Introduction
With the increasing integration and complexity of electronic devices, addressing the thermal performance of integrated circuits is crucial. The JEDEC solid state technology association supports the semiconductor industry by proposing test methods and product standards, which are available for free download after registration. STMicroelectronics datasheets specify maximum and minimum operating temperature conditions, defining thresholds beyond which permanent damage is probable and perimeters where electrical parameters are guaranteed within specifications.
Specifying operating temperature ranges relative to ambient or junction temperature is common practice. Depending on the device and its power consumption, vendors use either ambient or junction temperature. This document details the thermal management for ST25R39xx based applications and how to operate the devices within their specified temperature conditions. Applicable products are listed in Table 1.
| Type | Products |
|---|---|
| ST25 NFC readers | ST25R3911B, ST25R3912, ST25R3913, ST25R3914, ST25R3915, ST25R3916, ST25R3920. |
Glossary
| Acronym | Description |
|---|---|
| DUT | Device under test / silicon package |
| IC | Integrated circuit |
| IR | Infrared |
| NFC | Near field communication |
| NMOS | N-channel metal oxide semiconductor |
| PCB | Printed circuit board |
| PMOS | P-channel metal oxide semiconductor |
| TA | Ambient temperature. Represented as TA |
| TJ | Junction temperature. Represented as TJ |
Thermal systems definitions and basic concepts
This section details basic definitions and concepts related to thermal systems, as applied to silicon-based integrated circuits (ICs).
Thermal system definitions
The following thermal definitions are from JEDEC Standard No. 51-13:
- Black body: A perfect radiator or absorber of infrared radiation.
- Emissivity: The ratio of radiant energy emitted by a surface to that emitted by a blackbody at the same temperature.
- Heating current: Current supplied to the device-under-test to cause junction temperature rise.
- Heating power: The product of heating current and heating voltage, causing junction temperature rise.
- Junction temperature: The temperature of the operating portion of a semiconductor device.
- Thermal characterization parameter: A parameter characterizing package behavior. Commonly used parameters are ψJT and ψJB (defined in JESD51-2 and JESD51-6) measuring the temperature relationship between junction-to-top and junction-to-board. Units are °C/W, but they are not resistances as the temperature difference is divided by total power, not power between specific areas.
- Thermal equilibrium: A condition where no heat-producing power is applied, and the device junction temperature (TJ) equals the ambient temperature (TA) in the device's vicinity (thermal steady-state at zero applied power).
- Thermal resistance: A measure of steady-state heat flow from a higher temperature point to a lower temperature point, calculated by dividing the temperature difference by the heat flow.
Thermal model
The JEDEC committee JC-15 provides guidelines for modeling (JESD15) and measuring/reporting (JESD51) thermal behavior. Document "JESD51-0-Methodology for Thermal Measurement of Component Packages" offers a standard method for determining junction temperature under specific conditions.
The Junction temperature is defined as:
TJ = TJ0 + ΔTJ
In most cases, TJ0 equals the ambient temperature.
Under defined conditions for a specific environment, the change in junction temperature is:
ΔTJ = PD × RθJX = PD × θJX
Where:
- PD is the power dissipated in the device (heating power) in Watts.
- RθJX (or θJX) is the thermal resistance from the device junction to the specific environment, in °C/W.
The specific environment can be ambient (RθJA) or case (RθJC). This approach is used to set up a thermal model.
NFC reader PCB thermal model
The thermal model includes thermal resistances such as RJCT (junction-to-case-top), RCTA (case-top-to-ambient), RJCB (junction-to-case-bottom), RCBPCB (case-bottom-to-PCB), and RPCBA (PCB-to-ambient).
Figure 1. Thermal cross-section: This diagram illustrates heat flow paths from the silicon junction through the case (top and bottom) and the PCB to the ambient environment, indicating various thermal resistances (RJCT, RCTA, RJCB, RCBPCB, RPCBA) along these paths.
