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22 SMT Magazine • April 2014 capillaries for the absortion of humidity. An- other aspect is the content of the polymers and plastics, and the possibility of the release of chemical species during processing or service that could have influence on reliability. A typi- cal example is the use of various types of fire re- tardants, which can release fractionalized com- pounds causing corrosion reliability issues (see section on fire retardant and degassing in Part 2 of this article). Metallic materials on the PCBA are of prime interest for corrosion reliability, as electrochem- ical corrosion can occur on these parts lead- ing to material loss or several other problems, which leads to the failure of the device in a short span of time. Basically metals/ alloys used on the PCBA vary in their reactivity towards the environment, determined by its nobility. Metals like gold are noble (find in metallic form in the earth crust), while metals like Cu, Ni, Sn, Pb, Ag, etc., are less noble than gold and has a tendency to go back to its native state as stable com- pounds once they come in con- tact with an environment. Even gold can corrode under some conditions on the PCBA surface in- volving high potential bias and contaminations such as chlorides. From the aqueous corrosion point of view, many of the tiny components or conduction lines are made of active metals/ alloys, which also can have opposite potential bias when the device is functional. Together with humidity it can cause corrosion as shown in Figures 1 and 2. Some of the important com- ponents on a PCBA, which are susceptible to various types of corrosion, are briefly described below with a discussion on the materials' make- up and possible climatic reliability issues. Bare printed circuit board and manufacturing process The PCB is, in general, the substrate for mounting and connecting electronic compo- nents, while it mechanically supports the com- ponents and other devices from mechanical and thermal vibrations. A PCB consists of conduc- tive pathways embedded in a glass epoxy poly- mer or other types of laminates. The conduc- tion lines on a PCB are made of copper, while the non-conducting substrate, for example, is made from glass epoxy polymer. However, the surface of the PCB on which the components to be mounted will have an surface finish added to the base copper lines, where the type of sur- face finish depends on the type of application or product. A PCB can be single-sided, double- sided, or multilayered. Functional electrical circuits and PCBAs are formed by interconnecting pack- aged integrated circuits with other devices using printed circuit boards. As such, these boards can be viewed as the second-level packaging. In- dividual devices are attached to the PCB using mainly two types of soldering techniques namely wave soldering and reflow soldering process. Wave soldering is used for through-hole components with legs, while reflow is used for surface mount compo- nents. Manual hand soldering is used only for special compo- nents that cannot be soldered using either of the above process. Tin-lead alloys were the predominant choice for soldering until re- cently, but due to the ban on lead, lead free sol- dering technology is in place today. Printed circuit board surface finish and me- tallic combinations: Most commonly employed PCB surface finishes are: electroless nickel-gold (ENIG), immersion tin, hot levelled solder fin- ish (either Sn-Pb solder or lead-free solder al- loy), immersion silver, and copper finish. In the case of gold surface finish PCB (ENIG), gold is not directly plated onto copper, as this can eas- ily form diffusion couples that produce various types of intermetallics at the interface. In order to avoid this problem, gold surface finish PCBs are the ENIG type: copper and the gold layer are separated by 5–6 micrometers of electroless nickel. A nickel layer provides both a diffusion CliMaTiC rEliaBiliTY OF ElECTrONiC DEviCES aND COMpONENTS continues fEATurE From the aqueous corrosion point of view, many of the tiny components or conduction lines are made of active metals/alloys, which also can have opposite potential bias when the device is functional. " "