BS EN 4875-2020 pdf free download.Aerospace series — Surface treatments — Test method for measurement of electrical contact resistance
The surface roughness of the test specimen impacts the contact resistance result. Test specimens having rough surfaces will yield to lower resistance values when subjected to a contact electrode pressure due to coating fracture. This reasoning can also be applied to the contact electrode.
Before testing, the test specimens shall be cleaned with materials that remove soil without damaging the coating (e.g with isopropyl alcohol and soft cloth).
Test specimens shall be stored under clean and dry conditions.
7 Procedure
7.1 Measuring conditions
Measurements shall be performed after the coating is in a steady state according to the process specification (some process specifications may specify times between processing and testing).
The ambient temperature and relative air humidity should be between 15 °C and 35 °C and less than 70 % respectively.
The measurement shall not be carried out at a distance less than 5 mm from the edges of the specimen.
The measurement shall be performed on an area free of drilling, marking, identification and scratches.
7.2 Electrodes
The contact electrodes shall be in copper or pure silver plated copper (use of pure silver plated copper electrode is recommended).
For copper electrode, the polishing shall be done as a minimum with a P1200 metallographic abrasive paper.
For pure silver plated copper electrode, the polishing shall be done cautiously.
In both cases, the electrodes shall exhibit a polish mirror surface.
The measurement of the roughness is mandatory when specified by the OEM/manufacturer.
Due to the risk of loss of the flatness of the surface/parallelism of plots due to the wear of the electrode during preparations/measurements, it is recommended to check the flatness of the surface/parallelism of plots using a calibrated thin film (50 µm max.) which allows to identify a deviation of measurement.
The electrodes shall be cleaned with materials that remove soil without damaging the plating (e.g with isopropyl alcohol). The electrodes shall be handled during all cleaning and subsequent operations so that mechanical damage or contamination is avoided.
The functioning of the assembly of electrodes is controlled by pressure on a reference specimen in gold or silver plated sheet, with an electrical surface resistance ≤ 0,05 mΩ. The deviation shall be checked at the beginning of each day of test and after 50 measurements.
When using an ohmmeter, a measured value of 0 Ω shall be indicated on the measuring apparatus. If necessary, the measuring apparatus is adjusted to 0, otherwise the indicated value should be deducted.
After having removed the assembly of electrodes from the surface, the ohmmeter should be adjusted to ∞.
7.3 Measuring the electrical contact resistance on specimen
Arrangement of current and voltage connections to electrode and specimen shall be as mentioned in Figure 1.
The electrical contact resistance is measured after 10 s minimum of pressure application and with a test current of (1,0 ± 0,1) A.
The applied load shall be as described in Table 1. For specific applications, other requirements can be agreed between purchaser and supplier.
Unless otherwise specified in the process specification, the minimum number of specimens for electrical contact resistance testing is 3 (three) (corresponding to 30 measurements) before and 3 (three) others (corresponding to 30 measurements) after ageing. All test specimens have to be processed in the same batch. Each measurement area will be identified according to Figure 2.
8 Results of electrical contact resistance
8.1 General
Each individual measurement shall be recorded and the result shall be expressed as resistance per surface unit (square inch or square millimeters).
Electrical contact resistance testing results will vary depending on the substrate material and should be compliant to the process specification requirement.
8.2 Representativeness of the results
The average result of each test specimen shall be calculated without taking into account the two extreme values, and the normality shall be monitored (using the normal law).
For the qualification of the process, the worst value of average result shall be taken as the reference.BS EN 4875 pdf download.