The thermal resistances RJCT, RJCB, and RCBPCB are determined by the silicon and package. RCBPCB and RPCBA depend on the PCB and environment. For simplification, it's assumed no temperature gradient exists between the top and bottom of the PCB due to thermal vias and the exposed pad's connection to PCB copper, ensuring efficient heat transfer and low RCBPCB.
Figure 2. Thermal Model equivalent circuit: This figure presents an electrical circuit analogy for the thermal model. It shows a voltage source representing the temperature difference between junction and ambient (ΔTJA), with various resistors representing thermal resistances (RJCB, RJCT, RCBPCB, RPCBA, RCTA).
Power dissipation
The thermal model includes resistances like RJCT, RCTA, RJCB, RCBPCB, and RPCBA. The ST25R3916 and ST25R3911B, often in VFQFPN32 packages, have quoted thermal characteristics such as RJCT: 16.95 °C/W and RJCB: 0.85 °C/W. To solve the thermal model, device power dissipation, settled case-top temperature, settled PCB-bottom temperature, and settled junction temperature during continuous field operation must be assessed.
The ST25R3916 and ST25R3911B have three main blocks contributing to power consumption:
- Analog and digital logic: Required during operation. Power consumption is calculated as PAL = VDD × IAL, where VDD is the supply voltage and IAL is the current consumption of the active sections.
- Internal voltage regulator: Power dissipation is calculated as PREG = (VDD - VDD_RF) × IDRV. This is the voltage drop across the regulator multiplied by the current flowing through it (IDRV = IVDD - IAL).
- Transmitter driver stage: Power dissipation is calculated as PDRV = IDRV2 × (RDRV_NMOS + RDRV_PMOS), representing the power dissipated by the driver stage's NMOS and PMOS transistors.
Figure 3. ST25R3916 power consuming blocks: This block diagram shows the main power-consuming components of the ST25R3916, including the Analog and Digital block, Tx Regulator, and Tx Driver, with their respective power and current inputs/outputs.
To measure driver resistance, a specific procedure for the ST25R3916 is required, involving setting registers and executing a power-up sequence. At each RFO, one transistor is active. For example, at RFO1, the NMOS is enabled, and at RFO2, the PMOS is active, as illustrated in Figure 4.
Case Temperature measurement
Figure 4. ST25R3916 simplified driver stage: This schematic depicts a simplified driver stage with PMOS and NMOS transistors connected to VDD_DR and GND_DR, with outputs RFO1 and RFO2. It illustrates how current flows through these transistors.
NMOS driver resistance is measured between RFO1 and GND_DR, and PMOS driver resistance between RFO2 and VDD_DR, preferably using a four-wire measurement. The power dissipated by the driver is calculated using PDRV = IDRV2 × (RDRV_NMOS + RDRV_PMOS).
The procedures for ST25R3916 also apply to ST25R3911B. Once the power dissipated by the three blocks is known, the heating power (PD) flowing through the case to the ambient can be determined. To solve the thermal model, junction and case temperatures must be measured during operation.
There are two common methods for measuring package temperature:
- Thermocouple: A thermocouple with a round sensor body is placed on the package surface. Only a small part contacts the DUT, leading to potential cooling of the sensor and DUT by the ambient. Thermal compound paste is used to maximize heat transfer but can add thermal boundaries. Thermocouples are useful in closed environments.
- Infrared camera: An IR camera (e.g., FLIR ETS320) measures temperature differences precisely, but absolute temperature measurements have a tolerance of +/- 3 °C. Considerations include reflections from heat sources, the camera itself being a heat source, and the need for a 45° angle to the lens. To determine the +/- 3 °C offset, the DUT must reach ambient temperature, and the difference between the measured temperature and ambient temperature is the offset. Emissivity is a critical parameter, detailed in Appendix A.
IR camera temperature offset
The temperature offset can be assessed by storing the DUT at ambient temperature, measuring its temperature with an IR camera, and measuring the ambient temperature with a thermometer. The difference is the camera's offset.
