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August 2015 • SMT Magazine 29 exposed traces route to the board's edge for elec- trical access to volume between component and board. Background to the Problem In electronics manufacturing, solder paste is deposited onto PCBs and used to produce solder connections between discrete devices and the PCB. The assembly is heated above the melting point of the solder and allowed to cool. This process causes the flux to volatilize and ulti- mately vaporize. During volatilization, the flux removes oxides and permits the solder to chem- ically bond with the PCB and component. Ide- ally, the flux then vaporizes and is removed by dissipating into the surrounding atmosphere, leaving only solder behind. Practically, however, there is commonly a re- sidual component of flux remaining—flux that never fully vaporized or was prevented from dissipating. This flux residue, depending on its chemical constituents, can have varied impacts on the final assembly. Many fluxes are designed to leave trace levels of inactive residues, render- ing them relatively harmless. Conversely, flux is designed to attack metals, which makes leaving volatile/active fluxes on a PCB problematic. There is a wide range of proprietary chemi- cal makeups when it comes to fluxes. Regard- less of flux type, the warrantying agent of the final product must have a clear understanding of the risks involved in producing and selling its products. Included in these risks are the det- rimental effects of contamination sources. The clear warning sign of that risk manifests itself through the presence of ionic components near solder terminations. These compounds have the capability of migrating under a voltage bias, which produces a current. An even more dra- matic result of the presence of ionic compounds is the creation of dendrites. Dendrites serve as semi-permanent conduc- tive paths between previously unconnected PCB HOW CLEAN IS CLEAN ENOuGH TO ACHIEvE RELIABLE ELECTRONIC HARDWARE? continues FeAture Figure 3: Sensors placed under bottom termination. figure 4: active residues leading to leakage currents.