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SMT-Sept2017

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24 SMT Magazine • September 2017 allow electrochemical failures to occur, such as corrosion, reduction of insulation resistance, and, worst of all, electrochemical migration (Figure 1). If too much flux is applied to a re- work site, some of the flux can move to nearby parts of the board that will not be exposed to the heat of the soldering tool. This often hap- pens when flux is dispensed in large squeeze bottles with coarse tips. All those spikes in the SIR of the unheated flux are caused by the for- mation of dendrites between conductors. So, how do unheated flux residues (and oth- er contaminants) allow electrochemical prob- lems to happen? A diagram showing these fail- ure mechanisms is given in Figure 2. Many flux residues are hygroscopic, that is they absorb moisture from the air. This forms an electrolyt- ic film on the board. The presence of this film REWORK AND RELIABILITY: LESS IS MORE! Figure 1: SIR measurement of heated and unheated liquid, low-solids, no-clean flux. Figure 2: Electrochemical failure mechanisms caused by contaminants on circuit boards.

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