Temperature measurement
Silicon temperature can be measured using an IR camera with an open package IC, requiring emissivity and offset calibration. Alternatively, the ESD protection diode on the RFO1 pin can be used. This method involves forcing 1 mA through the diode with a multimeter (e.g., Keysight 34401A) and measuring the diode voltage, which decreases as temperature increases. Calibration is needed for each device. To correlate voltage drop with junction temperature, the device is heated to a steady state without dissipating power. The ambient temperature at this steady state equals the junction temperature.
Figure 5. IR camera setup: This diagram shows an IR camera and lens positioned to measure the temperature of an ST25R3916 IC on a PCB at a 45° angle.
Figure 6. RFO1 Diode Measurement: This image displays a Keysight 34401A multimeter connected to an ST25R3916 Discovery Board for measuring the RFO1 diode voltage.
The output driver must be disabled during measurement using a specific firmware sequence (setting tx_en bit to 0, d_resX bit to 1, and tr_am bit to 1). An external signal triggers the measurement device after a short timeout.
Junction Temperature assessment
The ST25R3916-DISCO board serves as a reference for measuring the VFQFPN32 package performance. To isolate heat sources, antenna and matching components are replaced by a 2.2 nF capacitor and a 47 Ω potentiometer (TE Connectivity Passive Product: 3-1625931-8). The potentiometer allows precise loading adjustment and power dissipation control. The capacitor prevents DC current between RFO ports.
Figure 7. Potentiometer impedance curve: These are Smith charts showing impedance measurements of the DUT, illustrating S11 parameters over a frequency range.
The ST25R3916 VFQFPN32 DUT on the ST25R3916-DISCO MB1414 PCB (4 layers, 70x31.5mm NFC area) features 9 thermal vias connecting the GND plane across layers.
Figure 8. ST25R3916 VFQFPN32 DUT – Top view: A photograph showing the top side of the ST25R3916 VFQFPN32 DUT on a PCB, with labels pointing to connection points like RFO diode GND and RFO diode input.
Figure 9. ST25R3916 VFQFPN32 DUT – Bottom view: A photograph showing the bottom side of the ST25R3916 VFQFPN32 DUT on a PCB, highlighting the PCB measuring area and the RFO potentiometer.
Figure 10. Potentiometer: A photograph of a potentiometer with a knob, connected to a 2.2 nF capacitor.
Self-heating model definition at 253 mA
A potentiometer is used to set a 253 mA driver output current, typical for NFC applications. Building a thermal model involves six steps:
- Measuring ambient temperature and IR camera temperature offset.
- Measuring self-heating temperature.
- Measuring electrical parameters.
- Calculating dissipated power.
- Defining the thermal model.
- Calculating overall thermal resistance.
Temperature offset measurement
To calibrate temperature measurements between the IR camera and thermocouple, the DUT is stored at ambient temperature until it reaches a steady state. Measurements are taken:
- Thermocouple: TAMB_thermocouple = 23.8 °C
- IR camera: TAMB_IR = 26.3 °C (as shown in Figure 11)
The temperature offset is calculated as: Toffset = 23.8 °C - 26.3 °C = -2.5 °C. The thermocouple temperature is the reference.
Figure 11. Ambient temperature measurement (ε = 0.95): A thermal image displaying ambient temperature measurement, showing a color gradient with a peak temperature of 26.3 °C and a scale from 24.9 °C to 28.9 °C. The IR camera shows a temperature 2.5 °C higher than the actual value.
Measurement of the self-heating at 253 mA
To measure heat dissipation, the device operates in its highest energy-consuming mode, typically continuous wave output. This is configured by setting register 0x02 to 0xC8 (enabling oscillator, receiver, transmitter) and register 0x27 to 0x70 (lowest driver resistance). The IC package surface temperature is monitored until it stabilizes.
Figure 12. Top case temperature (ε = 0.95): A thermal image showing the top case temperature of the IC during self-heating, with a peak reading of 43.8 °C. The actual top casing temperature is 41.3 °C.
The bottom side of the PCB is measured similarly, using an emissivity of 0.9. The bottom PCB temperature is 40.9 °C, as shown in Figure 13.
