Datasheet
SLLS173F − JANUARY 1994 − REVISED APRIL 2006
11
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
THERMAL CHARACTERISTICS OF IC PACKAGES
Θ
JA
(Junction-to-Ambient Thermal Resistance) is defined as the difference in junction temperature to ambient temperature
divided by the operating power
Θ
JA
is NOT a constant and is a strong function of
D the PCB design (50% variation)
D altitude (20% variation)
D device power (5% variation)
Θ
JA
can be used to compare the thermal performance of packages if the specific test conditions are defined and used.
Standardized testing includes specification of PCB construction, test chamber volume, sensor locations, and the thermal
characteristics of holding fixtures.
Θ
JA
is often misused when it is used to calculate junction temperatures for other
installations.
TI uses two test PCBs as defined by JEDEC specifications. The low-k board gives average in-use condition thermal
performance and consists of a single trace layer 25 mm long and 2-oz thick copper. The high-k board gives best case in−use
condition and consists of two 1-oz buried power planes with a single trace layer 25 mm long with 2-oz thick copper. A 4%
to 50% difference in Θ
JA
can be measured between these two test cards
Θ
JC
(Junction-to-Case Thermal Resistance) is defined as difference in junction temperature to case divided by the
operating power. It is measured by putting the mounted package up against a copper block cold plate to force heat to flow
from die, through the mold compound into the copper block.
Θ
JC
is a useful thermal characteristic when a heatsink is applied to package. It is NOT a useful characteristic to predict
junction temperature as it provides pessimistic numbers if the case temperature is measured in a non-standard system and
junction temperatures are backed out. It can be used with Θ
JB
in 1-dimensional thermal simulation of a package system.
Θ
JB
(Junction-to-Board Thermal Resistance) is defined to be the difference in the junction temperature and the PCB
temperature at the center of the package (closest to the die) when the PCB is clamped in a cold−plate structure. Θ
JB
is only
defined for the high-k test card.
Θ
JB
provides an overall thermal resistance between the die and the PCB. It includes a bit of the PCB thermal resistance
(especially for BGA’s with thermal balls) and can be used for simple 1-dimensional network analysis of package system
(see Figure 16).
Surface Node
q
JC
Calculated/Measured
Junction
q
JB
Calculated/Measured
PC Board
q
CA
Calculated
Ambient Node
Figure 16. Thermal Resistance