In electronics systems, the case temperature (sometimes referred to as top temperature) of a component often is easy to measure. Fortunately, the component case temperature is very close, both physically and thermally, to the component junction temperature. This means that you can estimate the component’s junction temperature by measuring it’s case temperature during operation. You can use the thermal parameter Psi-JT (ΨJT) for this purpose since it is unique for a particular component under typical use conditions, and is generally provided by the component manufacturer. This article details the proper method to determine the component junction temperature by measuring the case temperature. By carefully addressing component temperature, you can ensure operation within the thermal limits of the component.
Using Psi-JT to calculate junction temperature
When the temperature at the top of the component is measured, the junction temperature can be calculated using the equation:
TJ = TC + ΨJT * Pdiss
Or: Junction temperature = case temperature + Psi-JT * power dissipated
This equation is defined by JEDEC standard JESD51-2A. Note that the equation uses the device’s total power dissipation. Therefore, it is not necessary to know the power distribution going to the top of the package or to the board.
Psi-JT is not the same as Theta-JC
One important thing to note is that Psi-JT is not an actual or physical thermal resistance, but is a thermal characterization parameter. Theta-JC is another commonly used term that recently has been clarified in the industry as Theta-JC,top when referring to the top surface of the package. Psi-JT and Theta-JC are not the same parameter. Theta-JC,top should not be used in this manner to estimate junction temperature. The Psi-JT value is specific to every IC component and heat flow profile, and should be provided by the supplier in the datasheet. One common misconception is that the parameter Theta-JC can or should be used to estimate component junction temperature. Theta-JC gives misleading results as it tends to be a significantly larger value than Psi-JT. Originally Theta-JC was devised to allow estimation of the thermal performance of a package when a heat sink is attached, or to be used in a two-resistor package thermal resistance model. The Theta-JC,top test method forces almost all of the test device’s power through the top surface of the package.
Psi-JT is measured on a standard test board and allows the heat generated in the component die to flow normally along preferential thermal conduction paths, which more closely represents the energy flow in a typical application environment.
In the lab
To use Psi-JT to calculate the component junction temperature, you must have a functioning device, the ability to measure the device case temperature, and the known device power dissipation. The goal of the temperature measurements is to physically measure the ‘hottest’ case temperature on a functioning device. Several common methods to do so are with a thermocouple, with an IR camera or with an IR gun. Each method has its benefits and limitations. Some trial and error will be necessary to determine the best method for each system configuration.
The most common and cheapest method for case surface temperature measurement is to use a type K or T thermocouple. The thermocouple bead is to be attached to the component’s surface using conductive epoxy or thermal tape to ensure that the bead properly touches the case surface. One limitation of using a thermocouple is that for smaller components, the thermocouple bead and wire may act as an additional heat sink to the device. To minimize this impact, use the smallest gauge of thermocouple wire possible (such as 36 gauge, or 40 gauge for Type T thermocouple). Use only enough adhesive to ensure proper contact, place the thermocouple wire diagonally along the device surface, then tape the wire to the PCB. For a single-die device, place the thermocouple at the geometric center of the package. For a multi-die device, it is best to find the hottest case temperature before affixing the thermocouple. This can be accomplished by carefully touching the thermocouple to the device at several locations to determine the hottest spot and best final placement for the thermocouple. An example of proper thermocouple placement is shown in Figure 1.
Figure 1: Case temperature measurements using a thermocouple.
A second method to determine case temperature is through the use of an IR camera or IR gun. Calibrate the IR camera per the camera manufacturer’s instructions. Then carefully choose the correct emissivity value to represent the package top surface. Focus the IR camera by using the clarity of the device leads as a determining factor. Figure 2 shows a properly focused IR camera image. An IR gun is also a means to measure component case temperature. However, care must be used to ensure that the IR gun reading is from the hottest location on the device.
Figure 2: Case temperature measurements using an IR camera.
Factors impacting Psi-JT
Since Psi-JT requires the package top to be measured, factors that impact the heat transfer to the top of the package will also affect this parameter. In general, packages where more heat travels to the top of the package will have higher Psi-JT values. Conversely, most components dissipate heat primarily into the printed circuit board (PCB), meaning that the Psi-JT value is fairly small.
Smaller packages have less area for heat spreading. Consequently, they have higher values of Psi-JT compared to larger packages. Table 1 lists Psi-JT values for a typical device in a 3-mm x 3-mm and 9-mm x 9-mm QFN package at zero airflow.
Table 1: Psi-JT values as a function of package size for typical devices
A package with a heat spreader will have a higher Psi-JT value than a package without a heat spreader because the hot spot on top of the package is spread out. Table 2 shows Psi-JT values for a 23 mm PBGA package with and without a heat spreader.
Table 2: Psi-JT values as a function of heat-spreader.
A second factor that could impact Psi-JT value is airflow over the package. Increasing the airflow over a device typically causes the package top to be cooled at a faster rate and increases the value of Psi-JT. Table 3 shows the Psi-JT values for a typical device in a 3-mm x 3-mm QFN package as a function of airflow.
Table 3: Psi values as a function of airflow.
Using the Psi-JT metric and these practical lab-based measurement techniques, it is possible to effectively and accurately estimate the die junction temperature of a component. Part 3 in this series explains a method to estimate die junction temperature using the Psi-JB parameter. This article includes how the Psi-JB value can be used during the system design phase to estimate if a selected component will be able to function within thermal limits in a given system. More information can be found by reviewing their definitions in the JESD51 series of JEDEC standards, or reviewing application notes such as those found at www.ti.com/thermal.
About the Authors
Matthew Romig currently serves as the Packaging Technology Productization Manager in the Analog organization at Texas Instruments, where he is responsible for packaging technology development and implementation for TI’s broad range of Analog product lines and packaging technologies. He has developed specialties in thermal analysis, flip chip packaging, and power management packaging. He is also a member of TI’s Group Technical Staff. Matt received his BSME from Iowa State University, Ames, Iowa. He can be reached at email@example.com.
Sandra Horton, Analog Packaging Group, Texas Instruments is a leader in thermal modeling and has served as co-chair of the TI Thermal Council since 2007. She supports customer and development models which includes automotive products. Sandra received her BSME at Texas A&M University. She can be reached at firstname.lastname@example.org.
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