Technical Protection from the Elements Part III: Corrosion ...

©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.


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



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



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



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



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



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



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.



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

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