospheric mixing. In turn this means higher human exposure to aerosol particles, and increased health concerns. The particles produced from the ubiquitous pollution sources of the urban areas have two-fold effects. First, they affect the global climate and local weather conditions, and second, they have significant health impacts, and reduce the average lifetime of the exposed human population. Therefore, it is important to study the urban area and its pollution sources and ensure that we have the tools needed to carry out this research. The urban environment is highly transient, with strong temporal and spatial variations in the aerosol particle concentration. Aerosol particles are emitted among other sources from combustion processes. These processes release both particles directly and indirectly, as precursor gases that can later form large numbers of the minutest of particles in the air. In this thesis I study the properties of urban air quality during wintertime, with a special focus on the aerosol component, and more specifically the counting and sizing technology. The first portion of the thesis describes our finding regarding the urban atmosphere in two European cities, Helsinki and Budapest. In Helsinki we present findings of the different sources of pollution including strong regional sources, such as long-range transport as well as localized sources, like residential wood combustion and highways. We also describe how the pollutants disperse rapidly from local highways to background concentrations and attempt to quantify some of the traffic emissions. In Budapest we show how the local particle population consists of two distinct modes: one corresponding to non-hygroscopic fresh traffic emissions, and the other to more aged aerosol with both higher hygroscopicity and volatility. The latter part of the thesis discusses the state of current particle counting and sizing technology. The particle population in urban areas, tends to evolve rapidly, exhibit high particle number concentrations, and comprises of extremely small particles. To meet these measurement needs, we present the time response characterization of numerous commercial particle counters and discuss the metrology of response time measurement. We go on to compare some of the novel sub-3 nm particle counters developed in recent years and their properties, and finally describe the development and field testing of a purpose-built particle sizer for the sub-10 nm size range. Our findings show how in the sub-3 nm size range, the particle composition can influence the detection efficiency of condensation particle counters.Kaupunkialueiden ilmanlaatuun vaikuttavat useat päästölähteet, mukaan lukien liikenne, puun pienpoltto sekä erilaiset teolliset prosessit. Kaupunkialueilla ilmansaasteiden päästölähteitä on ympäröiviä alueita tiheämmin, minkä vuoksi kaupunkialueiden ilmassa on usein korkeita pitoisuuksia niin hiukkasmaisia kuin kaasumaisia päästöjä. Kohonneet saastepitoisuudet yhdistettynä kaupunkien korkeaan väestötiheyteen johtaa suurempaan ihmisaltistukseen päästöille ja kohonneeseen negatiivisten terveysvaikutusten riskiin. Hiukkaspäästöjen on osoitettu olevan terveydelle haitallisia ja johtavan globaalisti huomattavaan eliniän lyhenemiseen. Terveydellisten vaikutusten lisäksi kaupungeissa syntyvillä hiukkaspäästöillä on vaikutus paikalliseen ja alueelliseen ilmanlaatuun. Ilmakehän pienhiukkasia muodostuu kaupungeissa muun muassa liikenteen polttoprosesseissa. Useiden paikallisten päästölähteiden johdosta yksi kaupunki-ilman erikoispiirteistä onkin huomattava ajallinen ja alueellinen vaihtelevuus ilmansaasteiden pitoisuuksissa. Hiukkasilla on myös huomattavia ilmastollisia vaikutuksia, joita ei kaikkia tunneta. Tämän vuoksi on tärkeää tutkia hiukkaslähteitä ja hiukkaspopulaation kehitystä kaupunkiympäristössä. Tässä väitöskirjassa tutkin kaupunkialueiden ilmanlaatua talvikaudella, keskittyen aerosolihiukkasiin, ja eritoten näiden koonmääritys- ja laskentateknologiaan. Väitöskirjan alkuosassa käsitellään kaupunki-ilman tutkimustuloksia kahdesta eurooppalaisesta kaupungista, Budapestista ja Helsingistä. Helsinkiin sijoittuvissa tutkimuksissa tarkastellaan eri päästölähteitä kuten liikennettä ja puun pienpolttoa, ja kuvataan näiden lähteiden hiukkaspäästöjen kemiallisia ja fysikaalisia eroja. Kuvaan myös miten turbulentissa kaupunki-ilmassa liikennepäästöt sekoittuvat nopeasti ympäröivään kaupunki-ilmaan muodostaen jyrkästi laskevan pitoisuusgradientin päästölähteiden ympärille. Budapestissa suoritetussa tutkimuksessa osoitetaan miten liikenteen päästöt erottuvat niin kooltaan, pitoisuuksiltaan kuin hygroskooppisilta ominaisuuksiltaan alueellisesta tausta-aerosolista. Jälkimmäinen osa väitöskirjasta käsittelee hiukkaslaskureiden ja hiukkaskoon määritykseen liittyvää teknologiaa. Kaupunki-ilmassa on tyypillisesti hyvin korkea pienhiukkasten lukumääräpitoisuus, josta pienimmät, alle 10 nm pienhiukkaset, muodostavat usein valtaosan. Kaupunkialueilla muutokset hiukkaslukumääräkokojakaumassa ja kokonaishiukkaspitoisuudessa voivat olla nopeita. Kaupunkiympäristöön soveltuvien hiukkaslaskureilla tulee siis olla hyvä pitoisuusvaste ja lyhyt vasteaika. Väitöskirjassa vastaan näihin teknologisiin haasteihin määrittämällä erilaisten hiukkaslaskureiden vasteaikoja ja metodologian vasteajan mittaamiseen. Väitöskirjassa kuvataan myös uusien hiukkaslaskurimallien havaintotehokkuutta, ja siihen vaikuttavia tekijöitä alle 3 nm kokoluokassa. Esittelen myös uuden alle 10 nm hiukkaskokoluokittelijan karakterisoinnin sekä tuloksia laitteen toiminnasta ulkoilmamittauksissa. Tuloksissa korostuu hiukkasten kemiallisen komposition vaikutus alle 3 nm hiukkasten lukumääräpitoisuuden ja koon määrittämisessä