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Development and Application of Copper-Graphite Composite Thermal Control Cores for High-Reliability RF System PCBs
Keywords: copper-graphite, thermal, rf system pcb
A description will be given of recent developments, requirements, and applications for composite CTE-matched thermal control cores for manufacturing complex multilayer organic printed circuit boards which provide: a. High in-plane thermal conductivity; b. High through-plane thermal conductivity; c. Selected CTE values tailored to match semiconductor device CTE (specifically GaN, SiC, or Si) for direct die solder attachment d. Suitability for use as replacements for heavy copper thermal layers within conduction-cooled high-reliability multilayer PCBs. This presentation will identify market requirements and needs and describe development and applications of large panel forms of very thin copper-graphite composites manufactured to meet targeted thicknesses. Comparative data for published and measured thermal conductivity for related materials examined during the development and evaluation phase will be presented. A program undertaken by a large aerospace manufacturer for development of thermal control core materials for complex multilayer printed circuit boards (PCBs) required that a number of goals be met for this program. These goals include thermal expansion management, relatively high thermal conductivity (in-plane and through-plane), weight reduction, warpage control, suitability for PCB manufacturing equipment and industry-standard processes, and export suitability. Development of such thermal control core materials for conduction-cooled printed circuit boards with as many as 21 layers, to which high heat flux GaN RF and related power semiconductors must be mounted with direct die solder attach, has recently been completed for the initial target material requirements. A second development phase for a stretch goal of target material requirements is now underway. Development and manufacturing processes evaluated for these composite materials required manufacturing in large sheets to meet standardized PCB industry 'drop in place' requirements, to replace heavy copper layers within such PCBs. Testing and analysis of PCB manufacturing processes utilizing these composite panels were conducted by TTM Technologies. Manufacturability prove-out has now been completed with two high-reliability PCB manufacturing companies and one defense agency facility. Complex multilayer RF PCBs have been manufactured for seven different programs and are now in test. A manufacturing cost reduction program for core materials is in place with completion targeted for first volume production lots. Apparent thermal conductivity will be shown with calculated values based on board layer modeling and with empirical data for completed thermal core PCBs, with good correlation. Tested thermal conductivity values also correlate well with program target values. Targets for CTE values, thermal conductivity, warpage potential, and weight reduction critical to high performance, high reliability organic PCBs for high heat flux GaN and SiC RF devices will be reviewed. Note: Materials described meet US ITAR requirements and are suitable to meet current EU legislative requirements.
David L. Saums, Principal
Amesbury, MA

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  • ASE
  • Canon
  • Corning
  • EMD Performance Materials
  • Honeywell
  • Indium
  • Kester
  • Kyocera America
  • Master Bond
  • Micro Systems Technologies
  • MRSI
  • Palomar
  • Promex
  • Qualcomm
  • Quik-Pak
  • Raytheon
  • Specialty Coating Systems
  • Technic