17
TSPC750A/740A
2128A–HIREL–01/02
AdhesivesandThermal
InterfaceMaterials
Figure8.
ThermalPerformanceofSelectThermalInterfaceMaterial
Athermalinterfacematerialisrecommendedatthepackagelid-to-heatsinkinterfaceto
minimizethethermalcontactresistance.Forthoseapplicationswheretheheatsinkis
attachedbyspringclipmechanism,Figure8showsthethermalperformanceofthree
thin-sheetthermal-interfacematerials(silicone,graphite/oil,floroetheroil),abarejoint,
andajointwiththermalgreaseasafunctionofcontactpressure.Asshown,theperfor-
manceofthesethermalinterfacematerialsimproveswithincreasingcontactpressure.
Theuseofthermalgreasesignificantlyreducestheinterfacethermalresistance.Thatis,
thebarejointresultsinathermalresistanceapproximately7timesgreaterthanthether-
malgreasejoint.
Heatsinksareattachedtothepackagebymeansofaspringcliptoholesintheprinted-
circuitboard(seeFigure7).Thisspringforceshouldnotexceed5.5poundsofforce.
Therefore,thesyntheticgreaseoffersthebestthermalperformance,consideringthe
lowinterfacepressure.
Theboarddesignercanchoosebetweenseveraltypesofthermalinterface.Heatsink
adhesivematerialsshouldbeselectedbaseduponhighconductivity,yetadequate
mechanicalstrengthtomeetequipmentshock/vibrationrequirements.
0
0.5
1
1.5
2
0
10
20
30
40
50
60
70
80
Silicone Sheet (0.006 inch)
Bare Joint
Floroether Oil Sheet (0.007 inch)
Graphite/Oil Sheet (0.005 inch)
Synthetic Grease
Contact Pressure (psi)
S