In the chip specification sheet, there are three most helpful values for heat dissipation design: power consumption, temperature requirements, and thermal resistance parameters. The temperature of a chip is usually named according to multiple temperature concepts such as chip junction temperature, shell temperature, bottom temperature, and top temperature based on different location points. The junction temperature of a chip is usually represented by Tj, which is the temperature of the die surface of the chip (junction temperature), and the subscript J is the abbreviation for Junction; Tc is the temperature of the chip packaging surface, and the subscript C is the abbreviation for English Case. The basic thermal resistance characteristic parameters of chips include junction to air thermal resistance Θ JA; Shell to air thermal resistance Θ CA; Crust thermal resistance Θ JC; The thermal resistance of the junction plate is four, including Θ JB.

Θ JA - junction to environment thermal resistance is the thermal resistance from the die surface of the chip to the surrounding environment, measured in ℃/W. The surrounding environment is usually considered the ultimate destination of heat. The JA depends on the IC packaging, circuit board, air circulation, radiation, and system characteristics. When designing for forced convection, the influence of radiation can be ignored. Normally, the Θ JA in the chip specification book will have corresponding environmental ventilation conditions and installed heat sink settings. Different testing settings can lead to significant differences in the test results of Θ JA.
Θ CA - Shell to environment thermal resistance is the thermal resistance from the chip packaging surface to the surrounding environment, measured in ℃/W. Obviously, Θ CA and Θ JA have similar physical meanings, except that the temperature on the chip side becomes the temperature on the chip packaging surface.
JC - Crust thermal resistance is the thermal resistance from the die surface of the chip to the packaging shell, which can be regarded as a specific point on the outer surface of the packaging. Θ JC is one of the key parameters of chip thermal characteristics and an important reference index for heat dissipation enhancement design of chips.
JC depends on the packaging material (lead frame, molding material, die bonding material) and specific packaging design (die thickness, bare solder pads, internal heat dissipation vias, thermal conductivity of the metal material used). For packages with pins, the reference point of Θ JC on the housing is located at pin 1, which extends from the plastic housing. In standard plastic packages, the measurement position of Θ JC is at pin 1. This value is mainly used to evaluate the performance of the heat sink. When testing the thermal resistance of the crust, the testing device will force all the heat of the chip to dissipate from the top of the chip (i.e. the bottom of the chip is insulated).
JB - junction thermal resistance refers to the thermal resistance from the chip junction to the circuit board, and is another key parameter for chip heat dissipation enhancement design. JB quantified the thermal path from the chip Die to the circuit board, expressing the heat transfer resistance from the internal heat of the chip to one side of the single board. JB includes thermal resistance from two aspects: the thermal resistance from the die surface of the chip to the reference point at the bottom of the package, and the thermal resistance of the circuit board that runs through the bottom of the package. When testing the thermal resistance of the junction board, the testing device will force all the heat of the chip to dissipate from the bottom of the chip (i.e. the top of the chip is insulated).
Dual thermal resistance model
The dual thermal resistance model frequently mentioned in commonly used thermal simulation software refers to the use of two thermal resistances, Θ JB and Θ JC, to describe the heat transfer characteristics of chips.

To summarize the packaging characteristics at the chip level, there are three main paths for heat transfer. Firstly, heat is conducted from Die through the packaging material (Mold Compound) to the surface of the device, and then dissipated to the surrounding environment through convective/radiative heat transfer; Secondly, heat is transferred from Die to the solder pad, and then dissipated through convection/radiation by the printed circuit board connected to the solder pad; Thirdly, Die heat is transferred to the PCB for heat dissipation through leads and pins.
The thermal characteristic parameters in actual situations
In addition to these thermal resistance characteristic parameters Θ that have strict requirements for testing conditions, the chip also has a thermal resistance PSI that is not constrained by strict testing scenarios. The thermal physics meanings of PSI and Θ are similar, both referring to thermal resistance. PSI refers to the thermal resistance of the chip in practical applications, while Θ refers to the thermal resistance when heat is transferred entirely along the top or bottom due to the application of insulation measures on one side.
Taking the thermal resistance and thermal characteristic parameters of crust as an example, due to the fixed thermal conductivity of the material itself, in the testing scenario of PSI JT, heat will not be completely transmitted along the top because no coercive measures are applied.
In actual electronic system heat dissipation, heat will be transmitted from the top and bottom of the package or even from the surrounding area, rather than necessarily from a single direction. Therefore, the definition of PSI is more in line with the measurement conditions of actual systems. Similar to the thermal resistance of chip packaging, PSI also has multiple values, such as the thermal resistance between the chip junction and the top of the chip, PSI JT, and the thermal resistance between the chip junction and the bottom, PSI JB. The test value of PSI is not fixed and is related to the heat dissipation enhancement methods used in the chip. When the heat sink used for testing is larger, more heat will be dissipated along the top, often resulting in higher values being measured. Therefore, unless the testing conditions for thermal characteristic parameters can be determined, it is generally not recommended to use this value to understand the thermal characteristics of the chip packaging in previous designs.

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