1、2010 ASHRAE 207ABSTRACT The European Union (EU) directive 2002/95/EC “on theRestriction of the use of certain Hazardous Substances in elec-trical and electronic equipment” or RoHS was implementedin July 2006. However, this was only the first of many RoHS(-like) regulations that have been passed or a
2、re being consideredin many countries. The aim being shared by almost all RoHSlegislation is the elimination of lead in electronic products.These policies are now generally referred to as the RoHSDirective and are often referred to as “Lead-Free” legislation.A printed circuit board, or PCB, is used t
3、o mechanicallysupport and electrically connect electronic components usingconductive pathways laminated onto a non-conductivesubstrate. PCBs have conducting layers on their surface typi-cally made of thin copper foil which if left unprotected, willoxidize and deteriorate. Research has shown that pri
4、ntedcircuit boards made using lead-free materials can be moresusceptible to corrosion than their tin/lead counterparts andit was soon discovered that lead-free products with immersionsilver (ImmAg) surface finish will creep corrode in high sulfurenvironments. The majority of creep corrosion failures
5、occurred on hard disk drives (HDD), graphic cards, and moth-erboards in desktop or workstation systems (only those withImmAg PCB finish were affected).Corrosion-induced failures are frequent in electronicsproducts used in industrial environments. Now even in envi-ronments previously considered relat
6、ively benign withregards to electronics corrosion are experiencing seriousproblems as a direct result of RoHS compliance. Data centersin many urban locations have reported failures of servers andhard disk drives due to sulfur corrosion. Gaseous contami-nation can result in intermittent equipment gli
7、tches,unplanned shutdowns, or failure of critical systems that oftenresult in significant business and financial loss.Desktop and laptop computers, servers, data communi-cations (datacom) equipment and other information technol-ogy (IT) equipment are now at risk due to RoHS. There areindications tha
8、t this may even trickle down into personalcomputers and electronic devices.Manufacturers have to comply with RoHS if they want tocontinue in to do business in the EU, China, etc., and manyhave taken the ImmAg route as their path to compliance. Thishas taken care of one issue but has presented new ch
9、allengeswith regards to equipment reliability.INTRODUCTIONIn 1998, the European Union (EU) discovered that alarm-ingly large amounts of hazardous waste were being dumpedinto landfill sites. Trends also indicated that the volumes werelikely to grow 3-5 times faster than average municipal waste.This h
10、ighlighted a massive, and growing, source of environ-mental contamination. In order to address these issues, the member states of theEU decided to create the Waste Electrical and ElectronicsEquipment (WEEE, 2002/96/EC) directive, whose purposewas to:1. Improve manufacturers designs to reduce the cre
11、ation ofwaste,2. Make manufacturers responsible for certain phases ofwaste management,3. Separate collections of electronic waste (from other typesof waste), andWhats Creeping Around in Your Data Center?Chris MullerMember ASHRAEChris Muller is the Technical Director at Purafil, Inc., in Doraville, G
12、A.OR-10-023 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted
13、without ASHRAEs prior written permission. 208 ASHRAE Transactions4. Create systems to improve treatment, refuse, and recy-cling of WEEE.The WEEE directive laid the groundwork for additionallegislation and a proposal called EEE (Environment of Elec-trical caustic gases, such asammonia; and oxidizing
14、gases, such as ozone. Of these, theacidic gases are of particular concern. For instance, it takesonly 10 ppb (28.98 g/m3)of chlorine to inflict the sameamount of damage as 25,000 ppb (17.40 mg/m3)of ammonia.Each site may have different combinations and concen-tration levels of corrosive gaseous cont
15、aminants. Performancedegradation can occur rapidly or over many years, dependingon the specific conditions at a site. Common sources of corro-sive gases are shown in Table 1. Descriptions of commonpollutants and a discussion of their contributions to equipmentperformance degradation follow.Active Su
16、lfur Compounds. Active sulfur compoundsrefers to hydrogen sulfide (H2S), elemental sulfur (S), andorganic sulfur compounds such as the mercaptans (R-SH).When present at low ppb levels, they rapidly attack copper,silver, aluminum, and iron alloys. The presence of moistureand small amounts of inorgani
17、c chlorine compounds and/ornitrogen oxides greatly accelerate sulfide corrosion. Note,however, that attack still occurs in low relative humidity envi-ronments. Active sulfurs rank with inorganic chlorides as thepredominant cause of atmospheric corrosion.Sulfur Oxides. Oxidized forms of sulfur (SO2,
18、SO3) aregenerated as combustion products of fossil fuels and from autoemissions. Low parts per billion levels of sulfur oxides cancause reactive metals to be less reactive and thus retard corro-sion. At higher levels, however, they will attack certain typesof metals. The reaction with metals normall
19、y occurs whenthese gases dissolve in water to form sulfurous and sulfuricacid (H2SO3and H2SO4).Nitrogen Oxides (NOX). Some common sources of reac-tive gas compounds (NO, NO2, N2O4) are formed as combus-tion products of fossil fuels and have a critical role in theformation of ozone in the atmosphere.
