LGA775 Socket Heatsink Loading
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Thermal/Mechanical Design Guide 61
Appendix A LGA775 Socket Heatsink
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A.1 LGA775 Socket Heatsink Considerations
Heatsink clip load is traditionally used for:
• Mechanical performance in mechanical shock and vibration
⎯ Refer to Section
5.7.1 above for information on the structural design strategy for the
Intel RCBFH-3 Reference Design
• Thermal interface performance
⎯ Required preload depends on TIM
⎯ Preload can be low for thermal grease
In addition to mechanical performance in shock and vibration and TIM performance, LGA775
socket requires a minimum heatsink preload to protect against fatigue failure of socket solder
joints.
Solder ball tensile stress is originally created when, after inserting a processor into the socket, the
LGA775 socket load plate is actuated. In addition, solder joint shear stress is caused by
coefficient of thermal expansion (CTE) mismatch induced shear loading. The solder joint
compressive axial force (F
axial
) induced by the heatsink preload helps to reduce the combined
joint tensile and shear stress.
Overall, the heatsink required preload is the minimum preload needed to meet all of the above
requirements: Mechanical shock and vibration and TIM performance and LGA775 socket
protection against fatigue failure.
A.2 Metric for Heatsink Preload for ATX/µATX Designs
Non-Compliant with Intel Reference Design
A.2.1 Heatsink Preload Requirement Limitations
Heatsink preload by itself is not an appropriate metric for solder joint force across various
mechanical designs and does not take into account for example (not an exhaustive list):
• Heatsink mounting hole span
• Heatsink clip/fastener assembly stiffness and creep
• Board stiffness and creep
• Board stiffness is modified by fixtures like backing plate, chassis attach, etc.
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