Many connector samples pass initial factory tests with satisfactory normal force and contact resistance. However, after 1 3 years of long term service, failures such as intermittent disconnection, unstable signals and excessive contact resistance occur. Upon troubleshooting, no corrosion, rust or external damage from mating/unmating is found. The culprit is often stress relaxation of spring terminals.
Stress relaxation is a slow and hidden failure mode. It does not cause immediate connector malfunction, but gradually depletes the contact force of springs over time and eventually destroys long term contact reliability. The issue of stress relaxation is particularly prominent for high speed connectors, automotive connectors and AI server connectors, which operate under sustained compression at elevated temperatures for extended periods.
I. What is Spring Terminal Stress Relaxation
Once spring terminals are fully assembled, they stay bent, with constant elastic stress stored internally. Under the combined influence of temperature and time, slow plastic slip takes place inside the metal material, and the internal stress gradually decreases over time. For terminals, this directly manifests as continuous attenuation of normal force.
Nearly unchanged deflection → gradual stress reduction → lower contact normal force
It differs from permanent set: Permanent set: After the external load is removed, the spring cannot return to its original position. Stress relaxation: Hardly any visible change or deformation can be seen on the terminal, yet the clamping force has silently dropped.
II.Four Key Causes of Stress Relaxation
1. Ambient Temperature (the most critical accelerating factor) Higher temperatures increase the mobility of metal atoms, and the rate of stress relaxation rises exponentially. Automotive connectors, server internal connectors and power connectors operate continuously at 85 125°C. Their spring terminals relax much faster than those working at ambient temperature. High temperature does not burn out terminals instantly; it gradually depletes spring elasticity day after day.
2. Excessively high initial stress level To achieve greater normal force in design, the working stress of springs is set very close to the material yield strength. A higher initial stress leads to a faster relaxation rate. Many engineers assume higher normal force equals better reliability, yet this inadvertently creates hidden relaxation risks.
3. Insufficient relaxation resistance of base materials Copper alloys vary greatly in anti relaxation performance: Brass: poor resistance, rapid elasticity loss at high temperatures, unsuitable for long term high temperature service Phosphor bronze: moderate performance, for general purpose connectors at room temperature Beryllium copper & CuNiSi copper nickel silicon alloy: excellent performance, the top choice for high temperature and high reliability applications Plating (gold plating, silver plating) cannot eliminate stress relaxation. Coatings only prevent corrosion and fail to stop elasticity loss within the base copper alloy.
4. Long term constant compression Once mated, connector springs remain deflected all year round with no chance of springback. Vibration accelerates fatigue fracture, while sustained static load is the breeding ground for stress relaxation.
III. How Stress Relaxation Gradually Destroys Contact Reliability
Initial Stage (0 to several hundred hours) Sufficient normal force and low contact resistance are maintained. All connector specifications pass inspection, and hidden risks cannot be detected through short term testing.
Mid Stage (several hundred to several thousand hours) Stress decays slowly and normal force starts to drop. It has not yet fallen below the minimum allowable contact force, so the equipment still operates normally.
Final Failure Threshold Normal force decreases below the minimum contact force threshold. The micro protrusions on the two mating surfaces can no longer be fully flattened. Oxide films and contaminants separate the metal substrates, causing a sharp rise in contact resistance.
Ultimate Failure Symptoms
- · Intermittent signals and sporadic errors that disappear after rebooting
- · Local overheating on high current connectors
- · Packet loss and bit errors for high speed signals
- · Occasional and non recurring failures, which are difficult to reproduce and locate during on site troubleshooting
IV. Methods to Suppress Spring Stress Relaxation at the Connector Design Stage
1. Reasonably control working stress and avoid approaching the yield limit A safety margin shall be reserved for the working stress of springs, and the working stress should not be brought infinitely close to the material yield strength. Finite element simulation shall be adopted to calculate the maximum stress of terminals, and the working stress shall be confined within the long term safe range permitted by the material.
2. Select high temperature and anti relaxation copper alloys For high temperature and long service life applications, nickel silicon copper, beryllium copper and other alloys with superior anti relaxation performance are preferred, rather than simply increasing plating thickness.
3. Appropriately increase contact force margin Where structure permits, design the initial normal force higher than the minimum required value and reserve a “normal force degradation margin” to offset pressure loss caused by stress relaxation over years of service.
4. Predict ageing life via thermal simulation Apply the Arrhenius ageing model and perform high temperature accelerated ageing tests to evaluate the residual normal force of terminals after 10 year and 20 year service under target operating temperatures, so as to predict long term reliability in advance.
5. Optimize structures to compensate for spring force degradation For certain high reliability connectors, adopt dual spring configurations or increase cantilever length to optimize stress distribution, reduce stress per unit area and slow down the relaxation rate.
V. Recommended Test and Verification
Short term insertion withdrawal tests alone are insufficient to identify hidden risks of stress relaxation. For high reliability projects, additional tests below are recommended:
- · High temperature stress relaxation test (mated condition maintained for an extended duration at elevated temperature; normal force re measured after cooling)
- · Long term accelerated ageing test
- · Contact resistance comparison test before and after ageing
Summary
Stress relaxation acts as an invisible “chronic killer” of connectors. Numerous seemingly random and non reproducible poor contact failures originate from the slow degradation of contact spring normal force over many years. When designing long service life connectors for AI computing, automotive and other high reliability applications, stress relaxation, along with normal force and signal integrity, must be included as a core verification item for terminal design.
Post time: Sep-05-2026