Psi or Theta: Which One Should You Choose?

Mar 1, 2008 12:00 PM
By Roger Stout, ON Semiconductor

Over the years, standards organizations have undertaken numerous efforts to standardize the test methods that are used to characterize the thermal performance of semiconductor devices. Groups such as Semiconductor Equipment and Materials International (SEMI), the Electronic Industries Alliance (EIA) and the Joint Electron Device Engineering Council (JEDEC) have developed several standards or specifications that define methods of measuring a variety of thermal characterization parameters.

For example, the SEMI organization drafted standards for measuring the thermal resistance of ceramic packages, integrated circuit packages and semiconductor packages under different environmental and test conditions. These standards defined the familiar theta (θ) terms for thermal resistance such as θ_JA and θ_JC.

But recognizing certain limitations of these standards, EIA and JEDEC later came along and defined their own standards for measuring the performance of semiconductor devices. In the process, they clarified the applicability and narrowed the scope of the existing theta terms for thermal resistance, while also creating a new set of thermal characterization parameters, symbolized by the Greek letter psi (Ψ). As with theta, the psi terms usually carry subscripts that reflect measurement conditions.

As the EIA/JEDEC standards describe, the psi and theta terms are related but have different meanings and implications for use. Now that both terms are appearing more frequently on device data sheets, it's imperative that system designers understand the distinctions between the terms and how these terms are defined, so that they understand how the device vendors are characterizing their parts.

This article will first review, in some detail, the nuances of the standards themselves. It will then illustrate, by using a relatively simple system model, why these seemingly subtle differences in definitions between theta and psi are, in fact, profound. Armed with that knowledge, designers can make more accurate predictions about the thermal performance of the semiconductor devices in their systems.

Thermal Parameters Defined

Many of the now-familiar thermal-resistance parameters were originally defined by the SEMI organization. Three standards in particular are worth noting. In quoting excerpts from these standards, I have italicized certain passages to highlight some of their limitations.

1. SEMI G30-88 “Test Method for Junction-to-Case Thermal Resistance Measurements of Ceramic Packages.” This test method deals only with junction-to-case or mounting surface measurements of thermal resistance and limits itself to heatsink and fluid bath testing environments. The heatsink mounting method for measuring junction-to-case thermal resistance is a conservative measure of the package's ability to transfer heat to the ambient environment, because heatsinking is provided only on one side of the package, whereas the fluid bath mounting method has the potential for equally cooling both sides of the package.

2. SEMI G38-0996 “Test Method for Still- and Forced-Air Junction-to-Ambient Thermal Resistance Measurements of Integrated Circuit Packages.” This test method deals only with junction-to-ambient measurements of thermal resistance and limits itself to still- and forced-air convection testing environments.

3. SEMI G68-0996 “Test Method for Junction-to-Case Thermal Resistance Measurements in an Air Environment for Semiconductor Packages.” The measurement results are usually different from the results obtained by testing in the fluid bath environment described in SEMI G30-88 and in SEMI G43-87 “Test Method for Junction-to-Case Thermal Resistance Measurements of Molded Plastic Packages,” not summarized here.

From these definitions, it should be clear that it is very difficult to define a thermal parameter that's going to apply to a package under all circumstances. Yet, for some reason, device manufacturers — even the ones following these standards — tend to gloss over this fact in their data sheets.

However, in the industry at large there was a recognition that the SEMI standards were inadequate. Consequently, in the early 1990s, EIA/JEDEC (www.jedec.org), the developer of standards for the solid-state industry, drafted its own comprehensive thermal standards.

As can be seen in the following excerpt from JESD51-2 “Integrated Circuits Thermal Test Method Environmental Conditions — Natural Convection (Still Air),” published in 1995, JEDEC recognized the same issues as SEMI:

“The purpose of this document is to outline the environmental conditions necessary to ensure accuracy and repeatability for a standard junction-to-ambient (θ_JA) thermal resistance measurement in natural convection. The intent of θ_JA measurements is solely for a thermal performance comparison of one package to another in a standardized environment. This methodology is not meant to and will not predict the performance of a package in an application-specific environment.”

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