Determination of the current carrying capacity of contact springs is usually performed by measuring the temperature increase. The upper temperature limit is set by the temperature stability of the spring and the insulation material, the current limit is set accordingly.
But as miniaturization continues, the direct measurement of the temperature gets more difficult. Not every part of the contact spring is accessible, so hot spots cannot be detected reliably. The need for a custom fixture and the presence of the temperature sensors create a different thermal environment for the probe.
The measurement principle described here avoids these problems by taking the mechanical changes of the spring material. CuBe is hardened at about 315 °C and loses its hardness if it is exposed to high temperatures. Our measurement system consists of a 3-axis positioning system, a contact site with interchangeable metal sheets, a semiconductor force sensor with amplifier and digital multimeter, and a high current tester. The contact spring to be tested is mounted on the 3-axis system and the spring characteristics are measured. Then the metal sheet is contacted with the specified target deflection and current is applied. The high current tester is capable of generating current pulses up to 277 A with the shortest pulse length of 10 ms. The pulse/pause ratio can be programmed individually. Afterwards the spring characteristics are measured again. If the current has led to a significant heating of the probe, the change of the spring force is detected.