20、 They are also believedto have a catalytic effect on corrosion of base metals by chlo-rides and sulfides. In the presence of moisture, some of thesegases form nitric acid (HNO3) that, in turn, attacks mostcommon metals.Inorganic Chlorine Compounds. This group includeschlorine (Cl2), chlorine dioxide
21、 (ClO2), hydrogen chloride(HCl), etc., and reactivity will depend upon the specific gascomposition. In the presence of moisture, these gases generatechloride ions that, in turn, attack most copper, tin, silver, andiron alloys. These reactions are significant even when thegases are present at low ppb
22、 levels. At higher concentrations,many materials are oxidized by exposure to chlorinated gases.Particular care must be given to equipment that is exposed toatmospheres which contain chlorinated contaminants.Sources of chloride ions, such as bleaching operations, seawa-ter, cooling tower vapors, and
23、cleaning compounds, etc.,should be considered when classifying industrial environ-ments. They are seldom absent in major installations.Hydrogen Fluoride (HF). This compound is a memberof the halogen family and reacts like inorganic chloridecompounds.Ammonia and Derivatives. Reduced forms of nitrogen
24、(ammonia, NH3), amines, ammonium ions (NH4+) occurmainly in fertilizer plants, agricultural applications, andchemical plants. Copper and copper alloys are particularlysusceptible to corrosion in ammonia environments.Photochemical Species. The atmosphere contains a widevariety of unstable, reactive s
25、pecies that are formed by thereaction of sunlight with moisture and other atmosphericconstituents. Some have lifetimes measured in fractions of asecond as they participate in rapid chain reactions. In additionto ozone (O3), a list of examples would include the hydroxylradical as well as radicals of
26、hydrocarbons, oxygenated hydro-carbons, nitrogen oxides, sulfur oxides, and water. Because ofthe transient nature of most of these species, their primaryeffect is on outdoor installations and enclosures. In general,metals are only slightly susceptible to photochemical effects.However, ozone can func
27、tion as a catalyst in sulfide and chlo-ride corrosion of metals.Strong Oxidants. This includes ozone plus certain chlo-rinated gases (chlorine, chlorine dioxide). Ozone is an unsta-ble form of oxygen that is formed from diatomic oxygen byelectrical discharge or by solar radiation in the atmosphere.T
28、hese gases are powerful oxidizing agents. PhotochemicalFigure 2 Short-circuit on PCB caused by corrosion. 2010, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional
29、 reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. ASHRAE Transactions 211oxidation - the combined effect of oxidants and ultravioletlight (sunlight) - is particularly potent.Hydrogen sulfide (H2S), sulfur dioxide (
30、SO2), and activechlorine compounds (Cl2, HCl, ClO2), have all been shown tocause significant corrosion in electrical and electronic equip-ment at concentrations of just a few parts per billion in air.Even at levels that are not noticed by or harmful to humans,these gases can be deadly to electronic
31、equipment. Most of theodor threshold levels are much higher than the levels at whichcorrosive damage will occur.THE NATURE OF CORROSIONWhen discussing electronic equipment, the corrosion ofcopper, silver, or composite materials gives the same result: adisruption of electrical current flow. The sever
32、ity of the envi-ronment (i.e. the types and levels of gases, humidity, andtemperature) will determine the speed at which corrosionforms and how soon corrosion-related effects may appear.Sign of Corrosion For the purposes of this paper, corrosion can be thought ofas two distinct types: the first bein
33、g the more conventionalTable 1. Sources of Reactive Environmental Contaminants (ISA 1985)Constituent Symbol Category Common SourcesAcetic acid CH3COOH Gas Semiconductor manufacturing, wood and wood products, photo developingActive organic nitrogen N2GasAutomobile emissions, animal waste, vegetable c
34、ombustion, sewage, wood pulpingAmmonia NH3GasMicrobes, sewage, fertilizer manufacture, geothermal steam, refrigeration equipment, cleaning products, reproduction (blueprint) machinesArsine AsH3Gas Semiconductor manufacturingCarbon C Solid Incomplete combustion (aerosol constituent), foundryCarbon mo
35、noxide CO Gas Combustion, automobile emissions, microbes, trees, wood pulpingChloride ions Cl Liquid Aerosol content, oceanic processes, ore processingChlorine, Chlorine dioxide Cl2, ClO2GasChlorine manufacture, aluminum manufacture, paper mills, refuse decompo-sition, cleaning productsEthylene C2H4
