R_θJA (Junction-to-Ambient Thermal Resistance) is defined as the difference in junction temperature to ambient temperature divided by the operating power.

R_θJA is not a constant and is a strong function of:

• the PCB design (50% variation)
• altitude (20% variation)
• device power (5% variation)

R_θJA can be used to compare the thermal performance of packages when 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. R_θJA 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 R_θJA can be measured between these two test cards.

R_θJC (Junction-to-Case Thermal Resistance) is defined as the 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 the die, through the mold compound into the copper block.

R_θJC is a useful thermal characteristic when a heat sink is 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 R_θJB in 1-dimensional thermal simulation of a package system.

R_θJB (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. R_θJB is only defined for the high-k test card.

R_θJB provides an overall thermal resistance between the die and the PCB. It includes a bit of the PCB thermal resistance (especially for BGAs with thermal balls) and can be used for simple 1-dimensional network analysis of package system, see Figure 12-2.

Figure 12-2 PCB Thermal Resistances