Figure 13. Bottom PCB temperature (ε = 0.90): A thermal image showing the bottom PCB temperature during self-heating, with a peak reading of 40.9 °C.
After turning off the NFC field (register 0x02 to 0xC0), the multimeter measures the diode voltage. The DUT is heated by a thermal system (MPI ThermalAir TA-5000A) until the diode voltage matches the calibration value. For ST25R3916, registers 0x00 to 0x07 and register 0x01 bit "sup3V" must be set. A diode voltage of 0.33498 V corresponds to 42.9 °C. The junction temperature rise is ΔTJunction = 42.9 °C - 23.8 °C = 19.1 °C.
Measurement of the electrical parameters
The measured temperatures are used to complete the thermal model. Key electrical parameters measured are:
- VDD = 4.975 V
- VDD_RF = 4.759 V
- IVDD = 274 mA
- IAL = 21 mA
- IDRV = 253 mA
- RDRV_NMOS = 1.95 Ω
- RDRV_PMOS = 1.73 Ω
Power dissipation calculation
Dissipated power is calculated using the measured electrical parameters:
- Analog and digital logic: PAL = VDD × IAL = 4.975 V × 0.021 A = 0.105 W.
- Internal voltage regulator: PREG = (VDD - VDD_RF) × IDRV = (4.975 V - 4.759 V) × 0.253 A = 0.055 W.
- Transmitter driver stage: PDRV = IDRV2 × (RDRV_NMOS + RDRV_PMOS) = (0.253 A)2 × (1.95 Ω + 1.73 Ω) = 0.236 W.
Total dissipated power: PTOT = PAL + PREG + PDRV = 0.105 W + 0.055 W + 0.236 W = 0.395 W.
Power consumed by the device: PIN = VDD × IVDD = 4.975 V × 0.274 A = 1.363 W.
Output power: POUT = PIN - PTOT = 1.363 W - 0.395 W = 0.968 W.
Calculating the thermal model
The dissipated power (PTOT) is emitted by the silicon and flows through thermal resistances to the package and ambient, as shown in Figure 1. Calculating the thermal model is optional but recommended for evaluating and optimizing thermal performance. The paths through the bottom and top are parallel, splitting power according to their thermal resistance. Lower thermal resistance leads to lower temperature increase.
Measured temperatures are added to the thermal model shown in Figure 14. The top package temperature is 43.8 °C. The temperature difference to ambient (TCTA) is calculated:
TCTA = 43.8 °C - 26.3 °C = 17.5 °C.
The junction-to-case temperature difference (ΔTJCT) is:
ΔTJCT = ΔTJ - TCTA = 19.1 °C - 17.5 °C = 1.6 °C.
The same procedure applies to bottom PCB temperatures.
Figure 14. Thermal model including Temperatures and PTOT: This diagram shows a thermal circuit with temperature differences (ΔTJA, ΔTCTA, ΔTJCT, ΔTPCBA, ΔTJCB) and thermal resistances (RJCB, RJCT, RCBPCB, RPCBA, RCTA), indicating a total power dissipation (PTOT) of 0.395W.
Calculating the overall thermal resistance
To solve the thermal model, the power flow through RJCT is calculated: PTOP = ΔTJCT / RJCT = 1.6 °C / 16.95 Ω = 0.094 W.
Using the known power dissipated through the top case, RCTA is calculated: RCTA = TCTA / PTOP = 17.5 °C / 0.094 W = 185.38 °C/W.
Power dissipated through the bottom: PBOT = PTOT - PTOP = 0.395 W - 0.094 W = 0.300 W.
Thermal resistance between PCB bottom side and ambient (RPCBA) is calculated: RPCBA = TPCBA / PBOT = 14.6 °C / 0.300 W = 48.622 °C/W.
The thermal resistance between the exposed pad and PCB bottom side (RCBPCB) is calculated using the known bottom case power dissipation: RCBPCB = (ΔTJPCB - ΔTJCB) / PBOT = (4.5 °C - 0.300 W × 0.85) / 0.300 W = 14.136 °C/W.
