Transient Determination of Rjc
JEDEC 51-14 entitled "Transient Dual Interface Test Method for Measurement of Rjc for Semiconductor Devices with Single Heat Flow Path" uses the comparison of two transient tests on a component that has a single, dominant, conduction-heat-flow path. One test is performed with a low resistance case-to-sink thermal interface material (TIM) and another with a high resistance TIM. The point at which the two transient profiles diverge corresponds to the case-to-heat sink interface. The resistance at this point is thus the thermal resistance, junction-to-case, or Rjc.
This method compares the two transient characteristics using both the cumulative structure functions (CSF) and the heating curves (HC) to determine the impedance at the transient divergence point. The impedance at the divergence is assumed to be the junction-to-case thermal resistance, Rjc, since the divergence is caused exclusively by a difference in the case-to-sink interface. The reference temperature for this method is the temperature controlled cooling liquid temperature that supplies the heat sink. This reference temperature choice avoids an interface-thermocouple which is commonly influenced by minute mechanical positioning which can be difficult to control consistently.
This standard recommends using comparisons of “thermal grease” to “dry contact” interfaces. The problem with dry contact is that the contact interface resistance is extremely variable depending on the flatness of the mating surfaces and the particular interface contact pattern. The cause of this variability is simply that air is an excellent thermal insulator and small variations in the location and amount of this air causes substantial and unpredictable variations in the contact resistance. This in turn can create anomalous transient comparisons due to variations in the geometry of the heat flow across the air-entrained irregular interface. Logically, it would seem that a comparison based on a high-performance grease and low-performance grease would offer better consistency by eliminating the potential variations of the dry contact associated variable amounts of air entrained within the thermal transfer area.
JEDEC Std 51-14 includes a precise computational means for determining the HC divergence point while the CSF divergence point is less precisely defined. In either case, the divergence point is determined with a numerical procedure rather than a graphical procedure. The following plots are provided for graphical reference and visualization only. The red lines on the plots show the numerically determined divergence points corresponding to the Rjc determined.
JEDEC 51-14 Rjc determination with HC comparison: 0.518 C/W
JEDEC 51-14 Rjc determination with CSF comparison: 0.546 C/W
For this example, the Rjc determined from the HC comparison is 0.518 C/W versus 0.546 C/W for the CSF comparison. The JEDEC method provides a means to decide which value should be choosen as the final Rjc based on the type of die attach used in the device. In this case the Rjc determined by HC comparison is the correct result.
It should be noted that TIMs offering the greatest performance difference are more desirable since it makes the divergence point more pronounced and more accurately determined. Also, there is often some variation in the Rjc determined based on the specific selection of the two TIMs. More information on this topic can be found in the the white paper, Component Characterization, Transient to Steady State, section 10.5.