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©LightingGlobal–September2013
Introduction
The environmental stresses placed on pico‐powered
lighting products are often fairly extreme. Most aredesignedtoseedailyexposureto intensesunlightandthe associated thermal cycling. They are oftenmobile
devices and will continually be moved from onelocation to another with the very real possibility ofregular drops and spills. They get dirty from ground
contactandroughhandling,andmanywillbeexposedtowaterintheformofrainfall,moistureintheair,andgroundwatercontact.
Theelectronicnatureofpico‐poweredlightingproducts
coupledwith their typical serviceenvironment createsanatmospherethatcanbeveryconducivetocorrosion.Thebatteries,electroniccircuitboards,LED lights,and
multiple external connectors (for wires betweenproductcomponents)areallpotentiallyvulnerable.Thefailuremechanismsdescribed in thisarticleareknown
toaffectelectronicdevices in circumstances similar tothoseseenbypico‐powered lightingproducts,but it isnotknownifdevicesinthefieldhaveseenallofthese
typesoffailure.
CatastrophicandCorrosiveFailuremodesAnyeventthatcausesaproducttostopworkingcanbe
considered catastrophic. For the purposes of thisdiscussion, catastrophic failure modes are ones thatimmediatelyorveryquicklycausetheproducttofail.A
failed drop test is a good example of a catastrophicfailure – the productmay stop functioning if droppedonto a hard surface because a wire solder joint
detachesfromtheprintedcircuitboard(PCB)insidetheenclosure. Another example might be related tophysical ingress, where a stick or sharp object enters
the housing and destroys the small lens on the LEDpackage,killingthelightoutputinstantly.
Water exposure, on the other hand, may or may notcausecatastrophicfailure.Waterdoesnotimmediatelydestroyallelectronicsorelectronicdevices.Cellphones
tend to catastrophically fail after one full submersion(due to damaged suffered by the sensitive screenelements), but many pico‐powered lighting products
will continue to function immediately afterelectronicsareplacedinwater.Thismaynotbetrueforprolongedorrepeatedexposures,however,asthecombinationof
waterandchargedelectrical circuits tends topromotecorrosion and is certainly capable over time ofdestroyingcircuit (andproduct) functionality.Whether
a product fails in this circumstance depends on anumberof variables including thedegreeand locationof water exposure, duration, temperature, and the
voltagedifferenceofadjacentconductors.
Figure1.CorrosiononaLightingAfricatestproduct
TechnicalNotesIssue14Sept2013
ProtectionfromtheElementsPartIII:CorrosionofElectronicsThisTechnicalBriefingNoteexaminescorrosionmechanismscommontoelectronicassembliesandsummarizesconformalcoatingandpottingtechnologiesusedtomitigatecorrosiveactioninfinishedpico‐poweredlightingassembliesInformationcontainedinthisarticlebuildsonpreviousTechnicalNotesavailableontheLightingGlobalwebsite.
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
ElectronicCorrosionmechanisms
There are several corrosion mechanisms common toelectronic assemblies. Corrosion occurs in a variety ofconditionswheremetals are exposed to humidity and
water, and some corrosion mechanisms are madeworsebythepresenceoflowvoltageelectricpotentialsbetweenadjacentmetalsurfaces.Thetimerequiredfor
corrosion to affect a product is highly variable and aresult of many environmental, temperature, andexposure factors. Some of the following corrosion
mechanisms have been observed directly in pico‐powered lighting products tested by Lighting Globalwhileothersareknowntooccurinelectronicsgenerally
buthavenotbeenconfirmedintestedproducts.
Electrolyticcorrosion
This type of corrosion typically presents the greatestrisk of damage to pico‐powered lighting products.
Electrolytic corrosion is caused by small electriccurrentsthatflowbetweenadjacentmetalconductorsin the presence of an applied voltage and a liquid
electrolyte(Figure2).