The overall thermal resistance (RTH) is calculated as: RTH = ΔTJA / PTOT = 19.1 °C / 0.395 W = 48.394 °C/W.
This RTH value is specific to the board shown in Figures 8 and 9. Different packages, PCB layouts, or ambient conditions will alter RCBPCB and RCTA, impacting RTH.
Self-heating model definition at 350mA
With the thermal model solved, junction temperature can be estimated for different operating conditions. For example, increasing driver current from 253 mA to 350 mA requires these steps:
- Define electrical parameters.
- Calculate dissipated power.
- Calculate junction temperature.
Define electrical parameters
Electrical parameters are defined or estimated. For a 350 mA driver current, measured parameters include:
- VDD = 4.982 V
- VDD_RF = 4.585 V
- IVDD = 371 mA
- IAL = 21 mA
- IDRV = 350 mA
- RDRV_NMOS = 1.95 Ω
- RDRV_PMOS = 1.73 Ω
Calculate the dissipated power
Using these parameters, dissipated power for each block is calculated:
- Analog and digital logic: PAL = VDD × IAL = 4.975 V × 0.021 A = 0.105 W.
- Internal voltage regulator: PREG = (VDD - VDD_RF) × IDRV = (4.982 V - 4.585 V) × 0.350 A = 0.137 W.
- Transmitter driver stage: PDRV = IDRV2 × (RDRV_NMOS + RDRV_PMOS) = (0.350 A)2 × (1.95 Ω + 1.73 Ω) = 0.451 W.
Total dissipated power: PTOT = PAL + PREG + PDRV = 0.105 W + 0.137 W + 0.451 W = 0.692 W.
Power consumed: PIN = VDD × IVDD = 4.975 V × 0.373 A = 1.846 W.
Output power: POUT = PIN - PTOT = 1.846 W - 0.692 W = 1.154 W.
Junction temperature calculation
Using the total thermal resistance (RTH = 48.394 °C/W), the junction temperature rise (ΔTJA) is calculated:
ΔTJA = PTOT × RTH = 0.692 W × 48.394 °C/W = 33.48 °C.
At an ambient temperature of 25 °C, the junction temperature (TJ) will be:
TJ = ΔTJA + TAMB = 33.48 °C + 25 °C = 58.48 °C.
The maximum allowable ambient temperature (TAMB_max) for a maximum junction temperature of 125 °C is:
TAMB_max = TJ_max - ΔTJA = 125 °C - 33.48 °C = 91.52 °C.
Self-heating measurement at 350 mA
Assumptions for VDD, VDD_RF, and IVDD at 350 mA are based on real measurements. The estimated self-heating can be compared to real measurements using the following steps:
- Measurement of ambient temperature and IR camera temperature offset.
- Calculation of top and bottom temperature.
- Measurement of top and bottom temperature.
- Measurement of junction temperature.
Measurement of the ambient temperature and IR camera temperature offset
- TAMB_thermocouple = 24.8 °C
- TAMB_IR = 27.2 °C
- Toffset = 24.8 °C - 27.2 °C = -2.4 °C
Calculation of the top and bottom temperature
Thermal resistances are calculated: RTop = RJCT + RCTA = 16.95 °C/W + 185.38 °C/W = 202.33 °C/W. RBot = RJCB + RCBPCB + RPCBA = 0.85 °C/W + 14.136 °C/W + 48.622 °C/W = 63.608 °C/W.
Dissipated power through the top case (PTOP) is calculated: PTOP = PTOT × RTop / (RTop + RBot) = 0.692 W × 63.608 °C/W / (63.608 °C/W + 202.33 °C/W) = 0.165 W. PBOT = 0.692 W - 0.165 W = 0.526 W.
Junction-to-case temperature rise (ΔTJCT) is: ΔTJCT = PTOP × RJCT = 0.165 W × 16.95 °C/W = 2.805 °C. Case-to-ambient temperature rise (TCTA) is: TCTA = PTOP × RCTA = 0.165 W × 185.38 °C/W = 30.674 °C.
