1What is thermal resistance?

The thermal resistance of MOSFET (R)_th）It is a parameter used to demonstrate the heat dissipation capability of a device.

During the operation of the device, a large amount of noise is generated inside the chipcalorie，These heat will be transferred to the substance in contact with it, such as the metal conductor adhered to itThe thermal resistance is a parameter used to measure the strength of the heat resistance ability of the chip, which includes the framework and the plastic encapsulation material that surrounds it, which can hinder the heat transfer of the chipR_thIts unit is℃/W，The smaller the better.

Thermal resistance is divided into two types:R_thj-aandR_thj-cThat isR_thj-a(Juntion-to-Ambience)andR_thj-c(Junction-to-case);

The former represents the thermal resistance from the chip surface to the surrounding environment, with the chip as a system and all other conditionsAs another system, (packaging, metal frame, circuit board, air circulation, etc.) is mainly used qualitatively to evaluate the heat dissipation capability of devices, as shown in Figure 1;

 

The latter represents the thermal resistance experienced during heat transfer from the chip surface to the surface of the plastic package, which is only related to the packaging materialRelated to plastic packaging materials and frames, the heat dissipation capability of the device can be quantitatively evaluated, as shown in Figure 2.

 

2、 Understand

There is a perfect correspondence between thermal parameters and electrical parameters, just like thermal resistance and electrical resistance, as shown in the following table:

 

His publicformula1 is:

 

among which,λ is the thermal conductivity of the medium; D is the thickness of the medium; A is the cross-sectional area of the medium;

According to the deterministic formula,The thermal resistance of MOSFET devices is mainly related to the following factors:

①Chip area

The larger the area of the chip, the lower the thermal resistance.

For example, a larger areaMOSFET chips, with the same packaging shape, usually have lower thermal resistance than chips with smaller areas.

②Chip thickness

The thinner the chip, the more heat it generateschipinternal(PN junction)pass tochipThe shorter the distance on the surface, the smaller the thermal resistance.

③chip material

Thermal conductivity of different semiconductor materialsλ is different, the higher the thermal conductivity, the lower the thermal resistance.

For example, silicon（The thermal conductivity of silicon (Si) is lower than that of silicon carbide (SiC), so MOSFET chips made of silicon materials have higher thermal resistance compared to silicon carbide.

④Package outline

Different packaging shapes have different thermal conduction paths, heat dissipation areas, material characteristics, and structural designs, resulting in different thermal resistances.

formula2：

 

 

among which,TaIndicates the ambient temperature or crust temperature,TbIndicate junction temperature, △TIt is the temperature difference between the two sides of the medium;PIt is the heat flow passing through the medium;

InIn MOSFET devices, P represents the power consumption generated when current flows through the device, and thermal resistance can be measured according to the definition formula.

IIImeaning

powerMOSFET devices typically operate at high voltages and currents, making them prone to heat generation. If this heat is not dissipated in a timely manner, it can cause an increase in internal temperature and, in severe cases, permanent damage to the device.

Therefore, improving its heat dissipation capabilityReduce the thermal resistance of the deviceIt is very important for the normal operation of the device.

Taiyi Semiconductor CompanyFor many years, we have been deeply cultivating powerIn the field of MOSFET and analog chips, this time we will use Taiyi's characteristic modelsTN3401PSAUsing the model of "Perfect Replacement for the Common 3401MOS Tube in the Market" as an example, demonstrate the method our company uses to test this parameter.

4test method

Known thermal resistance（Definition of Rthfor：

ambient temperatureTb is usually easy to obtain, generallydesignated25℃，And the key parameter is junction temperatureHow can they obtain it?

Before that, an experiment is needed to measure that at a fixed junction temperatureP MOSFET devicesTN3401PSANumerical values of key parameters(There is V_TH、V_FSD、RD_ONwait）Then, by changing the junction temperature, obtain a set of data on the variation of this parameter with the junction temperature, and then obtain a quantitative relationship curve between the parameter and the junction temperature. Then, in the later stage, by using the value of this key parameter, the corresponding junction temperature can be obtained.

① Draw a relationship curve chart:

This article uses testingV_F(TN3401PSAVoltage drop of parasitic diode）The method of obtaining the relationship curve with temperature changes is as follows:

 

Translate into EnglishTN3401PSAPut the tube into a container filled with oil, heat the oil to raise the overall temperature of the device, and wait for the overall temperature of the device to rise to the same level as the oil temperatureAfter a certain period of time, when the system temperature stabilizes, the internal junction temperature is equal to the oil temperature. At this point, test the corresponding V at this temperature_FValue, by changing the oil temperature value, test the corresponding values for different temperaturesV_F, you can get a temperatureV_FThe change curve chart.

After experimentation, the curve graph obtained is as follows:

 

② Test thermal resistance:

withtestR_thj-cFor example, at room temperature conditions（Tc=25 ℃), place the MOSFET device flat on the heat dissipation plate, and apply a specific pulse power signal to the device through a power output device (P) to heat it up. Collect the characteristic parameter values of the device for each cycle, and when the parameter values reach equilibrium (indicating that the heat generation of the device is equal to the heat dissipation, and the junction temperature does not change with time)，Record the temperature of the device case at this timeT_a.

Based on the previous junction temperature verification test curve, the corresponding junction temperature can be obtainedT_bAt this point, we can calculate the finalTN3401PSAThe thermal resistanceR_thj-cValue:

Based on the feature parameter values collected for each cycle, we can obtainTN3401PSAThe curve of transient thermal resistance over time under different duty cycle conditions,asthe figure below，The duty cycle widens from bottom to top in sequence：

 

From this figure, it can be seen that:

initial stage: Increased thermal resistancephase，At this stage, the temperature of the chip itself is unbalanced,The temperature at the PN junction is higher than that of other parts, thereforeHeat comes from inside the chipPN junctionConducted to the surface of the chip,itsthermal resistancesizeMainly composed of internal components of the chip(Material)decision;

intermediate stageThe thermal resistance increases, and heat is conducted from the chip to the packaging shell, mainly through solder(Solder paste, etc.)The thermal resistance is determined by factors such as packaging materials;

Steady-state stageThe thermal resistance tends to a constant value, and heat passes through all heat conduction paths (chips)→ Solder → Packaging → Shell) is emitted into the environment, and the thermal resistance of each layer is activated, corresponding to the sum of the thermal resistance of each layer.

Conclusion:

Tai Yi CharacteristicsMOS productsTN3401PSAThe thermal response curve reflects the internal heat conduction mechanism through the change in thermal resistance in the time dimension;

This test parameter is not only used for Taiyi Company'sTN3401PSAThe precise measurement of thermal resistance of pipes can guide the optimization of thermal response packaging design and the investigation of process defects, enabling us to conduct multiple inspections at the process levelTN3401PSAImproved it step by step to become a distinctive product with superior quality compared to other manufacturers.