Figure2.Electrolyticcorrosiondiagram
Themetalconductorsserveasthetwoelectrodesinan
electrolytic cell, and small water droplets and surfacewatervaporcanfunctionastheelectrolyte.Metalionsfrom one electrode (the anode) dissolve in the
electrolyteandaredepositedontheadjacentelectrode(thecathode)(Figure3).Thewaterinthecellmusthaveionic components necessary for the reaction. These
“contaminants” often come from the processing andhandlingofthecircuitboardorarebyproductsofother
product assembly steps and subcomponentmaterials.Sources of contamination include flux residues,
cleaners, processing gases, coatings, and many circuithandling steps where salts, acids, and other ionicelementshaveaccesstotheboardandcomponents.
Manufacturers may be tempted to encapsulate, coat,or insulatespecificanodicpointsintheirassembliestoprevent metal ions from entering solution. This
approachshouldbehandledwithcaution,andspecifictesting should be performed, as any defects in thecoating and anode exposure can lead to highly
accelerated local corrosion of the anode if anelectrolyticpathisavailableforthereaction(Figure4).Identifying the electrochemical anode can also be
+ ‐‐+conductorconductor contaminatedwater
corrosionzone
Figure 3. Electrolytic corrosion of a Lighting Globaltestproduct.Theanodeispreferentiallycorrodedasmetal ions enter an electrolyte solution formed by
water vapor between the LED leads. The leads aretinplatedsteelasevidencedbytheironoxidestainsontheanodeandelectrolytepathonthePCB.
Figure4.Corrosion on theanode solder padunderaconformalcoating. Note theincompletecoverage of thesolderonthepad
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
confusing. A battery in the system can have onebattery terminal OR the other serving as an anode
depending on whether the battery is charging ordischarging.
Galvaniccorrosion
Galvaniccorrosion,distinct fromelectrolyticcorrosion,occurswhentwodissimilarmetalscomeincontactwith
one another in the presence of water (Figure 5). Thiscanoccurwithsolderjoints,onplatedconductors,andcan be particularly problematic on conductors used in
switchesandplugcontacts.
The two dissimilarmetals in the galvanic cell create anaturalvoltagepotentialwhichcandrivethecorrosionprocess – no externally applied voltage is necessary.
Again,anelectrolytewith ionicconductors isrequired,andthiscancomefromwatervapor,condensation,andwaterimmersionoftheconductors.
Figure5.Galvaniccorrosionofdissimilarmetals
Electrochemicalmigration
Electrochemicalmigrationoccurswhenthinconductivedendrites form across adjacent conductors, shorting
the connection. This can occur in an electrolytic cellformedbetweenadjacentpinsonan integratedcircuit(IC) package, or between thinly spaced copper circuit
lines.Dendritescanformfromsilver,copper,tin,lead,or a combination of metals, and growth rates can bequitehigh.Failures fromdendritegrowthcanoccur in
less than a day under the right environmentalconditionsandcircuitlayout(Figure6).
CreepCorrosion
Creep corrosion forms from copper sulfide oxidationproductson the surfaceof a PCB. Theoxide layer canformontopofthesoldermaskandistypicallyseenin
environmentswithelevatedsulfurlevels(Figure7).
conductor#2
conductor#1
contaminatedwater
corrosionzone
Figure 6. A conductive dendrite short circuitunderneathconformalcoating.
Figure7.CreepcorrosionofPCBvias
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
FrettingCorrosion
Frettingcorrosionaffectsnon‐nobleelectrical contacts(especially tin) undergoing small cyclical oscillations
caused by vibrations at the metal‐metal contactsurfaces.Thesesmalloscillationsbreakaparttheoxidesthat naturally form on the metal, and because the
relative movement of the opposing surfaces is verysmall these oxides can build up between the contactsand lead to an increase in the contact resistance. The
normal wiping action of plugging/unplugging thecontact will typically clear the oxides away, but thiscorrectivemeasureisnotavailableforinternalwire‐to‐
wire or wire‐to‐board connectors. Using noble metal(e.g. gold‐plated) contacts or high insertion forcecontacts (where the mated parts are held firmly in
place)willpreventmostcasesoffrettingcorrosion.
