User manual

Advantech SOM-Express Design Guide
Chapter 8 Heat Sink Recommended Design 101
Chapter 8 Heatsink Recommended Design
8.1 Material of Heatsink
The thermal conductivity of the heatsink's material has a major impact on cooling
performance. Thermal conductivity is measured in W/mK; higher values mean better
conductivity.
As a rule of thumb, materials with a high electrical conductivity also have a high
thermal conductivity.
The following materials are commonly used for heatsinks:
Aluminium. It has a thermal conductivity of 205W/mK, which is good (as a
comparison: steel has about 50W/mK). The production of aluminium
heatsinks is inexpensive; they can be made using extrusion Due to its
softness, aluminium can also be milled quickly; die-casting and even cold
forging are also possible. Aluminium is also very light (thus, an aluminium
heatsink will put less stress on its mounting when the unit is moved around).
Copper. Copper's thermal conductivity is about twice as high as aluminium -
almost 400W/mK. This makes it an excellent material for heatsinks; but its
disadvantages include high weight, high price, and less choice as far as
production methods are concerned. Copper heatsinks can be milled, die-cast,
or made of copper plates bonded together; extrusion is not possible.
Combination of Aluminium and Copper. To combine the advantages of
aluminium and copper, heatsinks can be made of aluminium and copper
bonded together. Here, the area in contact with the heat source is made of
copper, which helps lead the heat away to the outer parts of the heatsink.
Keep in mind that a copper embedding is only useful if it is tightly bonded to
the aluminium part for good thermal transfer. This is not always the case,
especially not with inexpensive coolers. If the thermal transfer between the
copper and the aluminium is poor, the copper embedding may do more harm
than good.
Silver. Silver has an even higher thermal conductivity than copper, but only by
about 10%. This does not justify the much higher price for heatsink production
- however, pulverized silver is a common ingredient in high-end thermal
compounds
Alloys. Alloys have lower thermal conductivity than pure metals, but may have
better mechanical or chemical (corrosion) properties.
8.2 Thermal Interface Material
It is important to understand and consider the impact the interface between the
processor and heatsink base has on the overall thermal solution. Specifically, the
bond line thickness, interface material area, and interface material thermal
conductivity must be selected to optimize the thermal solution. It is important to
minimize the thickness of the thermal interface material, commonly referred to as the
bond line thickness. A large gap between the heatsink base and processor die yields
a greater thermal resistance. The thickness of the gap is determined by the flatness
of both the heatsink base and the die, plus the thickness of the thermal interface
material (i.e., thermal grease), and the clamping force applied by the heatsink
attachment method. To ensure proper and consistent thermal performance, the
thermal interface material (TIM) and application process must be properly designed.