
14
PC8240
2149A–HIREL–05/02
The board designer can choose between several types of thermal interface. Heat sink
adhesive materials should be selected based upon high conductivity, yet adequate
mechanical strength to meet equipment shock/vibration requirements. There are several
commercially-available thermal interfaces and adhesive materials provided by the fol-
lowing vendors:
Heat Sink Selection Example
For preliminary heat sink sizing, the die-junction temperature can be expressed as
follows:
T
J
= T
A
+ T
R
+ (
θ
JC
+
θ
INT
+
θ
SA
) x P
D
Where:
T
J
is the die-junction temperature.
T
A
is the inlet cabinet ambient temperature.
T
R
is the air temperature rise within the computer cabinet.
θ
JC
is the junction-to-case thermal resistance.
θ
INT
is the adhesive or interface material thermal resistance.
θ
SA
is the heat sink base-to-ambient thermal resistance.
P
D
is the power dissipated by the device.
During operation the die-junction temperatures (T
J
) should be maintained less than the
value specified in Table. The temperature of the air cooling the component greatly
depends upon the ambient inlet air temperature and the air temperature rise within the
electronic cabinet. An electronic cabinet inlet-air temperature (T
A
) may range from 30 to
40
°
C. The air temperature rise within a cabinet (T
R
) may be in the range of 5 to 10
°
C.
The thermal resistance of the thermal interface material (
θ
INT
) is typically about 1
°
C/W.
Assuming a T
A
of 30
°
C, a T
R
of 5
°
C, a TBGA package
θ
JC
= 1.8, and a power consump-
tion (P
D
) of 5.0 watts, the following expression for T
J
is obtained:
Die-junction temperature: T
J
= 30
°
C + 5
°
C + (1.8
°
C/W + 1.0
°
C/W +
θ
SA
) x 5.0W
For preliminary heat sink sizing, the heat sink base-to-ambient thermal resistance is
needed from the heat sink manufacturer.
Though the die junction-to-ambient and the heat sink-to-ambient thermal resistances
are a common figure-of-merit used for comparing the thermal performance of various
microelectronic packaging technologies, one should exercise caution when only using
this metric in determining thermal management because no single parameter can ade-
quately describe three-dimensional heat flow.
Dow-Corning Corporation
Dow-Corning Electronic Materials
PO Box 0997
Midland, MI 48686-0997
Chomerics, Inc.
77 Dragon Court
Woburn, MA 01888-4014
Thermagon Inc.
3256 West 25th Street
Cleveland, OH 44109-1668
Loctite Corporation
1001 Trout Brook Crossing
Rocky Hill, CT 06067-3910
800-248-2481
Internet: www.dow.com
781-935-4850
Internet: www.chomerics.com
888-246-9050
Internet: www.thermagon.com
860-571-5100
Internet: www.loctite.com