Intel
®
Core
TM
2 Duo E6400, E4300, and Intel
®
Pentium
®
Dual-Core E2160 Processor—LGA775
Socket Heatsink Loading
Intel
®
Core
TM
2 Duo E6400, E4300, and Intel
®
Pentium
®
Dual-Core E2160 Processor
TDG October 2007
44 Order Number: 315279 - 003US
Appendix A LGA775 Socket Heatsink Loading
A.1 LGA775 Socket Heatsink Considerations
Heatsink clip load is traditionally used for:
• Mechanical performance in mechanical shock and vibration
• Thermal interface performance
— Required preload depends on TIM
— Preload can be low for thermal grease
In addition to mechanical performance in mechanical shock and vibration and TIM
performance, the 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 (Faxial) 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 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 other factors such as:
• 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.
Simulation shows that the solder joint force (F
axial
) is proportional to the board
deflection measured along the socket diagonal. The matching of Faxial required to
protect the LGA775 socket solder joint in temperature cycling is equivalent to matching
a target MB deflection.
Therefore, the heatsink preload for LGA775 socket solder joint protection against
fatigue failure can be more generally defined as the load required to create a target
board downward deflection throughout the life of the product.
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