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PCB007-Aug2019

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54 PCB007 MAGAZINE I AUGUST 2019 to prevent leaks of corrosive chemicals onto the floor. Spillage from a process tank, pipe, chemical mixing area or etcher must be antici- pated and methods provided to contain, col- lect, analyze, and process the liquids. Automation Computerized process control systems can be used for panel handling and process bath monitoring to prevent unexpected decomposi- tion of a process bath, controlled rinse flow, and uniform panel withdrawal from each pro- cess bath. Since these systems require a sig- nificant capital expense for initial installation, typically, only large PCB companies will find this to be a cost-effective alternative. DI and Soft Water for Rinsing Natural contaminants found in water used for production purposes can contribute to the volume of waste produced. Silicates are a known contaminant in PCB chemistries. When using pretreated rinse water, the water require- ments for each rinse are reduced. Printed Circuits Handbook: Sixth Edition [2] states that many water supplies contain high levels of dissolved ionic minerals and possible colloidal materials that cause rejects in board production. Some of these impurities are cal - cium, silica, magnesium, iron, and chloride. Typical problems caused by these impurities are copper oxidation, residues in the plated through-holes (PTH), copper-to-copper peel - ing, staining, roughness, and ionic contami- nation. Equipment problems due to these im- purities include, but are not limited to, water line and w ater spray nozzle clogging, corro- sion, and other mechanical breakdowns. Pro- cess baths should be made using deionized water. The presence of organics in water can ad- versely affect etching and another bath perfor- mance. Very good water may contain no more than 2.0 ppm of total organic carbon. The best plating practices suggest using good water quality for critical rinsing operations and high yields. While what is considered good water quality is not precisely defined, here are com- monly used criteria: • Total dissolved solids (TDS) 4–10 mg/l (or ppm) • Conductivity 8–30 microsiemens/cm • Carbonate hardness 3–15 ppm • Chloride 2.0 ppm • Turbidity 1.0 NTU Somewhat lower quality is acceptable for less critical applications (deburring) while some other operations (for example, the developer rinse) require better water, such as water con- taining only 0.5–5 ppm of TDS (0.1–1 MEG). However, this depends on several factors. One article [3] explained, "Aqueous dry film resists are susceptible to over development. If left too long in the developer, the exposed re- sist will be chemically attacked and will par- tially disintegrate…A short residence time in the development chamber helps minimize re- sist swelling." Additional swelling that may cause adhesion failure must be avoided in the rinsing cham- ber. Distilled or deionized water used in the first developer rinse may rapidly penetrate the resist due to osmotic pressure. This may dilute the higher ionic strength developer solution trapped in the resist. To avoid this, the first rinse should have, according to this article [3], a relatively high ionic concentration by adding salts. Water hardness in the first rinse of 140–350 mg/L of CaCO3 is adequate for most work. If the rinse does not have sufficient hardness, use an acidic second rinse. Another article [4] claimed that sufficient hardness must be available in the water used for the developer working solution makeup and the developer rinse for some resists. In those cases, magnesium sulfate has been add- ed to the water when sidewall definition and resist toughness needs improvement. Other critical rinses are: • The accelerator • The catalyst • The last rinse on the electroless copper line • Before and following nickel • The gold and palladium electroless/electroplating baths

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