36、Gas Fruit, vegetable, cut flower storage & transportationFormaldehyde HCHO GasWood products, floor & wall coverings, adhesives, sealants, photo develop-ing, tobacco smokeHalogen compounds HBr, HI Gas Automotive emissionsHydrocarbons (alcohols, aldehydes, ketones, organic acids)HC, THC GasAutomotive
37、emissions, fossil fuel processing, tobacco smoke, water treat-ment, microbes. Many other sources, both natural and industrial, paper millsHydrogen chloride HCl Gas Automobile emissions, combustion, oceanic processes, polymer combustionHydrogen fluoride HF GasFertilizer manufacture, aluminum manufact
38、ure, ceramics manufacture, steel manufacture, electronic device manufacture, fossil fuelHydrogen sulfide H2S GasGeothermal emissions, microbiological activities, fossil fuel processing, wood pulping, sewage treatment, combustion of fossil fuel, auto emissions, ore smelting, sulfuric acid manufacture
39、Inorganic dust SolidCrystal rock, rock and ore processing, combustion, blowing sand and many industrial sourcesMercaptans S8, R-SH Gas Foundries, sulfur manufactureOxides of nitrogen NOxGas Automobile emissions, fossil fuel combustion, microbes, chemical industryOzone O3GasAtmospheric photochemical
40、processes mainly involving nitrogen oxides and oxygenated hydrocarbons, automotive emissions, electrostatic filtersSulfur dioxide SO2, SO3GasCombustion of fossil fuel, auto emissions, ore smelting, sulfuric acid manu-facture, tobacco smoke 2010, American Society of Heating, Refrigerating and Air-Con
41、ditioning Engineers, Inc. (www.ashrae.org). Published in ASHRAE Transactions 2010, Vol. 116, Part 1. For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAEs prior written permission. 212 ASHRAE Transactionscorrosi
42、ve attack where the acid gases react with the metalsthemselves to form non-conductive salts and second typebeing “whisker growth”.Creep Corrosion. Any place where a non-precious metalmeets the atmosphere, corrosion may occur. Some metalsundergo self-limiting, or passive corrosion. Once an oxide orsu
43、lfide layer forms, it will not grow any further. The layer ofcorrosion products effectively insulates the underlying basemetal from the environment, and further corrosion isprevented.In active corrosion, the corrosion layer is not self-limiting.The base metal will continually corrode, and the corros
44、ionproduct will tend to spread out from its point of origin. Any pre-plated or clad contact (coated before stamping) will have bareedges. These edges are free to corrode, and the corrosion prod-uct can grow from the edges and slowly spread across thesurface as shown in Figure 3. This is known as “cr
45、eep corro-sion” (Anon F 2008). Creep corrosion is a mass-transport process during whichsolid corrosion products migrate over a surface. Eventually,the corrosion product will interfere with the electrical connec-tion by creating unacceptably high levels of contact resistance.Pollutants such as chlori
46、ne, hydrogen sulfide, and sulfur diox-ide are known to promote creep corrosion. For componentswith noble metal pre-plated leadframes, creep corrosion is apotential reliability risk for long-term field applications (Zhaoand Pecht 2005). Pore Corrosion. In an ideal world, surface platingswould be unif
47、ormly thick, continuous, and stress-free. In thereal world, there will always be some imperfections in the plat-ing. In many cases, platings will have pores, through which thebase metal will be exposed. The degree of porosity depends onthe plating thickness, application method, base metal rough-ness
48、, and base metal cleanliness. As plating thicknessincreases, there is less likelihood of a pore extending all theway to the base metal. In a very thin plating layer (on the orderof those used for gold), the likelihood is high that many poreswill extend all the way through this layer. Platings over b
49、asemetals with rough surfaces show more porosity than thoseover smooth surfaces. Dirt or oxide on the surface of the basemetal can also lead to the appearance of pores.If the base metal is exposed to a corrosive environmentthrough these pores, pore corrosion can occur. Base metalcorrosion can occur entirely in the pores, where it may behidden from view, or the corrosion product may creep acrossthe surface (Anon F 2008). An example of this is shown inFigure 4. Pore corrosion is promoted by the many of the samegases that promote creep corrosion.Via Corrosion. When a multi-layer circuit
