THERMAL SOLUTIONSeGRAF PRODUCTS FOR LED APPLICATIONSINTRODUCTIONsubset of conduction resistance,which itself is onlypenetration of light emitting diodes(LEDs)intoone of the mechanisms by which heat transfer takesmainstream lighting applications such as generalplace(the others being convection and radiation).lighting and liquid crystal display backlight units isInasmuch as different cross-sectional areas can existmanaging the heat generated by the LEDs.Lightin the thermal circuit (e.g.LED-to-board,board-to-output,component lifetime,and power consumptionheat sink)minimizing thermal spreading resistance inof the LEDs are all influenced by the ability tothose cross-sectional areas can be key to minimizingminimize the junction temperature of the LED,andthe thermal resistance from the junction to theeach of these in turn impacts the degree to whichambient environment(R).LED-based lighting products can achieve their costtargets to displace incumbent lighting technologies.Hot Spot ReductionThe temperature of the ambient air can be a limitingVendors from all parts of the value chain(luminairefactor in the extent to which the LEDs can beand LED manufacturers,printed circuit boardcooled.In many cases,the local ambientmanufacturers,heat sink suppliers),are attacking thistemperature that an LED experiences can be raisedproblem with a common objective:achieve LEDdue to the proximity of other sources of heat suchtarget junction temperaturesas driver electronics.This can be problematic forat a minimum cost.eGRAFlarge LED arrays where temperature uniformitythermal management productsbetween LEDs is key to optimizing light output.Inare able to play a unique rolesuch cases,thermal spreading via a heat spreaderin this effort due to the uniqueplaced between the heat source and the LEDs cananisotropic thermal propertiesreduce the local ambient temperature experiencedthat natural graphite offers.by the LED,i.e.can reduce the hot spot caused byNatural graphite heatGraphite Crystal Structurethe driver electronics.Likewise,LEDs themselvesspreaders have high in-planecan cause hot spots at touch surfaces,such as with athermal conductivity of upflash LED in a mobile phone.In this case,thermalto 500 W/mK andspreading via a heat spreader placed between thecomparatively low through-LED and the case can minimize the touchthickness thermaltemperature.conductivity of~3 W/mK.,resulting in superior thermalAnisotropic ThermalOther BenefitsConductivityspreading capability.ThisThermal spreading can also be a tool for optimizingcapability is relevant for direct cooling throughcost.Many designs exist where the extra volumeconduction and for hot spot reduction,both ofconsumed by a thick heat sink or the extra weightwhich are critical in many LED applications.resulting from that thickness is undesirable,orwhere the cost of a mechanical component(e.g.Cooling Through Conductionmaterial change from aluminum to steel or plastic)Figure I'shows a series resistance thermal circuit,that is also used as a heat sink can be optimized ifcommonly understood tothermal spreading in that component were able todepict various resistances tobe improved.These situations are prevalent in theheat flow encountered in anlighting market,where aesthetics,weight,and costLED application.Thermalcan be important to overall design.Thermalspreading resistance occurs asspreading optimization,then,should also be viewedheat flows by conductionas a means for mechanical and cost optimization.between a source and aFigure I_Thermal Resistancesink with different cross-Circuitsectional areas.It is aFor additional information,contact the GrafTech Sales Office:+I (800)253-8003 (Toll-Free in USA).+I(216)529-3777 (International).+(216)529-3922 (Fax)AM469-EG-ZS-LED APPLICATION NOTE Updated:02/28/2008 2008 GrafTech International Holdings Inc.All Rights Reserved.www.egraf.comTHERMAL SOLUTIONSNATURAL GRAPHITE PERFORMANCEunique form factor applications where someCooling Performanceflexibility is beneficial for the product design.TheNatural graphite heat spreaders offer lower thermalother property is the comparatively low weight ofresistance in cooling applications versus copper andnatural graphite (~30%of the weight of aluminum,aluminum heat spreaders where lateral surface area80%the weight of copper),which can offer value inavailable for spreading is high and where theapplications with severe weight limitations,such asthickness available is low,as demonstrated in Figurethose where shock and vibration requirements are2.Natural graphite can thushigh.be used to reduce junctiontemperatures for power LEDsin slim applications whereeGRAF PRODUCTS FOR LEDthere is little room for aAPPLICATIONS:eGRAFlarge heat sink.Naturalgraphite can also be used toSPREADERSHIELD is a sheet of flexible naturalenable the conversion from angraphite with in-plane thermal conductivity up to 500expensive heavy structuralFigure 2W/mK and through-thickness thermal conductivitycomponent that also acts as aof 3-10W/mK.thermal component(e.g.aluminum housing)to a lessexpensive combination of components (e.g.graphiteLED Applicationsspreader plastic housing)in a thickness-or weight-SPREADERSHIELD TM heat spreaders can beconstrained application.considered for any application in which temperatureuniformity hot spot reduction is required.In largeLED arrays,forHot Spot Reduction