Tinwhiskers
The advent of lead‐free technologies also gave rise toan increased risk for the formation of tin whiskers.Although not a form of corrosion, tin whiskers
nevertheless behave in much the same way as somecorrosion mechanisms. Very thin, hard whiskers are
knowntogrowoncomponentleadsthataretin‐plated(Figure 8). These whiskers are thought to arise as areaction to surface compression stress in the pure tin
coatingandcanshortcircuitadjacentleads.
Watercontaminationandioniccontent
Waterisakeydriverofcorrosioninelectroniccircuits.Water can reach sensitive circuit elements from
external exposure (i.e. rain or water can enter theproduct housing), water droplets can condense in thehumid environment often found inside these types of
products,andwatervaporcanbeabsorbedbydirtanddustonthesurfaceofcircuitboardsandexposedmetalconnectorsandinitiatecorrosionthatwouldotherwise
notoccuronacleancircuitboard.
Small concentrations of various contaminants aretypicallyall that is required toallowwater to serveasthe electrolyte in a corrosion process. These
contaminants often come from the processing of theproductduringmanufactureandassembly,eitherfromintentional exposures to process gases, solvents,
plating solutions, polymer formulations, and solderingflux residues, or through incidental contact withworkersorcustomershandlingandusingtheproduct.
Acidic and organic contaminations are particularly
damaging and can greatly accelerate corrosive activityandcorrosionrates.Aggressiveionsfromprocesssteps
willoftenleaveresidues,evenaftercleaning.Chlorides,sulfur compounds, salts, andany chemicals capableofproducing ionic (charged)compoundswill increasethe
corrosiveactionofthewaterelectrolyte.
Propercleaningandprocessingofproductcomponentsis helpful in lowering theelectrolytic contentofwaterexposures. However, since very small contaminations
are enough to enable corrosion, it is typically notsufficient (or wise) to rely on a clean circuit board topreventcorrosionuponexposuretowater(whichmay
itselfalreadyhaveenoughioniccontenttoserveasanelectrolyte).
Batteryterminals
Protecting batteries and battery terminals fromcorrosioncanbechallenging.Batterycontactsandwire
Figure8.Tinwhiskerformingbetweenresistorpads
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
leads will often present ideal locations for corrosionbecause of the applied voltage and water trapping
features of the physical assembly (Figure 9). Contactshavemultipletypesofmetalsandplatingfinisheswithgalvanic potentials. Attempts to insulate the exposed
surfacescanleavesmallspotsunprotectedandleadtoacceleratedlocalizedcorrosion.Somebatterytypescanventcorrosivegasesduringcharge/dischargecycles.
Protectingagainstcorrosivefailures
Designing an inexpensive pico‐powered lighting
product that can tolerate exposures to sunlight, dirt,humidity,rainandfrequentroughhandlingisadifficultengineeringtask.Eachcomponentmustbematchedto
theothersinthesystem,andallmustworktogethertoachievethegoalofcontinuedfunctionality.Noproductwilllastindefinitely,butproperdesignchoicescanhelp
avoid problems that arise as a result of harshenvironmentalexposures.
Materialcompatibility
Thevariousmaterialsused intheproducthousingand
internal components must be compatible with eachother. Someplasticsused for theproducthousingwillcontinue to off‐gas elements that can accelerate
corrosion.Vinyl(PVC)basedmaterials,forexample,canemit chlorine gases and other volatile organic
compounds (VOCs) that can be a source ofcontaminationforelectrolyteformationontheinternal
circuit boards. This can be a problem with very newparts used in the assembly that have not had time tooff‐gasVOCsafterinjectionmolding.
Coatings and finishes must also be compatible to
achieve good adhesion. Cleanliness is once againextremely important inassuring thatmaterialsbehaveaccording to design. Process residues must be
controlledandproperhandlingassuredthroughoutthemanufacturingandassemblyprocess.
PCBfinishes
Circuit tracesaremadeof copper.Boardsare coveredwitha solder resist (soldermask)on topof the traces
and plated with various finishes to protect the barecopper pads prior to component assembly and aid insolderingduringreflow.
Once assembled the PCB pads are covered by the
component leadsand solder. Theplatingmaydissolveinthesolderand/orformvariousalloysatthejunctionthatwillhavesometypeofgalvanicstructure.Viasand
otherboarddetails,however,arenotsolderedandmayhaveareaswheretheboardfinishisexposed.
