Printed circuit boards electroless plating

Electroless copper solutions underwent similar development during the same period (10). Early printed circuit boards used mechanically attached eyelets to provide electrical conductivity between the copper sheathing laminated on two sides of a plastic board. Electroless copper plating provided a less expensive, better conductive path, allowing much greater numbers and smaller sizes of holes. Later, electroless coppers even replaced the laminated bulk copper sheathing in the semiadditive and additive processes (see Copper). 

Electroless nickel—boron baths use sodium borohydride or dimethylamine borane [74-94-2] in place of sodium hypophosphite (see Boron compounds). The nickel—boron aHoy is brittle, highly stressed, and much more expensive than nickel—phosphoms aHoys. Nickel—boron is mainly used to replace gold in printed circuit board plating. 

Fig. 2. Multilayer printed circuit board composite. Constmction is multiple layers of epoxy—glass and foil copper. Foil copper outermost layer and drilled through-holes are sequentially plated with electroless copper, electrolytic copper, electroless nickel, and electroless gold.

The cost of the chemicals used in electroless copper plating is very low, rarely exceeding 2.78/m, except for fully additive processes. The principal costs of printed circuit board production arise mainly from handling steps and other operations. 

Manufacture of Printed Wiring Boards. Printed wiring boards, or printed circuit boards, are usually thin flat panels than contain one or multiple layers of thin copper patterns that interconnect the various electronic components (e.g. integrated circuit chips, connectors, resistors) that are attached to the boards. These panels are present in almost every consumer electronic product and automobile sold today. The various photopolymer products used to manufacture the printed wiring boards include film resists, electroless plating resists (23), liquid resists, electrodeposited resists (24), solder masks (25), laser exposed photoresists (26), flexible photoimageable permanent coatings (27) and polyimide interlayer insulator films (28). Another new use of photopolymer chemistry is the selective formation of conductive patterns in polymers (29). 

Currently there is much interest in modifying the surfaces of commodity polymers with a view to either functionalising the surfaces to provide for more exotic materials or subjecting the treated surface to electroless or electrolytic plating to produce, for example, printed circuit boards. 

Treatment methodologies for process solution wastes include pH adjustment, flocculation, precipitation of metals, dewatering and sludge drying. Chelated metals require special treatment to break the complexes. Chromium, cyanide, electroless plating, and printed circuit board wastes often need to be segregated from other streams in order to reduce waste volumes and avoid the chance that some complexed metals may escape the treatment system. 

ED has also been used to improve or maintain the quality of a plating bath and thus eliminate the need for periodic replenishment. An instructive example is shown in Figure 8.9 for the electroless plating of copper onto nonmetallic substrates such as printed circuit boards.10 The process illustrated utilizes an uncharged EDTA-Cu complex where the reduction of copper is driven by the oxidation of formaldehyde to formic acid. The formate ions and the sulfate ions introduced with the make-up copper are undesirable reaction by-products that are removed by ED. NaOH is added to adjust pH sufficiently to ionize the formic acid. The uncharged EDTA-Cu complex remains in the diluate stream, which is returned to the plating tank. 

Electroless copper The type chosen will depend on the method of image transfer as well as on the need for panel plating. These processes are readily automated. The largest percentage (estimates are as high as 95 percent) of printed circuit board manufacturers worldwide rely on the electroless copper method for hole metallization. 

IPC-4552, Specification for Electroless Nickel/Immersion Gold (ENIG) Plating for Printed Circuit Boards, October 2002. 

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