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→13.2.1.4 Ductility
To evaluate the ductility of gold layers, a bending test according to DIN 50 153 is usually employed. For certain applications the testing method is agreed upon between the coating manufacturer and the user. After bending the test sample over a pre-defined radius the surface layer in the bend area is examined microscopically. The detection of cracks or even delaminating is an indication of insufficient ductility
===13.2.2 Functional Properties===
====13.2.2.1 Frictional Wear====
For contact parts which are subjected to frictional sliding between each other, such as for example connector or sliding contact parts, the frictional wear is the determining factor for contact life and reliability. In general it is assumed that harder surfaces are more wear resistant. This usually is true for molten alloys, but not necessarily the fact for electroplated surface layers. As an example, low-carat molten AuCuCd layers exhibit despite a high hardness (HV 350) a higher mechanical wear than high-carat electroplated hard gold coatings (HV 120).
In the latter the incorporated carbon content acts as a lubricant, reducing wear significantly. Comparative tests of the mechanical wear of contact layers are After these additional testing will be performed, for example on actual connectors which then incorporate the real design characteristics in the connector contact area.
====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 ''(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.
Fig. 13.2: Computer controlled contact resistance measuring device (WSK Messtechnik)
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.
Fig. 13.3: Frequency distribution of the contact
resistance of a clean contact surface
(Ag rivet with electroplated hard gold layer;
test parameters: 10 mV, 10 mA, 10 cN)
Fig. 13.4: Frequency distribution of the contact
resistance of a contaminated contact surface
(Ag rivet with electroplated hard gold layer; test
parameters: 10 mV, 10 mA, 10 cN)
===13.2.3 Technological Properties===
====13.2.3.1 Adhesion Strength====
Good adhesion of the electroplated layer on the substrate is mandatory for the reliable function of a contact system. The adhesion strength between the electroplated surface coating and the substrate depends on many factors, such as the surface roughness of the carrier metal, the surface preparation, thermal expansion properties, ductility, and others. A difference in formability of layer and substrate metal can easily lead to separation of the coating layer. Rapid temperature changes can also lead to delaminating if the thermal coefficients of expansion of the coating and the substrate differ substantially. A prerequisite for good adhesion is the careful surface preparation prior to the actual electroplating, which is usually integrated into the electroplating process equipment.
There is no non-destructive test method which can be applied under practical use conditions. The methods used are mechanical tests such as a fold and bend test, or the wrap test. Often also an adhesive strip test according to DIN IEC 326 Part 2 is used. A transparent adhesive tape is applied to the electroplated surface and removed again after 10s by a uniform pulling action.
During this removal the coating layer is not allowed to come off the substrate, which means no substrate material is allowed to become visibly exposed.
Quite often temperature tests are also used to judge the adhesion strength. For these samples are exposed to elevated temperatures between 120 and 400°C over a defined time period (5 to 60 min). At insufficient bonding strength bubbles are becoming visible and delaminating may occur.
====13.2.3.2 Solderabilty====
In this context soldering is defined as using low temperature tin or tin alloys. A freuquently used method to determine the solderability is the immersion test. The coated material is inserted into a bath of the liquid solder and tinned for about 5 s. After this exposure 95% of the surface immersed must be wetted with the solder.
One of the requirements for complete wetting is also that the interface area between the substrate metal and the coating layer is not interrupted by oxides or other foreign substances.
Gold is known to be well solderable, however problems can occur when soldering onto thin gold layers. Since the gold quickly dissolves in the soldering alloy the viscosity of the liquid solder is increased and can lead to reduced wetting. Gold and tin also form intermetallic compound phases which lead to embrittlement and thus reduce the mechanical strength of the solder bond. In addition non-precious alloying components or co-deposited carbon can be problematic for good solderability.
====13.2.3.3 Bondability====
The wire bonding – a welding process of fine wires onto flat semiconductors and metal surfaces – was developed for contacting semiconductor
components (see Chapter 9). Depending on the application the reliability of the bond connections over extended time periods under difficult environmental conditions is of great importance. The quality of such connections can only be evaluated in destructive pull or shear tests.
*Pull Test
During the pull test a hook is inserted into the bond wire loop and the pull force is slowly increased to an amount that destroys the connection. During this test the weakest spot of the bond loop is determined. The actual delaminating between bond wire and substrate is deemed the most serious defect. The transition between bond spot and wire is a weak point because of the high deformation stresses exerted onto the wire and can also lead to break. A break in the actual wire is the most desirable result condition during the pull test.
Typical rupture pull forces measured are in the range of the rupture strength of the bond wire (i.e. 10 – 15 cN for a 25 μm diameter Au wire) and depend on the geometry of the loop.
*Shear Test
The shear test is used as an alternative to the pull test. A shear force is excerted onto the bond area. Separation then occurs between the bond layer and bond wire, or for a good bond connection within the bond wire.
*Aging Test
A higher difficulty for testing occurs if the initial bond test results from pulling or shearing were good at high forces, but later the bond connections show early failures due to aging.
One of the most severe tests is an exposure at a humidity level of 85% RH and at 85°C. At higher porosity levels in the coating the high humidity can lead to corrosion effects on the substrate material from corrosive gases in the atmosphere. For the most severe requirements a simulation test at 85°C and 85% RH is performed over a 500 h exposure time period.
