Lock-in Thermography

Thermalyze Thermal Image Analysis Software

Lock-in Thermography

Note: Lock-in Thermography is included only with Sentris and EL and is an optional item otherwise

Steady-state thermography is limited to detecting hot spots that heat up at least 100 mK (0.100°C) and dissipate at least 20 mW of power. This may be useful for locating shorts on high-power devices, but is inadequate for detecting lower power defects such as leakage current or hot spots in packaged devices. Steady-state thermography also suffers from poor spatial resolution as the heat from localized hot spots diffuses rapidly, blurring the location of the heat source.

Lock-in thermography is a process of automatically and repeatedly powering a device at regular intervals using a laboratory power supply and reed relays while the temperature response of the device is integrated and analyzed over time. Over many power cycles, the sum of thermal images captured while the device is unpowered are subtracted from the sum of thermal images captured while the device is powered.

Increasing the number of test cycles results in improved test sensitivity. Using this technique, hot spots that heat up less than 1mK (0.001°C) and dissipate below 100 µW can be detected. Weak sources of heat arising during normal operation of the device may even be detected.

Cycle Frequency can be set up to 15 Hz. Performing lock-in tests at lower frequencies improves test signal/noise due to higher device heatup. Higher frequency tests improve hot spot spatial resolution by reducing thermal diffusion into adjacent areas of the device.

Amplitude Image

The amplitude image, which displays total temperature increase on a device during power cycling and is commonly used to determine fault location in the x, y direction, as shown in the image below. Performing lock-in tests at lower frequencies improves test signal/noise due to higher device heatup. Higher frequency tests improve hot spot spatial resolution by reducing thermal diffusion into adjacent areas of the device.

Phase Angle

Phase angle represents the time delay between powering a device and subsequent heating on the surface of a device. Phase angle can be used to determine the depth of a fault inside a device. The amount of time delay, or phase angle, is dependent on the thermal conductance of the device and defect depth.

Phase angle is measured in units of degrees and has a range of 0° to -360°. A phase angle of 0° indicates device heating occurring immediately after power is applied and takes place at the surface of the device. Negative phase angle values, such as -120°, indicate device heating occurring at some time after power is applied below the surface. Larger negative values of phase angle indicate heating that occurs at even greater depths.

The amplitude image, which displays temperature rise, is commonly used to determine fault location in the x, y direction as shown in the image below. To determine fault depth, the phase image can be displayed. Phase represents the delay between device powering and the resulting device heating. Phase is displayed in units of degrees and has a range from 0° to -360°. A phase value that is close to zero indicates a very small delay between device powering and heating, as in the case when the fault is very near the surface of the device. More negative values of phase indicate longer delays, as when a fault is located below the device surface, as when a fault is located at a distance from the heat source or below the device surface. A phase of -360° indicates a delay that is equal to the time length of the cycle. In many cases, the relationship between defect depth and phase angle can be used to calculate the depth of the fault.

Cycle Images

During lock-in tests with a cycle time equal to 1 second or longer, each captured thermal image is added to one of 30 image "banks", depending on the time that the image was capturing during the cycle. For shorter cycle times, fewer image banks are used. Each of these image banks can be displayed, representing the average thermal image at each moment during the cycle. This feature enables the analysis of heat propogation during the cycle.

The cycle images below were produced with a lock-in frequency of 1Hz. Therefore, the progression of images represents the heating that takes place while power is turned on for 0.5 seconds. The time lapse between images is 33ms. The progression of images showing the cooling that occurs while power is turned off for 0.5 seconds can also be displayed.