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|Metallic TIM Testing and Selection for IC, Power, and RF Semiconductors|
|Keywords: Thermal , reliability, materials|
|An important determinant of device reliability is operating temperature control. Maintaining a semiconductor device at or below the maximum rated junction temperature (Tj) is accomplished through careful thermal management design and selection of well-performing thermal interface materials (TIM) that minimize efficiency losses in packaging and between the semiconductor device package and a heat sink or liquid cold plate. Minimizing these thermal resistances through the semiconductor package material stack and at the external case or package surface is an important aspect of maintaining operating temperatures within specified maximum values. Selection of a thermal interface material between the semiconductor device package and the heat sink or liquid cold plate for an RF semiconductor can also impact electrical signal performance. Therefore, use of a highly conductive (thermally and electrically) thermal interface will provide a uniform electrical and thermal transfer mechanism between the contacting surfaces. Indium metal foils have traditionally been utilized for RF semiconductors, for telcom, wireless, and aerospace applications for this reason. Transition within the RF and microwave markets from silicon to gallium nitride RF devices has also led to use of GaN devices with smaller footprints, equivalent or higher power output, and higher heat flux values, increasing the need for electrically and thermally conductive thermal interface materials. New forms of metallic TIM preforms have recently been developed and implemented for both integrated circuits and power and RF semiconductor applications. Certain applications require an easily reworkable TIM and this has also led to development of new material types. Different metals and alloys will be described, with accompanying data on bulk thermal conductivity and maximum suggested temperature range for each metal and alloy. Understanding performance of differing metals and alloys is important in selection of a well-performing TIM for high-temperature applications as well as traditional computing and telcom industry operating environments. This presentation will identify how thermal performance is significantly improved in these applications when a metallic TIM is used, showing empirical data for such a metallic TIM in comparison to use of a traditional silicone-based thermal grease as the TIM for GaN RF devices. Thermal performance data for the RF device will also be demonstrated, comparing thermal performance with a fully electrically-conductive path via the metallic TIM versus performance with a high performance silicone-based thermal grease, showing empirical data developed by a military/aerospace OEM in GaN RF module applications. Two different clamping forces were applied, with identical RF device packages in the same mounting system, for both a silicone-filled thermal grease that has been used for production applications by the OEM, as well as the thermal resistance data for a metallic TIM used with the same loading forces in the identical mechanical mounting system. (Graph) Comparative thermal resistance test values are shown for flange-mount RF power transistors dissipating 26W and 36W, respectively, with different clamping force loading and two different thermal interface materials. Test results demonstrate an improvement of 36% with a patterned metallic TIM versus the standard silicone-based Dow Corning DC340 thermal grease employed in earlier designs. (Courtesy: Rockwell Collins Inc.)|
|David L Saums, Principal