Case Temperature; Board Temperature; Stm32 Thermal Parameters; Table 2. Stm32 Thermal Resistances - ST STM32 Application Note

3.4

Case temperature

Case temperature is the temperature of the top of the package used as a chip carrier for the die. The JEDEC
standards specify that the temperature sensor must be placed at the center of the package top using 1 mm of
conductive epoxy.
If the package case-to-ambient thermal resistance is much higher than the package junction-to-case thermal
resistance (one order of magnitude at least) and as most devices dissipated power goes through the board, the
junction temperature can be conflated with case temperature if a certain incertitude is acceptable.
3.5

Board temperature

The board temperature, as defined by the JEDEC standards, is the temperature measured near the center lead of
the longest side of the device. The board temperature and the package junction-to-board thermal resistance are
very critical parameters when assessing the device thermal performance.
Under steady air conditions, most of the heat generated by the device is dissipated through the board. The heat
dissipated through the board can be 20 times higher than the heat dissipated through the package top. For
instance, for a JEDEC high-conductivity test board and under steady air conditions, 95 % of the device power
dissipation passes through the board and only 5 % is dissipated through the package.
3.6

STM32 thermal parameters

Most of STM32 datasheets give only Theta-JA thermal resistance, but some specify also Theta-JC and Theta-JB,
defined in the table below.
Thermal metric
Theta-JA
Theta-JC
Theta-JB
These parameters are determined under the following specific conditions that differ from the final application
conditions:
• Theta-JA: The JEDEC51-2 document Integrated Circuits Thermal Test Method Environmental Conditions -
Natural Convection (Still Air) describes the thermal test method to define Theta-JA.
• Theta-JB: The JEDEC51 document Integrated Circuit Thermal Test Method Environmental Conditions –
Junction-to-Board describes the thermal test method to define Theta-JB.
• Theta-JC: The MIL-STD-883E document Test method standard Microcircuits describes the thermal test
method to define Theta‑JC.
These thermal resistances must be carefully defined in semiconductor packages and devices context.
The most important point about these values, making them "theta", is that the total power dissipated by the device
flows between the two following "points":
• the junction
• ambient or board temperature
There are no extraneous parallel thermal paths in the system allowing some of the heat to "leak" away. All the
heat leaving the junction eventually arrives at or passes through the other point.
It is usually possible to know the total power dissipation of a device but it is far more difficult to know which
fraction of the heat flows out through the case top, versus the part that flows through the leads, and versus the
part that flows through the air gap under the package.
Theta-JC and Theta-JB are generally used as input for thermal simulation tools, that calculate which fraction of
the heat flows out through the top and the bottom of the device.
AN5036 - Rev 3
Table 2. STM32 thermal resistances
Tj − T A
Symbol Value
Pt
θ
JA
Tj − T C
P C
θ
JC
Tj − T B
P B
θ
JB
Case temperature
Main purpose
Used to rank package performance in JEDEC
environment
Used to rank package performance
Used in simulations with 2R model (2R =
two resistances)
Used to rank package performance
Used in simulations with 2R model
AN5036
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