LightingGlobaldoesnothaveclearevidencesuggesting
which board finishes will protect a pico‐poweredlighting product from corrosion. Immersion tin (ImSn)and lead‐free hot air solder leveled (HASL) finishes
containtinthatcanprotecttheunderlyingcopper.Goldfinishes(ENIG,NiAu)resistoxidationbutcanalsoformstrong galvanic couples. Gold finishes are expensive,
however, and immersion tin may be prone to tinwhiskers. Organic solderable coatings (OSP) andimmersionsilverhavebeenlinkedtocreepcorrosionin
high sulfur environments. Manufacturers areencouraged to investigate the use of different PCBfinishes to increase corrosion resistance and product
lifetime.
Figure9.Earlysignsofcorrosiononbatteryleads
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
ConformalCoatings
Conformal coatings can be applied on an assembledPCB to protect the underlying components from
moisture, dirt, vibration/impact, and temperatureextremes. There are several different types ofconformalcoatingmaterialsandapplicationprocedures
(Figures10,11).Eachhasadvantagesanddisadvantagesin different environments aswell as cost implications.Standards for conformal coating technologies can be
found in IEC 61086‐3‐1, IPC‐CC‐830B, MIL‐I‐46058C,UL746E,andUL94
All conformalcoatingsarepermeable toairandwatervaportosomedegree.Withtheexceptionofparylene,
the coatings do not serve as a true water barrier butrather limit the access of water molecules to ioniccontaminants that may then act as the electrolytic
transportcomponentsinadynamiccorrosionreaction.This is particularly important in situations where thecoating adhesion to the substrate fails and an
electrolyticreactionoccursunderneaththecoating.
Cleanliness is critically important to achieving goodadhesionbetweenthecoatingandPCBsubstrateand
avoidingproblemswithcorrosivefailures(Figure12).
Applicationtechniques
Spray–Multiplespraypassesatdifferentanglesare
necessarytocovercomponentsandavoidshading.
Dip – Provides good coverage under and aroundcomponent leads, but not available for all coatingtypes.
Brush–Goodfor lowvolumeandselectivecoating
on the PCB, but is prone to coverage errors andrequirestrainedpersonneltoavoidproblems.
TypesofconformalcoatingsAcrylic (Type AR) – Acrylic formulations have good moistureresistanceandoffereaseofrepair/rework.Theyareinexpensiveand the most common type of coating used for consumerelectronics.Epoxy (Type ER) – Epoxy formulations provide excellentabrasion and chemical resistance, but poorer moisture anddielectric performance. Epoxies are virtually impossible toremoveforreworkduetotheotherepoxybasedmaterialsonaboard (epoxy glass substrate and epoxy based plastics incomponenthousings),whichcanbevigorouslyattackedbytheapplicationofanepoxybasedstrippingagent.Urethane(TypeUR)–Urethaneformulationsprovideexcellentchemicalresistancecombinedwithgoodmoisture,temperatureanddielectricproperties.Thehigh levelofchemical resistance,however,canmakereworkdifficultandcostly.UV Curable (UV) Ultraviolet light curable coatings offerextremely fast cure speed and low VOC content as well asprovidingexcellentmoisture,dielectric,temperatureresistanceand chemical resistance. UV curable materials are particularlysuitedforhighvolume,selectivecoatingapplications.Silicone (Type SR) – Silicones have high and low temperaturecapability and have very good adhesion to the substrate.Available in soft formulations that have good stress relievingqualities.Parylene(TypeXY)–Sometimesreferredtoasagoldstandard,the deposition process uniformly covers all component edgesandpocketswithadurable,highlyresistantcoating.Paryleneisthemostexpensivecoatingtype.
Figure12.CorrosivefailureonacoatedPCB
Figure10.Conformalcoatingapplicationmethods
Figure11.Conformalcoatingapplicationmethods
©LightingGlobal–September2013
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Importantfactorsinchoosingaconformalcoating
Cost ‐ Costs vary by material choice and applicationtechnique. Acrylic is typically the least expensive (and
very common for these types of applications) whileparylene is the most expensive due to the vapordepositionapplicationprocess.
