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11-20 M68040 USER’S MANUAL MOTOROLA
HEAT SINK
PIN GRID 
ARRAY
µ MC68040RC25
NOTE: Do not cover up microprocessor markings with an adhesive mounted heat sink.
Figure 11-10. Heat Sink with Adhesive
All pin-fin heat sinks tested were made from extrusion aluminum products. The planar face
of the heat-sink matting to the package should have a good degree of planarity; if it has
any curvature, the curvature should be convex at the central region of the heat-sink
surface to provide intimate physical contact to the PGA surface. This heat sinks meet this
criteria. Nonplanar, concave curvature in the central regions of the heat sink results in
poor thermal contact to the package.
Although there are several ways to attach a heat sink to the package, it is easiest to use a
demountable heat-sink attachment called “E-Z attach for PGA packages” (see Figure 11-
33). A steel spring clamps the heat sink and the package to a plastic frame. Besides the
height of the heat sink and plastic frame, no additional height is added to the package.
The interface between the ceramic package and the aluminum heat sink was evaluated
for both dry and wet interfaces in still air. The thermal grease reduced the θCA quite
significantly (about 2.5 °C/W) in still air. An attachment with thermal grease provided
about the same thermal performance as if a thermal epoxy had been used.
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