Issue link: https://iconnect007.uberflip.com/i/1309864
66 PCB007 MAGAZINE I NOVEMBER 2020 ion-selective probes, and capacitance and con- ductivity sensors (Table 1). Conductivity The conductivity sensors will measure the amount of total dissolved solids in an electro- lyte. It is common to use them where the con- centration of a known salt, base, or acid must be determined. The concentration of these so- lutions will vary the resistance of the solution or the inverse of resistance: conductance. The conductivity sensor typical measurement is in mhos per centimeter (reciprocal of ohm-centi- meters). Lower conductivity ranges of 0.01–100,000 micromhos per centimeter are used for water purity, such as boilers and chillers or deion- ized water. A higher concentration of electro- lytes (50–1,000 millimhos per centimeter) use electroless probes to avoid the polarization ef- fects of electrolysis. Conductivity (dissolved ionic concentration) and pH (hydrogen-ion concentration) are quite common sensors used in the industry. Specific Ion Certain applications require that the activity of an ion in a solution be measured. This can be accomplished with an electrode designed to be sensitive to the ion whose concentration is being measured. These electrodes are similar in ap- pearance to those employed to measure pH but are constructed of glass-membrane electrodes, solid-membrane electrodes, liquid ion-exchange mem- brane electrodes, and sili- cone rubber-impregnated electrodes. The reference electrode is the same as that used for pH. The electrode output is read on a high im- pedance voltmeter similar to that used with pH elec- trodes. Many applications are possible using ion-selec- tive techniques. In printed circuits, measurements are made of copper and chlo- ride in acid sulfate plating baths, lead and flu- oroborate in tin-lead fluoroborate plating so- lutions, cyanide (ductility promoter) in elec- troless copper solutions, permanganate in desmear/etchback solutions, and the sulfate in nickel sulfate plating solutions for tab plating. The specific-ion electrodes are made for spe- cific ions, such as copper, chlorine, sulfate, etc. with their range of sensitivity in ppm and pre- ferred pH. Many times, specific ion electrodes use the same meter as pH electrodes. ORP/Redox ORP or redox measurements determine the oxidizing or reducing properties of a chemical reaction. A reduction is the opposite of oxida- tion. There can be no oxidation without an at- tending reduction. For example, a ferrous ion may lose an electron and become a ferric ion (gaining increased positive charge) if a reduc- tion of cupric to cuprous ions (which is the re- verse of this operation) occurs at the same time. As a sensor in automatic chemical solution control (ACSC), ORP, or redox is used in the control of the oxidizer in ferric chloride, cupric chloride and hydrogen peroxide/sulfuric acid etching, in measuring the Au(III) to Au(I) in gold-tab plating and the copper activity in elec- troless copper baths. 2. Specific Gravity A second universal sensor is specific grav- ity (SG). Although not a chemical analytic Table 1: The five main electrochemical sensors have a lot of useful features for PWB processes.