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4.1.3
Vent Free Area Ratio
Free area ratio (FAR) is defined as the total vent open area, divided by the vent area.
It is recommended that this vent have an effective open area of at least 53%. This
will maximize the effectiveness of the vent and should still contain EMI effectively.
Higher FAR's will improve airflow into the chassis but are at the expense of aesthetics,
safety, and increased EMI emissions. Lower open area percentages will degrade
thermal performance but may still be effective depending on the entire system design.
These are design choices the chassis designer must understand.
4.1.4
Dust Considerations
Depending on the geographic region the system is used in, dust accumulation can
pose a challenge as it can block fan inlets and degrade heat sink performance. For
these concerns, removable screens that may be cleaned by the user are the simplest
option. While there is associated air flow impedance by adding a screen, it is
generally the best option for dust prevention.
4.2
Environmental and Skin Temperature Limit
Recommendations
The table below summarizes the recommended environmental and skin temperature
limits that should be assumed for system design.
Table 4-1. Environmental and Skin Temperature Limit Recommendations
Maximum operating room temperature outside the system
High touch surface temperature limit
Low touch surface temperature limit
Exhaust air temperature limit
High touch surfaces can be described as surfaces of the system which contact the
user's skin for prolonged periods while operational, such as the touch screen. Low
touch surfaces can be described as surfaces of the system which the user touches only
briefly, such as a system's back when accessing I/O connectors. The system skin
temperatures are recommended to be at or below the values listed in the table above
for all ambient operating temperatures.
4.3
Motherboard Component Cooling
4.3.1
Memory Cooling
Stacked SO-DIMM memory is used in this design guide because this is the only
memory configuration that can meet the height limitations for this form factor and
40
Condition
Thermals & Acoustics
Value
35°C
45°C
50°C
70°C
Thin Mini-ITX Based PC System Design Guide
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