Case study
An accelerator card was throttling after thermal cycling because heatsink screws were relaxing. Class 12.9 screws with controlled friction held preload through 1,000 cycles.
1,000
Thermal cycles without throttle
12.9
Property class
0.13
Friction coefficient (µ)
−18 °C
Peak-to-peak temperature swing
Client: Data centre equipment manufacturer · Industry: PCB, Server & Data Center
The challenge
The card used M3 class 8.8 screws to clamp a 1.2 kg heatsink onto a GPU package through a phase-change thermal interface. After 300 thermal cycles between 25 °C and 85 °C, junction temperature had risen by 9 °C and the card was throttling under sustained load.
Teardown showed the screws had lost 25–35% of their installed preload. The thermal interface material had pumped out of the interface because clamp load was insufficient to keep it in place.
At a glance
Products specified
Machine Thread · Precoated · Standoffs
The approach
Each change addresses a specific failure mechanism rather than tightening a specification in general.
01
M3 class 12.9 screws replaced 8.8, raising the achievable preload. But class alone is not enough — friction scatter between lots was causing ±20% preload variation.
02
A zinc-nickel finish with a defined lubricant gave µ = 0.13 ± 0.02, verified by torque-tension testing per lot, so the torque specification produced a predictable preload.
03
A through-hardened washer spread the clamp load and prevented the aluminium heatsink from Brinelling under the head, which had been a secondary source of relaxation.
04
A micro-encapsulated adhesive was applied to the thread to prevent back-off under vibration during shipping, without affecting the installed preload.
05
Break-loose torque was measured at 85 °C after cycling, not at 23 °C, because the relaxation mechanism only appears hot.
The result
01
Junction temperature rise after 1,000 cycles was held to 2 °C, versus 9 °C on the original design.
02
Preload loss fell from 30% to under 6% after the same cycle count.
03
No throttling under sustained load in the qualification test.
04
The fastener specification — class, finish, µ and torque — was adopted as the platform standard for three card families.
Products used
These families carried the joint. A different combination may suit your assembly — the engineering method is the same.
Micro Screws · M0.5 – M6
Precision machine screws from M0.5 to M6 with rolled Class 6g threads, tight head concentricity and clean drive recesses for automated driving.
Micro Screws · M0.8 – M6
Screws with a factory-applied locking or sealing element, so the assembly line receives a ready-to-drive fastener with no liquid adhesive and no cure time.
Precision Components · M1 – M4
Male-female and female-female standoffs from M1 to M4, plus plain spacers, in brass, stainless and aluminium — with length tolerances that keep board stacks parallel.
Send the joint drawing, the cycle count and the failure mode. We will tell you whether the fix is a fastener, a tolerance or a design change.