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→13.2.2.2 Contact Resistance
The contact resistance is the most important functional property determining the reliability of a contact layer.
For the measurement of the contact resistance commercially available test instruments with applicable data analysis programs are used ''<xr id="fig:Computer controlled contact resistance measuring device"/> (Fig. 13.2)''. In a pre-set program the surface of a contact layer is scanned and probed. Most frequently a freshly cleaned contact rivet with a hard gold coated contact rivet is used as the probing contact.<figure id="fig:Computer controlled contact resistance measuring device">Fig. 13[[File:Computer controlled contact resistance measuring device.2: jpg|right|thumb|Computer controlled contact resistance measuring device (WSK Messtechnik)]]</figure>
The measuring voltage and current are usually in the range of < 20 mV and, 10 mA (DC or 1 kHz AC). The contact force is selected as 2, 5 or 10 cN,
depending on the application of the contacts. A histogram of the individual resistance data is usually used to show the frequency distribution of the data and serves as an indicator for the cleanliness of the contact surface. As shown in Fig. 13.3 for a gold contact layer, a narrow scatter range and symmetrical distribution of the contact resistance values is typical for clean and tarnish film free contact surface. In case of the contact surface being partially or totally covered with a tarnish film, characteristic asymmetrical contact resistance distributions are evident ''(Fig. 13.4)''. While the contact resistance distribution can show the presence of tarnish films, only surface analytical methods can clarify their type and composition.
(Ag rivet with electroplated hard gold layer; test
parameters: 10 mV, 10 mA, 10 cN)
===13.2.3 Technological Properties===