For the bottom temperature: Junction-to-case-bottom temperature rise (ΔTCBA) is: ΔTCBA = PBOT × (RJCB + RCBPCB) = 0.526 W × (0.85 °C/W + 14.136 °C/W) = 7.888 °C. PCB-to-ambient temperature rise (TPCBA) is: TPCBA = PBOT × RPCBA = 0.526 W × 48.622 °C/W = 25.591 °C.
Measurement of the top and bottom temperature
Figure 15. Top case temperature (ε = 0.95): A thermal image showing the top case temperature, with a peak reading of 57.8 °C and a scale from 30.0 °C to 62.0 °C. The measured temperature, adjusted for offset, is 55.4 °C. The calculated temperature is 55.473 °C.
Figure 16. Bottom PCB temperature (ε = 0.90): A thermal image showing the bottom PCB temperature, with a peak reading of 49.9 °C and a scale from 30.0 °C to 55.0 °C. The measured temperature, adjusted for offset, is 47.5 °C. The calculated temperature is 50.391 °C.
Measurement of the junction temperature
The diode voltage during self-heating is 0.318 V, correlating to a junction temperature of 58.6 °C when the device is in power-down mode (TJunction_measured = 58.6 °C).
The calculated junction temperature is: TJunction_Calculated = ΔTJA + TAMB = 33.48 °C + 24.8 °C = 58.28 °C.
The calculated and measured junction temperatures show good correlation. The model derived from the 250 mA load is used to calculate junction temperatures for different load scenarios and adjust parameters for specific operating conditions, such as maximum ambient temperature.
Thermal model of a WLCSP package
The WLCSP (Wafer Level Chip Scale Package) exposes the silicon directly to the ambient. Junction temperature is measured directly on the silicon, assuming negligible temperature gradient. The thermal model is simplified.
Figure 17. Thermal model of a WLCSP device: This diagram shows a simplified thermal circuit for a WLCSP package, featuring a single thermal resistance connecting the junction to the ambient.
There is one thermal resistance for junction-to-ambient transition through the silicon's top surface. On the bottom side, thermal resistance depends heavily on the board layout (via types, layers), influencing the ratio of power dissipated through the top versus bottom. Better bottom dissipation leads to lower overall thermal resistance and cooler device operation. The split between top and bottom dissipated power cannot be distinguished in this configuration; only overall thermal resistance is calculated.
Measurement of the ambient temperature and IR camera temperature offset
Figure 18. ST25R3916-BWLT DUT - top view: A photograph showing the top view of the ST25R3916-BWLT DUT (WLCSP package) on a PCB, similar to Figures 8 and 9.
Calculating thermal resistance involves these steps: ambient temperature and IR camera offset measurement, self-heating measurement, electrical parameter measurement, dissipated power calculation, and overall thermal resistance calculation.
For a CSP package, junction temperature measurement using an IR camera is straightforward, as illustrated in Figure 19.
Measurement of the self-heating
Figure 19. Ambient temperature measurement (ε = 0.97): A thermal image showing ambient temperature measurement, with a peak temperature of 28.0 °C and a scale from 26.6 °C to 30.7 °C. The temperature offset between the IR camera and ambient is calculated as 28.0 °C - 26.5 °C = 1.5 °C.
Junction temperature is measured directly on the top side of the package.
Figure 20. Top case temperature (ε = 0.97): A thermal image showing the top case temperature of the ST25R3916 IC (WLCSP package) during self-heating, with a peak reading of 45.1 °C. The actual top casing temperature is 43.6 °C.
Figure 21. Bottom PCB temperature (ε = 0.90): A thermal image showing the bottom PCB temperature for the WLCSP package, with a peak reading of 42.8 °C. The actual bottom temperature is 41.3 °C.
The diode measurement shows a voltage of 0.331V, correlating to a temperature of 43 °C. The measured case top temperature of 43.6 °C also corresponds to the junction temperature.
