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April 2014 • SMT Magazine 27 CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues fEATurE Component assembling and soldering: The next level of packaging is the introduction of components by the soldering process to pro- duce final assembly for the device. Additional materials and process related parameters are in- troduced at this stage, namely: (i) solder alloy to join the component on to the board, (ii) compo- nents with various shapes and material content, and (iii) chemicals (solder flux) used during the soldering process. Soldering carried out by wave, reflow soldering, or in some cases by selective soldering/hand soldering depending on the na- ture of the components. Apart from the solder alloy and components, an important factor to contribute from this stage to the corrosion reli- ability is the solder flux residues from the sol- dering process. The type, quantify, composition, and morphology of the flux residue introduced from the wave, reflow, and hand soldering pro- cess are different, and therefore interact differ- ently to the humid environments (See section cleanliness issue, leak current, and corrosion in Part 2). Figure 8 shows the wave solder flux residue on a PCBA resulting from the use of no- clean weak organic acid (WOA) based flux. Electronic components and corrosion reliability Chip capacitor: Practically capacitors for electronic applications are available in variety of forms, but the discussion here is limited to the most commonly used type today namely surface mount chip capacitor (SM capacitor). These are flat components which can be di- rectly surface mounted on to the PCB by the reflow soldering process. Due to its bipolar na- ture, the chip capacitors are found to cause cor- rosion problems on a PCBA, especially the fail- ure mode electrochemical migration. Also, the open circuit nature of a capacitor makes it very sensitive to leakage currents (e.g., if a condensed layer of water bridges the two electrodes). As the stored electrical energy can be released from the capacitor even after the potential bias has been switched off, a capacitor can cause corro- sion failures after the electronic device has been switched off. For example, an SEM picture of the cross- section of a SM capacitor is seen in Figure 5 [2] . The ceramic part of the SM capacitor is com- monly based on titanium dioxide or barium ti- tanate (Ba 2 TiO 3 ), with small additions of other elements to increase the dielectric constant. The electrodes of the comb pattern are made from nickel or silver-palladium [12] . The connec- ting electrodes has copper layer, which is cover- ed with a layer of nickel followed by a top layer of tin as shown in Figure 9. Both copper and nickel layers are made by thick film technique, figure 8: A view of the wave solder flux residue on a PCbA. Figure 9: cross-section of a SM chip capacitor showing comb electrode pattern and end elec- trodes.