Liquid Crystal Hot Spot Detection

Liquid crystal hot spot detection locates heat sources associated with the electrical failure of a device.

Hot spot detection is non-destructive and applicable whenever abnormal leakage or power is associated with a failure.  The goal is to precisely identify the location of abnormal heat.  The defect causing failure is often at or near the source of heat.  Liquid crystal hot spot analysis can easily detect a point source of 1 mW.   Under optimum conditions 10 µW can be located.

Common materials are well defined by a melting point (0°C for water) and a boiling point (100°C for water).  Nematic liquid crystals have two distinct liquid phases.  The temperature dividing the two phases is called the clearing point.   A test tube of liquid crystal is milky white below its clearing point but is as clear as water above its clearing point.  The optical transformation is instant and abrupt.

The optical transition is even more dramatic in a thin film of liquid crystal.   The microscope image of an object illuminated with polarized light and viewed through a crossed polarizer is very dark, because the crossed polarizer blocks most of the light.  However, a thin film of liquid crystal below its clearing point effectively twists or rotates the light as it passes through the film.  The image is quite bright, because the polarizer is not crossed.  When the temperature is increased above the clearing point, the liquid crystal behaves as a simple liquid, light is not rotated, and therefore is blocked by the crossed polarizer.  This is the basis of all liquid crystal hot spot detection procedures.

An IC coated with a thin film of liquid crystal and viewed through crossed polarizers appears mottled and bright.  If a spot of liquid crystal is locally heated above the clearing point temperature, that spot will become instantly black.   Depending on ambient temperature, 20 to 30 mW of power may be required to create a discernible spot.  If the ambient temperature is increased to a temperature just below the clearing point, 10 to 100 µW may be enough to produce a visible hot spot.

Device Failure Characteristics
Whenever a failure is characterized by abnormal leakage or high power supply current, liquid crystal hot spot detection is an appropriate technique.  The only circuit knowledge required is how to bias the device in the abnormally high current state.   The procedure is simplest when the leakage is between two pins such as input and ground, or VDD and VSS.  In such two pin cases, any hot spots detected are associated with the failure.

Liquid crystal is also effective when the circuit must be specifically biased to address the high current state.  Digital devices that fail IDDQ in specific logic states are perfect candidates for liquid crystal analysis.1

The capability and procedure of liquid crystal hot spot detection depends on the equipment available.  Liquid crystal will be used routinely and often if required equipment is constantly available.  See Equipment Considerations for details.

Hot Spot Detection Procedure
1.  Electrically characterize device to establish bias conditions for analysis.   Determine the maximum voltage possible without exceeding a breakpoint which introduces an unwanted current path. (See Figure 1)  With external heating, 1 mW of power is normally detectable.

2 .  While probes are in contact with device, apply a thin film of liquid crystal by touching the device surface with a small brush containing LC29R liquid crystal solution.  Brush is fine and soft enough to move around probes without disturbing contact.

3.  View the device under cross-polarized conditions.  The liquid crystal should appear mottled due to the optical properties of liquid crystal below the clearing point.

4.  Apply power to the device.  Use DC voltage to generate maximum heat.  Slowly increase the voltage to the maximum determined in Step 1.  Vary the DUT voltage about its maximum. (HSS1 automatically performs this function at a constant frequency.)  While viewing the device under cross-polarized conditions, increase the external heat until a hot (dark) spot appears or until the entire surface darkens.  If the hot spot is found, video tape or otherwise document the hot spot.  If the entire surface becomes dark, decrease the external heat until a mottled appearance returns, and continue scanning the surface.  A typical result is shown below.

5.  To an experienced analyst, failure to isolate a hot spot does not mean that “liquid crystal didn’t work”.  Helpful knowledge about the failure was achieved.  Specifically, if characterization (Step 1) revealed that nearly 1 mW of power exists, a hot spot would be detectable if the power were dissipated in a single spot within a few microns of the surface.  Failure to detect a hot spot suggests that the power is divided among several sites each too small to detect.  Similarly, an area phenomena such as surface inversion may have power density too low to heat the device.   Or the source of power is deep in the substrate too remote to heat the surface.

Figure (1)  Electrical characterization to determine the maximum voltage to use.

Typical Result
The mottled, colorful area is the natural appearance of a thin film of liquid crystal viewed under cross polarized conditions.  This film was applied by wetting the surface of the IC with LC29R.  The pulsing dark spot indicates where 1 to 2 mW of power from abnormal leakage heats the defect area above 29°C.  LC29R loses its unique optical characteristics at temperatures above 29 degrees, so the hot area appears dark.  (The damage to this IC was caused by ESD stress.)

hot spot_5alt

Procedure Variations
The procedure above assumes the most usual case that power to the defect is not excessive and is readily controlled.  For analysis of latchup, the power produced may be so high that the entire device becomes heated and masks the origin of the heat.   LC29R liquid crystal can still be used by externally limiting current flow or pulsing the device at a low duty cycle can reduce power.

Liquid crystals with a higher transition temperature (up to 100°C) can also be useful for high temperature applications. {Call for details}

H. Lin, M. Khan, T. Giao, “Dynamic Liquid Crystal Hot Spot Examination of Functional Failures on Production Testers,” Proceedings from the 20th International Symposium for Testing and Failure Analysis, 1994, p. 81.

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