Measurement of the electrical parameters
- VDD = 4.971 V
- VDD_RF = 4.734 V
- IVDD = 272 mA
- IAL = 21 mA
- IDRV = 251 mA
- RDRV_NMOS = 1.90 Ω
- RDRV_PMOS = 1.75 Ω
Calculation of the dissipated power
- Analog and digital logic: PAL = VDD × IAL = 4.971 V × 0.021 A = 0.104 W.
- Internal voltage regulator: PREG = (VDD - VDD_RF) × IDRV = (4.971 V - 4.734 V) × 0.251 A = 0.059 W.
- Transmitter driver stage: PDRV = IDRV2 × (RDRV_NMOS + RDRV_PMOS) = (0.251 A)2 × (1.90 Ω + 1.75 Ω) = 0.230 W.
Total dissipated power: PTOT = PAL + PREG + PDRV = 0.104 W + 0.059 W + 0.230 W = 0.394 W.
Power consumed: PIN = VDD × IVDD = 4.971 V × 0.272 A = 1.352 W.
Output power: POUT = PIN - PTOT = 1.352 W - 0.394 W = 0.958 W.
Calculating the thermal resistance
The thermal resistance (RTH) is calculated as: RTH = ΔTJA / PTOT = (43.6 °C - 26.5 °C) / 0.394 W = 43.420 °C/W.
Conclusion
This document outlines methods for measuring junction temperature and calculating self-heating for NFC applications. Thermal performance of integrated circuits depends on package thermal resistance and PCB characteristics (size, layout, stack-up). Larger packages with bigger thermal pads and PCBs with increased copper planes facilitate better heat dissipation and cooling.
Thermal performance is application-specific and cannot be generalized. Calculating dissipated power and solving the thermal model helps identify system contributors. Measurements in this document used continuous wave output. Dissipated power can be significantly reduced by using duty cycles for polling, where the on-time is much shorter than the self-heating settling time. For instance, a 100 ms on-time and 100 ms off-time cycle could halve the dissipated power of the regulator and TX-driver, substantially lowering junction temperature compared to continuous output.
Appendix A Emissivity
Emissivity is crucial for thermal imaging, describing a material's ability to emit radiation relative to a black body. Emissivity typically ranges from 0.1 to 0.95; polished surfaces have low emissivity, while oxidized or painted surfaces have higher emissivity. Human skin exhibits emissivity of 0.97 to 0.98.
Further information is in the ETS320 IR camera user manual. The manual describes a method to determine emissivity factors:
- Select a sample placement area.
- Determine and set reflected apparent temperatures.
- Place electrical tape with known high emissivity on the sample.
- Heat the sample at least 20 K above room temperature evenly.
- Focus and auto-adjust the camera, then freeze the image.
- Adjust level and span for optimal brightness and contrast.
- Set emissivity to that of the tape (typically 0.97).
- Measure tape temperature using Isotherm, Spot, or Box Average functions.
- Note the temperature.
- Move the measurement function to the sample surface.
- Change emissivity setting until the same temperature reading is achieved.
- Note this emissivity.
For this procedure, an unpowered ST25R3916-DISCO board was heated to 51 °C. The board uses an ST25R3916 QFN package, with the top side removed to measure silicon temperature.
Figure 22. Measurement on plastic tape ε = 0.97: A thermal image showing a measurement on plastic tape for emissivity calibration, with a peak temperature of 51.0 °C and a scale from 37.2 °C to 97.2 °C. The crosshair indicates the measurement point on the tape.
Figure 23. Measurement on the silicon top, adjust ε = 0.84: A thermal image showing measurement on the silicon top for emissivity calibration, with a peak temperature of 51.0 °C and a scale from 32.4 °C to 108 °C. The emissivity setting was adjusted to 0.84.
| Material | ε |
|---|---|
| PCB surface (solder resist) | 0.90 |
| IC VFQFPN32 package | 0.95 |
| IC WLCSP package | 0.97 |
Revision history
| Date | Version | Changes |
|---|---|---|
| 11-Jan-2021 | 1 | Initial release. |
