A laminated composite-material plate has been developed for use as a lightweight heat sink and mechanical support for rigid printed-circuit boards (PCBs) that hold surface-mounted electronic components. This composite-material plate is intended to replace a conventional aluminum heat-sink plate. The aluminum plate weighs 0.63 lb (0.29 kg), whereas the composite-material plate weighs only 0.36 lb (0.16 kg).
An advanced laminated composite is attractive as an alternative to aluminum in several respects:
- The orientations of fibers in the laminae can be selected to tailor the properties of the laminate.
- The bending stiffness of a typical advanced laminated composite material is eight times as great as that of aluminum; this is an important advantage because PCBs that hold surface-mounted components are relatively intolerant to flexing. (The intolerance to flexing arises because flexing can break the beads of solder used for attachment in surface mounting.)
- The density of a typical advanced composite material is two-thirds that of aluminum.
- The coefficient of thermal expansion (CTE) of a quasi-isotropic composite laminate is typically less than one-eighth that of aluminum. The lower CTE of the composite material, in concert with the CTE of the PCB material, promises an increase in fatigue lives of solder joints, and thus increased reliability.
- The effective thermal conductivity of an advanced composite material can be made to exceed that of aluminum; as a result, better heat-sink performance is attainable. As a consequence of better heat-sink performance, heat-generating components can be packed more densely; thus, a greater degree of miniaturization is possible.
The composite-material heat-sink plate (see figure) comprises (1) a quasi-isotropic laminated core of graphite fibers in a cyanate ester matrix and (2) a 0.002-in. (0.05-mm)-thick aluminum skin backed with a film adhesive. The laminated core consists of six laminae; the quasi-isotropy is achieved by stacking each lamina with its fibers at an angle of 60° with respect to those of the adjacent lamina. This fiber orientation was chosen because it optimizes the strength and stiffness characteristics of the laminate. Because the thermal conductivity is greatest along the fibers and it is desired to maximize widthwise thermal conduction in the heat sink, the fibers in the 0° plies are oriented along the width of the heat sink. The aluminum skin is wrapped around the laminated core to provide a continuous ground plane for the PCBs.
This work was done by Jill M. Holz, Lee Niemeyer, and David Puckett of Goddard Space Flight Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category. GSC-14142