SN74ACT244: The value of Junction-to-case (top) thermal resistance

The application report Semiconductor and IC Package Thermal Metrics (SPRA953) suggests that  ψ_JT is more useful for practical purposes:

The measurement procedure for ψ_JT is summarized from JESD 51-2 as follows:

Step 1. Mount a test package, usually containing a thermal test die, on a test board.
Step 2. Glue a fine gauge thermocouple wire (36 gauge or smaller) to top center of package.
Step 3. Dress the thermocouple wire along package to minimize heat sinking nature of thermocouple.
Step 4. Dissipate power in test die.
Step 5. Measure the test die junction temperature and thermocouple temperature.
Step 6. Divide the thermal gradient between the junction temperature and surface temperature by the dissipated power.

Why is ψ_JT not a true thermal resistance? In the above procedure, the heat energy generated by the test die is allowed to flow normally along preferential thermal conduction paths. The quantity of heat flowing from the die to the top of the package is actually unknown in the measurement, but is assumed to be the total power of the device for the purposes of ψ_JT calculation. Clearly, this assumption is invalid, but when calculated this way, ψ_JT becomes a very useful number because the experimental configuration is much like the application environment of the IC package. As such, the amount of energy flowing from the die to the top of the package during test is similar to the partitioning of the energy flow in an application environment. In comparison to Equation 4, the actual junction temperature can be very closely estimated using Equation 8:

T_J = T_C + (ψ_JT × Power)

Hello,

I will get back to you on the values; I may have to request the data. That App note Clemens posted is also a good read in case you are unsure which metric is best for your needs. 

I will let you know soon. 

regards,

Gabriel

Hello,

Thank you guys for your answer.
Thank you Gabriel, I will wait for your reply of the values,

Best Regards,
Tom

Hi Tom,

I have requested the standard data sheet thermal metrics and will get back to you as soon they are in. It can take up to three weeks. 

thanks,

Gabriel

Hi Gabriel,

Thank you very much and I will wait for you answer.

Best Regards,
Tom