Performanceexample,temperatureNatural graphite heatuniformity acrossspreaders are ideally suited forthe LEDs may behot-spot reduction across aSPREADERSHIELDcritical forSPREADERSHIELDvariety of small and large-sizeconsistent LED lifetimeelectronics applications.Forand light output.They can also be used to shieldexample,spreaders placed inthe LEDs from extraneous heat sources,or shieldthe display or keypad side of atouch surfaces from the heat generated by the LEDs.mobile phone3 can reduceIn some applications,they can be used as a uniquelyhot spots by as much as 8CFigure 3thin and light weight heat spreader/heat sink for(see Figure 3),while incooling of LEDs in smaller products such as mobileplasma display panels(see Figure 4)temperaturephones,PDA's,mobile gaming units,and notebookreduction of up to 9.5C hasBE6-075computer LED backlight unitsbeen demonstrated.Copper or aluminumProduct Usespreaders/foils and thermallySPREADERSHIELD heat spreaders are available in aconductive plastic films arevariety of thicknesses,grades of thermal conductivityall are comparativelyand coverings(plastic,adhesive,aluminum).ineffective for hot spotFigure 4Thermal,mechanical,and cost requirements of thereduction.application will dictate which grade of the spreaderswill be used.The spreaders are most often securedOther Considerationsto structural components with pressure sensitiveTwo other properties should be considered whenadhesive.They can be die-cut and bent into threedetermining the viability of natural graphite for andimensional forms (with some limitations).Pleaseapplication.One is its flexibility/conformability.see technical bulletin TB32I at www.egraf.com forDue to this property,in applications where themore detail on product variations available.spreader is interfacing to a structural component,forexample,a thermal interface material may beeliminated.This property can also be useful forFor additional information,contact the GrafTech Sales Office:+I(800)253-8003(Toll-Free in USA).+(216)529-3777(International)+I(216)529-3922(Fax)AM469-EG-ZS-LED APPLICATION NOTE Updated:02/28/20080 2008 GrafTech International Holdings Inc.All Rights Reserved.www.egraf.comTHERMAL SOLUTIONSZSPREADER heat spreaders offer a greateropportunity for cost reduction when the followingReliabilityapplication conditions exist:See technical bulletin TB32I atwww.egraf.comThe spreader is ableto be sized(lengtheGRAF®ZSPREADERTMand or width)ZSPREADER heat spreaders consist of Cu or Alindependent of PCBthermal vias embedded in 500 W/m-K naturalsize such thatgraphite.The viasspreader area isare sized andgreater than orFigure 5-ZSPREADER withpositioned accordingequal toPCBto the shape andlocations of the heattimes the area of PCB(see Figure 5)source(s),i.e.theThe conversion of a mechanical componentLED thermal slug.to which the MCPCB is attached to a lessThe vias minimizeSPREADERexpensive material (e.g.steel,plastic,thinner aluminum )can be consideredthrough-thickness thermalThe thermal interface material that isresistance at the LED,typically required to interface the MCPCBresulting in a low profileto the structural component can beheat spreader witheliminated,owing to the flexibility/superior coolingconformability of natural graphiteperformance.Theproduct is designed to beused with low-costProduct Useindustry standardZSPREADER with printedLEDs are reflow-soldered to industry standardprinted circuit boardsprinted circuit boards,typically using SMT processes(FR4,CEM3)as anwith a PCB panel.A hole is placed in the PCB belowalternative to metal core printed circuit boardsthe LED between the LED solder pads,through(MCPCB)for cooling of high power LEDs(>0.5 W),which the thermal via will contact the LED thermalresulting in lower junction-to-ambient thermalslug.The spreader is adhered post-reflow to theresistancebottom side of the PCB with a pressure sensitiveadhesive.After assembly,the LED thermal slug is inLED Applicationsdirect contact with the phase change material on topWith focus on thermal performance,ZSPREADERof the rivet.This contact is assured by the biasing ofheat spreaders provide the greatest advantage overthe rivet such that the rivet head is nominally locatedMCPCBs,when the following application conditions~0.003 inches above the PCB surface.The rivetexist:must then be heated to facilitate the change of phaseand complete the bond..Power LEDs (>0.5W)are usedArea available for thermal spreading isZSPREADER heat spreaders can be used ingreater than or equal to~1.5-2 in2 perapplications where LED-to-LED electricalisolation isLEDrequired(addressed through an anodization layer onThe application is limited by thicknessaluminum rivet)or where HIPOT isolation isand/or weight,or can benefit from reducedrequired (addressed through plastic coverings).Inthickness and/or weight.applications where the spreader is required to beThe mechanical component to which theflush with a surface,an extra layer of graphite can beMCPCB is mounted is a poor thermaladded.conductor (either very thin conductivematerial like aluminum,or plastic/steel)For additional information,contact the GrafTech Sales Office:+I (800)253-8003 (Toll-Free in USA).+I(216)529-3777 (International).+(216)529-3922 (Fax)AM469-EG-ZS-LED APPLICATION NOTE Updated:02/28/2008 2008 GrafTech International Holdings Inc.All Rights Reserved.www.egraf.com