PCB007 Magazine

PCB-Mar2017

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70 The PCB Magazine • March 2017 corrosion. One new development that may show promise both in preserving solderability and minimizing creep corrosion is the use of self-assembled monolayers. Self-Assembled Monolayers Self-assembled monolayers (SAMs) of al- kanethiols adsorbed onto clean metal surfaces have been the focus of research chemists and en- gineers for several years. These molecules have shown promise as a way to control oxidation of active metals such as copper and silver. In addi- tion, there is speculation that these SAMs may also be effective in bonding with copper under certain conditions and essentially acting as an organic solderability preservative (OSP). The molecules typically possess a function- al group that has an affinity for the substrate, also known as a head group, and a tail group. In forming a self-assembled monolayer, the head groups of molecules chemisorb to the sub- strate, arraying the tail groups to form a dense assembly that extends from the surface of the substrate. Known head groups include thiols, si- lanes, and phosphonates. In many applications, the tail group of the molecule is functionalized to provide the resulting monolayer with desired properties relating to, for example, wetting ad- hesion, chemical resistance, biocompatibility, and the like. Due to the strong affinity of the thiol head group to metal substrates, alkane- thiols have often been used in the formation of self-assembled monolayers. Alkanethiol self- assembled monolayers have found applications in electronics, for example, for modifying the surface properties of metal electrodes [1]. Why is this important? Many circuit components and printed boards used in electronic equipment are ex- posed to harsh environments. This description includes PCB assemblies under the hood of an automobile or in instrumentation and controls subjected to corrosive environments. The latter would apply to electrical units deployed in pa- per mills, clay modeling studios and other areas where sulphur in the atmosphere may come in contact with any susceptible exposed metals. A good example of corrosion is shown in Figure 1. Electroless nickel/immersion gold coatings are not the only finishes susceptible to creep corrosion. In Figure 2, immersion silver (with soldermask) exhibits corrosion. Specifically, the corrosion product seen here is copper sulphide. Creep corrosion is the mi- gration of copper sulphide across the circuit. For creep corrosion to occur, there must be ex- posed copper (on the PCB) and sulphur in the atmosphere. Silver sulphide will also form when exposed to sulphur in the environment. There IMPROVING SOLDERABILITY AND CORROSION RESISTANCE FOR FINAL FINISHES, PART 1 Figure 1: ENIG coated copper surface showing extensive creep corrosion. (Source: R.K. Veale, Rockwell Automation [2] .) Figure 2: Immersion silver deposit over copper exhibiting creep corrosion. (Source: IPC Process Effects Handbook 9121.)

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