Environmental protection ‐ For pico‐powered lighting
products, moisture (from humidity and rain) andelevated temperature (~50 C) are likely to be theprimary environmental threats. Under some design
circumstancesUVexposuremayalsobeanissue.Manycoatingmanufacturers suggest that their productswillhelp protect PCBs from humid environments butmay
not provide adequate protection from direct liquidwaterexposuresandsubmersions.
Application technique ‐ Spray, dip, brush, and vapordeposition(Figure10).Someapplicationtechniquesare
notavailableforallcoatingtypes.
Coverage – Proper coating thickness is essential forprotection. Too thin or too thick coats will exhibitservice problems. Pinholes, bubbles, wrinkles, and
delamination of the coating must be avoided. Liquidcoatings have trouble covering sharp edges on
components and any solder spikes where the coatingtendstothinoutduetosurfacetension.Thisisknownas “edge crawling” and can be avoided with multiple
thin coats and careful attention paid to the coatingviscositytomaintainadequatethickness.
Conformal coatings are usually applied to a circuitboard that has selective areas masked off. External
connectors and wire‐to‐board solder pads are oftenused during assembly after the conformal coat isapplied,andunlessfollow‐upprotectionisprovidedto
theseareas(e.g.encapsulation,seebelow)theymaybeatincreasedriskaspossiblecorrosionsites.
Rework/repair ‐ Some coatings are easier to reworkthanothers.Thecoatingmayneedtobechemicallyor
mechanically removed prior to rework andsubsequently reapplied. Some coatings allow solder
jointstobeburneddirectlythroughthematerial.
Curingsystems
Conformal coatings must be properly cured afterapplication. Some coating systems use a two‐partmixturethatcuresthroughachemicalreaction,others
are a single component that cures with exposure toheat,atmosphericmoisture,orUVexposure.
Problems can arise from improper curing of thecoating. Some process contaminants or component
materials can inhibit the cure of two part systems.TallcomponentscanshieldtheunderlyingcoatingfromUV curing lamps, and improper thermal cycling can
leavesomeareasuncured.
Conformalcoatings,wires,andconnectors
Wire‐to‐board solder joints and board‐mounted plug,headers, switches, and connectors are problematicwhenusedonconformalcoatedcircuitboards.Wireto
board solder joints can flex when the wires flex,breaking the coating and/or serving asmoisture entrypointsforwaterthattravelsalongwiresandcollectsat
the joint. When applied after the conformal coat,solder joints will not be protected unless additional
encapsulationisperformed.
Plugsandswitchesthatarenotcoatedpresentexposedmetal surfaces that can corrode in electrolytic orgalvanic reactions. Since these components have
intricate geometries and/or are required to serve anelectromechanical function, they often cannot becompletely coated and protected. Other means must
be used to protect these components such as correctcomponent placement (away from areas that mighttrap water), component selection, and the use of
platingtechnologiesresistanttocorrosion(goldplatedcontacts, while not totally immune to corrosion, arenevertheless more resistant to corrosion). Wiring and
Figure9.Conformalcoatingmaterialtechnologies
©LightingGlobal–September2013
ProtectionfromtheElementsPartII:CorrosionofElectronicsIssue14September2013
Figure13.UVinspectionrevealsuncoatedPCBareas
connector design is often one of the more difficultelementsofcorrosionmitigation.
Qualitycontrolofconformalcoatings
ManycoatingsystemsincludeUVindicatorsthatcanbe
used forpostcure inspectionof theencapsulatedPCBunder a UV inspection lamp (Figure 13). Inspection istypicallyperformedbyatrainedtechnicianwhochecks
for bubbles, voids, blisters, wrinkles, incompletewetting, and coverage errors in the final coat. Postinspection is critical to a successful conformal coating
operation, as small changes in the manufacturingprocess can trigger errors that would otherwise gounnoticed.
PottingandEncapsulation
Potting an electronic circuit board or device involves
filling anenclosure containing thedevicewith a liquidpolymerthatcurestoahardorsoftsolidencapsulant.This can provide a very high degree of environmental
protectiontothepottedcomponents.
