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Original question:

TMS320C6748: Thermal calculations

TMS320C6748: Heat dissipation

Archana Rao
Intellectual 915 points
Part Number: TMS320C6748

Hi,

We are using TMS320C6748EZWTD4 on our board (14 layer).  The core utilization is around 70% with maximum power consumption of 0.6W to 0.63W.

From each of the BGA ground pins, we have vias, which are then connected to ground planes.  These vias run from layer 2 to layer 13.

In most of the layers, we have relatively large copper pouring, all of which are connected to ground planes.

We would like to know how effective is the heat dissipation of this processor from the junction to board.  Will using large amounts of copper pouring effectively steer away the heat from the component?

Our thermal analysis team claims that the heat dissipation for this kind of BGAs will be mostly through junction to case rather than junction to board and hence copper pouring is not effective.

We have limitations on our board which does not permit us to use heat sinks.  Please help us to understand this.

Regards,

Archana Rao

  • 0
    TI__Mastermind 30110 points

    Hi Archana,

    This device appears to use a wire-bond package. Power is dissipated primarily into the PCB on wire-bond packages, and primary through the case/ lid on flip-chip packages. From the datasheet Table 8-2. Thermal Resistance Characteristics (PBGA Package) [ZWT], THETA-JC = 7.3 C/W. Flip-chips devices typically have this value below 1 C/W.

    We cannot estimate how well the PCB will conduct heat away from the chip. Additional copper is always better. You have to consider things like the size of the board and how the board dissipates the heat into the ambient surrounding. Is there a case? What is its material and finish? Does it have fins? Is there any airflow outside the case? Even its orientation and shape make a difference when convection is the only way for heat to move into the air. The only way to have an accurate analytical answer is use of 3D thermal modeling tools.

    You can also run some empirical tests to get some idea of the final THETA-JA. You simply need to run the device at a known steady-state, power consumption level for a long period of time and then measure the case temp and the ambient temp using thermo-couples. The resulting THETA-JA = (CASE_TEMP – AMBIENT_TEMP) / POWER with the temperatures in degrees Celsius and the power in Watts.

    If you assume that the board can dissipate all of the heat, a first-order approximation can be gotten from the THETA-JB number in the datasheet. Assuming an ambient of 30C and a power dissipation of 0.6W (and no heat loss through the package lid): THETA-JA = THETA-JB = 12.4 C/W = (CASE_TEMP - 30C) / 0.6W. The resulting CASE_TEMP is estimated at 37.4C; showing that the case operates about 7.4C above the ambient temp. Be sure to understand that the first-order estimate may not be very accurate but it provides an indication of thermal performance with a simple calculation.

    This app note is a good primer on the thermal coefficients and how to used them: http://www.ti.com/lit/an/spra953c/spra953c.pdf 

    Regards,
    Mark