Datasheet

Junction
PCBoard
q
JB
Calculated/Measured
SurfaceNode
AmbientNode
q
CA
Calculated
q
JC
Calculated/Measured
SN65HVD50-SN65HVD55
www.ti.com SLLS666E –SEPTEMBER 2005–REVISED OCTOBER 2009
APPLICATION INFORMATION
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:
• the PCB design (50% variation)
• altitude (20% variation)
• 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 it consists of a single copper trace layer 25 mm long and 2-oz thick. The high-k board
gives best case in-use condition, and it consists of two 1-oz buried power planes with a single copper trace layer
25 mm long and 2-oz thick. 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 applied to package. It is not a useful characteristic to
predict junction temperature because it provides pessimistic numbers if the case temperature is measured in a
nonstandard 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 as 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 26.
Figure 26. Thermal Resistance
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