Materials Characterization

- Contact Resistance Measurements -

The reliability of electronic systems is largely influenced by the ability of the many contacts and connectors within the assembly to maintain a connection resistance below a prescribed "threshold" value. Common degradation mechanisms, such as growth of oxide films, pore corrosion, and stress relaxation, can lead to increases in contact resistance and eventual failure of the system. Contact resistance measurements are used to assess the risks of using various contacts and connectors. The Automated Contact Resistance Probe (ACRP) at the CALCE was developed, with assistance from AMD and Bellcore, for this purpose.

Contact resistance measurements require the ability to measure very small fluctuations in resistance while simulating actual operating conditions. The ACRP ensures accurate measurement by using a computer-controlled loading system designed to apply a vibration-free normal force to the contact surface. This loading system has an adjustable loading speed and has a range of 0 to 400 grams, with a resolution of less than 2 grams. Contact resistance is measured using a computer-controlled, high resolution power supply and measurement unit, which can deliver 1nA to 1A of current at a voltage varying between 0.1mV to 110V. If contact measurements need to be made at a higher current rating, which may be warranted for a field application, an appropriate source unit can be added to the existing setup.

The ACRP conducts resistance measurements in real-time, enables measurement using actual contact geometries, and can simulate the repeated mating and unmating cycles experienced in separable connectors. This "wipe" motion is accomplished through automatic x-y positioning. This computer-controlled displacement has a resolution of 1.2 um and allows for multiple measurements of a single point (cyclic loading) or over an entire contact surface (automatic multi-point contact).

Cyclic Loading: This capability enables mating/unmating and loading/unloading test of connectors. The ACRP at CALCE can detect real-time change in the contact resistance vs. normal force relationship in each individual mating cycle.

Automatic Multi-Point Contact: This capability allows a determination of the distribution of contact resistance as a function of contact location. Multiple measurements are needed to obtain an accurate assessment of the contact resistance behavior. The environment is a critical factor in contact resistance and often accelerates common degradation mechanisms. The ACRP allows the user to measure contact resistance real-time while subjecting the connector to a range of temperatures and humidity designed to simulate or accelerate actual field conditions. This is critical because failure mechanisms, such as fretting corrosion, depend on the synergistic effect of micromotion, temperature, and humidity.

The use of a mixed flowing gas (MFG) chamber enables accelerated assessment of contact failure mechanisms due to corrosive environments.