142
Lattice Semiconductor
Data Sheet
September, 2002
ORCA Series 4 FPGAs
PackageThermalCharacteristics
Summary
Therearethreethermalparametersthatareincommonuse:ΘJA,ψJC,andΘJC.Itshouldbenotedthatallthe
parametersareaffected,tovaryingdegrees,bypackagedesign(includingpaddlesize)andchoiceofmaterials,
theamountofcopperinthetestboardorsystemboard,andsystemairflow.
Θ
ΘJA
Thisisthethermalresistancefromjunctiontoambient(theta-JA,R-theta,etc.):
whereTJisthejunctiontemperature,TA,istheambientairtemperature,andQisthechippower.
Experimentally,ΘJAisdeterminedwhenaspecialthermaltestdieisassembledintothepackageofinterest,and
thepartismountedonthethermaltestboard.Thediodesonthetestchipareseparatelycalibratedinanoven.The
package/boardisplacedeitherinaJEDECnaturalconvectionboxorinthewindtunnel,thelatterforforcedcon-
vectionmeasurements.Acontrolledamountofpower(Q)isdissipatedinthetestchip’sheaterresistor,thechip’s
temperature(TJ)isdeterminedbytheforwarddroponthediodes,andtheambienttemperature(TA)isnoted.Note
thatΘJAisexpressedinunitsof°C/watt.
ψ
ψJC
ThisJEDECdesignatedparametercorrelatesthejunctiontemperaturetothecasetemperature.Itisgenerally
usedtoinferthejunctiontemperaturewhilethedeviceisoperatinginthesystem.Itisnotconsideredatruether-
malresistance,anditisdefinedby:
whereTCisthecasetemperatureattopdeadcenter,TJisthejunctiontemperature,andQisthechippower.Dur-
ingtheΘJAmeasurementsdescribedabove,besidestheotherparametersmeasured,anadditionaltemperature
reading,TC,ismadewithathermocoupleattachedattop-dead-centerofthecase.ψJCisalsoexpressedinunitsof
°C/W.
ΘJA
TJ
TA
–
Q
--------------------
=
ψJC
TJ
TC
–
Q
--------------------
=