Encapsulationissimilartopotting.Apolymerisappliedto the protected component that surrounds andencloses sensitive parts. Unlike potting, however,
encapsulationdoesnotuseorretainanenclosureafterthe polymer cures. Removable molds are sometimes
used, or the encapsulant may have a high enoughviscosity that it canhold its shapeandpositionduring
the cure cycle. Back substrate solar panels are a goodexampleofencapsulationwhereanuncuredpolymerispoured over the solar cells on the substrate. The
viscosity and surface tension of the polymer keep itfrom running over the side of the substratewhile thepolymercures.
One‐part silicone pastes, dispensed from handheld
gunsormachineautomateddispensers,areoftenusedon PCBs to encapsulate connections and to securelarger components like electrolytic capacitors. These
formulations must be compatible with the boardcomponentsandmustbenon‐corrosive.Acetoxycuredsilicones will release acetic acid that may corrode
copper based metals and must be avoided – alkoxycured silicones are available that are specificallydesigned for this purpose and release non‐corrosive
alcoholvaporasacurebyproduct.
Conformalcoating,potting,andencapsulationproblems
Problems can arise from improper design andimplementation of potting and encapsulation
approaches. Many are related to mechanical stressplacedon theboardby thepottingcompound.Duringthe cure cycle, caremust be taken to avoid excessive
swelling and shrinkage of the potting material thatcould strain the encapsulated components. Aftercuring, differences in thermal expansion can stress
component leadsorwarp thePCBand lead tobrokensolderjoints.Thecoefficientofthermalexpansion(CTE)of the variousmaterials, including potting compound,
PCB,andcomponents,mustbematchedtoavoidstresscyclesthatoccurwithdailyheatingandcooling.Epoxypottingcompoundstendtobeharderandwillbemore
likely to stress the PCB and board components, whilesilicones as a class are softer and have higher stressrelievingqualitiesinthefinalassembly.
©LightingGlobal–September2013
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Problems may also occur when water gets under theencapsulation and/or in between the encapsulation
and components. This can result from inadequateadhesionorvoids inthecuredpotting.Wire leadsandconnectors provide electrical access to the
encapsulated components andmay also be a path forwater ingress under certain design geometry. Theedges of encapsulated solar panels and wire junction
boxes tend to be points of entry for water andcontaminants and are often protected by additionalencapsulationandmechanicalframeelements.
Conclusions
As product durability increases so too do therequirements for corrosion protection of the
component electronics. Corrosion prevention isachievedthroughpropersystemdesign,manufacturingcontrols,andproducttesting.Processqualitycontrolis
essential topreventingPCB contamination andproperadhesionofappliedcoatings.
In many cases, coatings and encapsulation materialswill be used to protect the sensitive electrical circuit
elements of a pico‐powered lighting product. Thesuccessor failureof these coatings involves anumberof factors including process quality control, material
selection,andspecificproductdesigngeometry.
No single approachwill be appropriate for all productdesigns. A careful study of the specific materials andgeometry of different products will lead to different
designdecisionsandmanufacturersareencouragedtoexplore novel approaches to increase the corrosionresistanceoftheirdesigns.
References
1.“CorrosionandProtectionofElectronicComponentsinDifferent Environmental Conditions ‐ AnOverview”,Vimalaetal,TheOpenCorrosionJournal,2009,2,105‐
112.“AreviewofCorrosionandenvironmentaleffectsonelectronics”, R. Ambat, Dept. of Manufacturing and
Management,TechnicalUniversityofDenmark3.“PreventingCorrosionofPCBAssemblies”,D.Culen,OnBoardTechnology,October2008
4.“Considerationsforselectingaprintedcircuitboardsurfacefinish”,R.SchuellerPhD,DfRSolutions5.http://www.empf.org/empfasis/dec04/coat1204.htm
6.http://www.empf.org/empfasis/2009/may09/conformal.html7.Informationonconformalcoatingsprovidedby
Humiseal(USmanufacturerofconformalcoatings)www.humiseal.com