LGA775 Socket Heatsink Loading
78 Thermal and Mechanical Design Guidelines
A.2.4 Board Deflection Metric Implementation Example
This section is for illustration only, and relies on the following assumptions:
72 mm x 72 mm hole pattern of the reference design
Board stiffness = 900 lb/in at BOL, with degradation that simulates board creep
over time
Though these values are representative, they may change with selected
material and board manufacturing process. Check with your motherboard vendor.
Clip stiffness assumed constant – No creep.
Using Figure 7-7, the heatsink preload at beginning of life is defined to comply with
d_EOL – d_ref = 0.15 mm depending on clip stiffness assumption.
Note that the BOL and EOL preload and board deflection differ. This is a result of the
creep phenomenon. The example accounts for the creep expected to occur in the
motherboard. It assumes no creep to occur in the clip. However, there is a small
amount of creep accounted for in the plastic fasteners. This situation is somewhat
similar to the reference design.
The impact of the creep to the board deflection is a function of the clip stiffness:
The relatively compliant clips store strain energy in the clip under the BOL preload
condition and tend to generate increasing amounts of board deflection as the
motherboard creeps under exposure to time and temperature.
In contrast, the stiffer clips stores very little strain energy, and therefore do not
generate substantial additional board deflection through life.
NOTES:
1. Board and clip creep modify board deflection over time and depends on board
stiffness, clip stiffness, and selected materials.
2. Designers must define the BOL board deflection that will lead to the correct
end of life board deflection
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