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THERMAL CHARACTERISTICS OF IC PACKAGES

SurfaceNode

Calculated/Measured

Junction

Calculated/Measured

PCBoard

Calculated

AmbientNode

SN65HVD485E

SLLS612C – JUNE 2004 – REVISED MARCH 2007

APPLICATION INFORMATION (continued)

(Junction-to-Ambient Thermal Resistance) is defined as the difference in junction temperature to ambient

temperature divided by the operating power

is NOT a constant and is a strong function of

• the PCB design (50% variation)

• altitude (20% variation)

• device power (5% variation)

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

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

can be measured between these two test cards

(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.

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

in 1-dimensional thermal

simulation of a package system.

(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

is only defined for the high-k test card.

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 17 ).

Figure 17. Thermal Resistance

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