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LAIRD TECHNOLOGIES 73
Finding
the right
PCB shop
RobertKranzfromLairdTechnologiesexplains
thedifferencebetweenFR4andthermally
conductivePCBs
SincethereareseveralnamesofthermallyenhancedPCB
substrates–insulatedmetal,metalcore,insulatedmetal
substrates,andthermallyconductivePCBs–theyarecollectively
referredtoasthermallyconductivePCBs(TCPCBs)inthisarticle.
Forthemostpart,FR4andTCPCBsprocessidenticallythrough
componentassemblyandPCBshops.Theapplicationmustrequire
heatremovaltogetthevalueofTCPCB.WhenpurchasingTCPCBs,
however,youwillnoticesomedifferencesinthestandardFR4PCB.
Thefollowingpointsshouldhelpguideyouinthepurchasing
process.
Ask your supplier
IntheFR4world,mostofthesubstrateinformationmeetsspecific
standards.PCBshopshaveagoodunderstandingofthese
requirementsandproperties.Sincetherearenostandardsfor
thermallyconductivesubstrates,however,youshouldobtain
relevantinformationdirectlyfromthesubstratesupplier.
Substratethermalconductivityandapplicationrequirementsare Why use thermally conductive PCBs?
notwellknownbymostPCBshops.Thereareseveralwaysto PCBsmadewiththermallyconductivesubstratesarethreetofour
measureasubstrate’sthermalperformance,whichmakesitdifficult timesasexpensiveasanequally-sizedFR4.However,theincreased
tocomparesuppliersdatasheets.Itisrecommendedthatyou costsassociatedwiththePCBneedtobeweighedagainstthecost
obtainmaterialselectioninformationdirectlyfromsuppliers. ofthewholeassembly.WhenTCPCBsubstratesareused,thePCB
Forthesesamereasons,thespecificsubstrateselectedshould andheatsinkcanbesmaller;andinsomecases,fewerchipsare
beindicated–notasubstratespecification–whensettingupaPCB requiredtodothesamejob.
designfile.Specvariationsfromsuppliers’datasheetscanbe MainapplicationsincludeDC/DCpowerconvertersorcontrollers,
easilyconfusedandoverlooked. motorcontrols,amplifiermodules,andLEDlightengines.
WhencreatingyourfirstTCPCBprototype,seekasupplier’s ThermallyconductivePCBsallowdesignstobesmallerinbothsize
referralforthePCBshoptheyworkwithoraskyourcurrentPCB andvolume.
supplieriftheyrunTCPCBsasanalternative.Itisimportantthat
youworkwithaPCBshopthathasexperiencerunningtheproduct. Single or multilayer?
Thissavescriticaltimeintheearlyprototypingstage.Oncethe Themostwidelyusedconstructionisasingle-layersubstratethat
productisdesignedandreadyformassproduction,givethePCBto isacombinationofbasemetal(typicallyaluminumandsometimes
yourstandardshopandaskyoursuppliertoadvisetheshopon copperwiththicknessof0.5–2mm),thermallyconductive
0
8
howtorunthesubstrates. dielectric,andcircuitcopperlayers.
Someapplicationsrequiremultilayercircuitsandhavetheoption
B
E
R
ofusingdouble-sidedcircuitpairsthatutilisethermallyconductive
M
substrates(DS,doublesided)orFR4(CCL,coppercladlaminates).
ThemostpopularconstructionforDSistwolayersthatisprinted
N
O
V
E
andetched,thenlaminatedtobasemetalusingthermally
P
E
conductivedielectricsheet.TheuseofatwolayerPCBallowsthe
circuittobeevensmallerbecausecircuitrycanbemovedtothe
E
U
R
O
secondlayer.
HybridthermalPCBsuseFR4CCLsubstratesinthesamewayas
DStobuildthePCB. FR4requiresthermalviastomovetheheat
U
R
C
N
G
fromthetoplayertothebottomlayerofthePCB.PTH(plated
S
O
throughhole)viasandthermalviasbasicallylookandareprocessed
C
S
thesameexceptthatoneconductselectricallyandtheotherconducts
N
I
thermally.
T
R
O
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