The case temperature is defined as the hottest temperature on the top of the device. In most instances, this is at the center of the top surface or lid of the device. The case temperature measurement can be performed with (in order of accuracy) an IR camera, a fluor-optic probe, a thermocouple, or IR gun with a maximum field view of 4-mm diameter just to name a few techniques. When a thermocouple is chosen as the technique to perform the measurement, a fine gauge wire (36 to 40 gauge, J or K wire) should be used to minimize the local cooling from the thermocouple. You should be aware that if the case temperature is measured by a gauge thermocouple larger than 36, the thermocouple sinks heat away from the surface, cooling the spot that is being measured, invalidating the calculation of Equation 8. The impact of using a heavy-gauge thermocouple to measure the package top surface can be very substantial, reducing the delta between the ambient and actual surface temperature by 50% or more. There can be error even when a 36 gauge or smaller thermocouple is employed.

If using a thermocouple, it should be attached to the center of the package surface (± 1 mm) with a bead of thermally conductive epoxy no larger than 2 × 2 mm on a side. Taping the thermocouple to the package surface is not recommended. To minimize the heat sinking nature of the thermocouple, the wire should be dressed along the diagonal of the package, down to the PCB surface, and over a minimum distance of 25 mm before lifting from the PCB. The thermocouple wire can be tacked to the PCB for this routing purpose by using a tape. Use of improper thermocouple wire gauge can create errors in the measurements of 5%–50%.

When using either an IR camera or IR gun, be sure to correct the reading for the emissivity of the surface being investigated. For details, see the documentation for the instrument being used.

Measuring case temperatures with heat sinks applied represents special challenges because the heat sink covers the surface to be measured. If you wish to measure the case temperature with a heat sink applied, the following procedure is recommended (see Figure 3-1).

1. Drill a hole less than 1 mm in diameter in the heat sink so the hole is at the center of the package when the heat sink is attached. Chamfer the end of the hole on the face of the heat sink that mates with the package. The chamfer should be sized to accommodate the epoxy that will attach the thermocouple to the package.
2. Thread a fine-gauge (less than 36-gauge) thermocouple through the drilled hole from the top of the heat sink.
3. Use epoxy to attach the thermocouple bead to the package top surface with thermocouple wire oriented perpendicular to the package top surface. Wait for the epoxy to cure.
4. Apply heat sink compound to the base of the heat sink.
5. With the thermocouple wire still threaded through the drill hole, carefully slide the heat sink onto the top of the package.

Figure 3-1 Installing a Heat Sink With Thermocouple