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TLV757P: RΘJA, RΘJC

Yuta Nishie
Genius 5015 points
Part Number: TLV757P

Tool/software:

Hi,

1. Please tell me the reason why there is a package whose value of RθJA is larger than RθJC and why there is a package whose value of RθJC is larger than RθJA in TLV757P.

2. Also, could you confirm whether my understanding of RθJA and RθJC is correct?

If the difference between the junction, which is the source of heat, and the package surface temperature is large, the thermal resistance value increases. (Thermal difference ÷ loss: The thermal difference increases as the loss increases.)

■RθJC measurement conditions

The package surface is at a low temperature because most of the heat is taken away by the cold plate, and the plastic material of the package has a low thermal conductivity, which makes it difficult for heat to pass through. Therefore, the thermal difference inevitably increases. In order to limit the outflow of heat from the bottom of the device at this time, it is mounted on a single-layer substrate (Low-K condition). When this definition was used in the past in JEDEC, it was under the condition that a heat sink was installed.

■Rθja measurement conditions

It is mounted on a four-layer substrate (High-K condition), and the outflow of heat from the bottom of the package to the direction of the thermal pad is dominant. (The reason is that the thermal conductivity of the "non-flowing" air from the package surface to the space is so low that almost no heat can be released. What can be released is radiation energy, but it is within an error range. Rθja measurement is performed under conditions where there is no flow of outside air. )

As described above, since the definition has a large difference in measurement conditions, Rθjc is rarely used (if there is a case where a heat sink is installed by insulating the bottom of the device, it is applied).

Best Regards,

Nishie

  • +1
    TI__Genius 14245 points

    Hi Nishie-san,

    As you mentioned, there can be a lot of variation in how RθJC(top) compares with RθJA for a specific devices. This is because there are several variables which can significantly alter the RθJC(top) value, such as the thermal properties of the encapsulation/mold compound even how close the die is to the top of the package (die can be thicker/thinner and the height at which the die is placed within a specific leadframe can result in the die being closer or farther away from the top of the package). 

    Your understanding of the two metrics seems to be correct. One note about RθJA, you mention "non-flowing air", this means there is no force convection (an active fan blowing air across the device), one small note is that there will be natural convection which is a physical effect that sees small air movement due to the heating of air molecules which then move due to the extra heat/energy. This is primarily just a technicality as this natural convection does not meaningfully improve thermal dissipation. 

    As a general comment, RθJC(top) can be a difficult Thermal Metric to apply to most designs since few use topside heatsinks (as you said, the thermal conductivity of the plastic is poor so the benefit is minimal) and RθJC(top) assumes all of the heat is transferred through the top of the package. In real applications heat is generally transmitted through the bottom of the package/pins to the PCB which acts as the primary heat sink. So if you are trying to estimate the junction temperature based on measuring the temperature at the top of the package it is better to use the ψJT thermal metric which accounts for the heat transmitting through all possible routes.