Microthermography or Hot Spot Detection

Microthermography or Hot Spot Detection - Page 2 of 2

When performing Liquid Crystal Hot Spot Detection, the following must be observed:

The correct amount of liquid crystal must be dropped on the die surface. Too thick a film of liquid crystal would appear uniformly black, making it impossible to detect nematic phase changes from temperature increases. Too thin a film makes the surface appear as streaks of dark and light grays, also making hot spots less visible. The perfect amount would make the general area of the die surface look rainbow-colored, which would offer the best contrast to a hot spot.

The bias must be chosen such that the defect site carries enough current to heat the liquid crystal at the hot spot, but not enough current to heat the entire liquid crystal film. If the hot spot generates so much heat that the entire die surface is darkened even at minimum power setting, the excitation must be 'pulsed' or oscillated. This will allow the analyst to locate the hot spot, which is the point of origin and return of the oscillating color contrast on the die surface.

The polarizing filters of the microscope must be adjusted to provide the best rainbow color for the liquid crystal film.

Care must be taken when interpreting the presence or absence of hot spots on the die. Although an abnormal hot spot very likely means that something wrong, the hot spot itself is not always the actual failure site. Some hot spots come from good components that are just forced to conduct high currents by an anomaly somewhere else in the circuit. It is important to complement microthermography results with those of other FA techniques in order to arrive at the right conclusion.

Figure 1. Photo of a hot spot during

liquid crystal analysis; note the rainbow

color of the liquid crystal

Microthermography is used for detecting the following: Dielectric Shorts or Breakdowns, Metallization Shorts, Junction Leakages, Mobile Ionic Contamination, etc.

Curve Tracing; LEM; Microprobing; FA Lab Equipment; Basic FA Flows;

Package Failures; Die Failures

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