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24 SMT Magazine • April 2014 CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues barrier and the necessary hardness, especially for contact applications. A thin layer of gold is plated onto the nickel layer using an electroless process called immersion gold plating. In this case the gold layer is plated on to the surface by ion exchange process with palladium activated nickel. With immersion Au finish, thickness of the gold layer is extremely low to the level of 50–100 nanometers. But gold layer on the electronic contacts are usually much thicker of the order of 500 nm—1 µm. In this case higher thickness is achieved by electroplating process. For a tin finish, immersion tin is mainly used, produced by plating a thin layer of tin onto the copper conducting lines. The tin lay- er in this case is usually not more than 1 µm. The immersion tin plating process is similar to immersion gold plating process, which is car- ried out by electroless process using an ion ex- change mechanism. Hot-levelled PCB surface finish PCBs are made by coating the solder ma- terial (with lead or lead-free) on to the copper lines by dipping the whole PCB into a molten solder bath followed by removal of excess mate- rial using an air knife. The thickness of the layer in this case is of the order of 1 micrometer. Irrespective of the overall surface finish, low-voltage, low-force connectors and contacts are usually present on a PCB for connecting the device with other parts of the device [9,10] . A va- riety of different configurations exist, but the most used substrates are copper, brass, bronze, or copper-beryllium. The use of copper alloys ensures some susceptibility to environmental degradation. To reduce interfacial resistance and corrosion, the substrate is plated with a nickel diffusion barrier and then with a precious metal (e.g., gold, palladium) [9,11] . Printed circuit board and electronic corro- sion: Important effects of a bare PCB on the cor- rosion reliability are the following: (i) source of contamination, (ii) water permeation through laminate and related corrosion issues, and (iii) corrosion issues related to multimetallic layers on the surface. Due to miniaturization, features on the PCBs are tiny and the smallest distances between the conducting lines on the external fEATurE figure 7: Trapped contamination in via holes and ion chromatographic analysis: (a) overview of vias, (b) localized contamination in a vias, and (c) levels of different ionic residues.