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Considerations for Developing A Standard for Measuring Theta jc in High and Low Resistance Packages
Keywords: Theta JC, Measurement, Standard
Abstract Many attempts have been made to develop a standard for extracting the junction-to-case thermal resistance, Theta jc, for electronic packages. However, presently there is not a consensus in the electronic cooling industry on the best method for making measurements even though it recognized that a standard is desperately needed. Unfortunately, there is no modern guideline for reporting Theta jc measurements. This study provides a historical review of attempts to develop a standard and efforts to improve Theta jc measurements. Also included in this study is a review of factors affecting Theta jc measurements including the design of the cooling heat sink, case temperature measurement methods, variation in thermal interface material (TIM II) application and repeatability in measurement methods. To quantify the effectiveness of the proposed standard, a round-robin test was conducted using three different styles of electronic packages. Data are compared to quantify how well measurements made in three different labs can be reproduced following the same standard. Concluding remarks are provided for basic requirements needed for the development of a standard using techniques and methods that are commonly available by moderately equipped thermal labs. Introduction Developing a testing standard requires selecting methods that can be replicated without a tremendous effort or expense using commonly available equipment while producing a scientifically sound result. With these requirements in mind, three packages were selected as shown in Figure 1. The packages include a carrier array ball grid array (CABGA) package, a TEPBGA-2 (embedded copper heat spreader) and a flip chip ball grid array (FCBGA) package. The three types of packages were tested at three different labs. At least four samples were tested per package type. Figure 1. Thermal test vehicles samples used for round-robin test. (a) CABGA, (b) TEPBGA-2, (c) FCBGA. The test facility was constructed using a CPU cooler as shown in Figure 2. Packages are mounted face down on the heat sink surface covered with a thin TIM II layer. A weight is placed on top of the back-side surface of the mother board. All hardware used to run tests are shared between test laboratories participating in the round-robin exercise. The variability and absolute theta jc values are compared to understand the variation in testing methodology employed by the following the same specified set of conditions and using the same thermal test vehicles. Two different methods are employed for measuring the case temperature. For low power devices, such as the CABGA package, the temperature sensor is flush mounted to the surface of the CPU cooler. For higher packages, such as the FCBGA package, the temperature sensor is flush mounted to the lid to avoid including the additional thermal resistance of the TIM II material. For the intermediate thermal resistance case, the impact of flush mounting a temperature sensor to the case is compared to a temperature sensor mounted in the CPU cooler. Figure 2. Theta jc Test fixture.
Jesse Galloway, VP Advanced Package Engineering
Amkor Technology
Tempe, AZ
USA


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