What is Lock-in Thermography?
Lock-in thermography is a process for detecting low-level localized heating in devices such as integrated circuits and semiconductors, caused by leakage current or resistive shorts.
Figure 1: Lock-in thermography hotspot detected on a semiconductor device using an Optotherm Sentris FA-MW system
By automatically and repeatedly biasing an electronic device at regular intervals, thermal images of the device are analyzed by software. High-sensitivity lock-in images are created to detect and locate low-level thermal hotspots. Localized heating less than 1 mK (0.001 °C) can be detected.
Common Faults Detected
- Leakage current in bare and packaged semiconductor devices
- Resistive shorts on bare circuit boards
- Power-to-ground shorts on populated circuit boards
Basic LIT System Components
Thermal Imaging Camera
Optotherm offers thermal imaging cameras in the long-wavelength and mid-wavelength ranges, each with its own advantages. The uncooled LWIR camera has a lower sensitivity (higher NETD), operates at frame rates up to 60 Hz, has spatial resolution down to 5 µm, and is less expensive. The cryogenically cooled MWIR camera offers the highest sensitivity, can capture images at higher frame rates (up to 1,000 Hz windowed), and can achieve spatial resolution of 2 µm (1 µm lens available soon), but is generally more expensive.
Figure 2: Optotherm MW640-15 MWIR camera
Voltage Source
Many different voltage sources can be used to bias the DUT; however, source measure units (SMU) can provide shorter slew rates and higher measurement resolution for current and voltage while a test is in progress.
Optotherm’s Thermalyze software integrates with and controls Keithley SMUs during an LIT test.
Figure 3: Keithley 2450 Source Measure Unit (image courtesy of Tektronix)
Computer with LIT Software
LIT software coordinates electronic device biasing with image capture, applying voltage at the beginning of each lock-in cycle and removing voltage at the end of each cycle. The LIT cycle frequency must be selected based on the device design and thermal characteristics.Higher test frequencies can accommodate thin devices and devices with high thermal conductivity. Lower test frequencies must be used with thicker devices to allow fault heating to conduct to the surface where it can be detected by the thermal camera. For example, frequencies of 1 Hz and higher are used with bare semiconductor devices. Frequencies of 1 Hz and lower are used with packaged devices and circuit boards.
Figure 4: Optotherm’s lock-in thermography software window
An LIT test outputs the results in three different formats: The single phase image(s) highlights heating at specific times during the test cycle; the amplitude image displays heating at any time during the cycle; the phase image maps the time within a cycle when internally generated heat reaches the surface.
LIT Test Procedure
Step 1
Connect the SMU output to the circuit containing the resistive short or leakage current. Electrical connection using probes, solder, and test clips are commonly used (see Figure 5).
Figure 5: Optotherm probe needles on a semiconductor device
Set SMU voltage and current limit high enough to detect the fault but low enough to prevent altering the fault or damaging the device. Stepping up from lower than expected limits is advised.
Use LIT software to send setup commands to the SMU, and to turn SMU output on briefly to confirm electrical connections have been made properly and that fault characteristics have not changed.
Step 2
Figure 6: Optotherm Thermalyze software instrument setup window
Start the LIT test. Thermal images are captured while voltage is cycled to the fault at the LIT test frequency. Typically, voltage is applied at a 50% duty cycle: ON during the first half of the cycle and OFF during the second half of the cycle. Joule heating is generated in the fault during the first half of the cycle and conducts through the device to the surface where its emittance can be detected by the thermal camera.
Step 3
Review LIT result images (single phase, amplitude, and phase images) to identify fault heating.
Single Phase Image (shown at -324°)
The single phase images display temperature changes at specific times within the lock-in cycle. These images are used to identify areas that heat up at different times within the cycle. 0° displays heat detection during the first half of the cycle (when a device is biased).
Amplitude Image
The amplitude image displays all temperature increases on a device at any time during the cycle. This image is commonly used to determine fault location in the x-y direction.
Phase Image
The phase image displays the phase angle of heating at each point in the image. This image represents the delay between powering a device and the resulting surface heating.
Step 4
Locate the source of fault heating using the thermal image overlay automatically generated during the test. Heating can also be located using software tools that measure distance between hot spots and known device features.
Learn more about lock-in thermography for electronics failure analysis at optotherm.com