Electroless copper 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). 

The ideal electroless solution deposits metal only on an immersed article, never as a film on the sides of the tank or as a fine powder. Room temperature electroless nickel baths closely approach this ideal electroless copper plating is beginning to approach this stabiHty when carefully controUed. Any metal that can be electroplated can theoretically also be deposited by electroless plating. Only a few metals, ie, nickel, copper, gold, palladium, and silver, are used on any significant commercial scale. 

Electroless nickel or nickel—lead alloys can improve the solderabiUty and braisabiUty of aluminum even when a continuous film is not present. Electroless nickel systems based on dimethylaminehorane reduciag agents are used to coat aluminum contacts and semiconductors (qv) ia the electronics iadustry. Newer uses iaclude corrosion-resistant electroless nickel topcoatings on electroless copper plating for radio frequency... 

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. 

SemiadditiveMethod. The semiadditive method was developed to reduce copper waste. Thin 5.0 lm (4.5 mg/cm ) copper foil laminates are used, or the whole surface may be plated with a thin layer of electroless copper. Hole forming, catalysis, and electroless copper plating are done as for subtractive circuitry. A strippable reverse—resist coating is then appHed. Copper is electroplated to 35 p.m or more, followed by tin or tin—lead plating to serve as an etch resist. The resist is removed, and the whole board is etched. The original thin copper layer is quickly removed to leave the desired circuit. This method wastes less than 10% of the copper. 

Copper was selected as the metallizing material because of its wide industrial application. Copper was then plated onto the preplate layer either by electroless or electroplating. A commercial solution was used for the electroless copper plating to obtain a 1.0-pm thick copper deposit. A 20 pm-thick layer of copper was deposited electrolytically under the following conditions. 

Figure 21.4 SEM micrographs of (a) as-received FRP substrate, (b) glow discharge activated (0.15 volume fraction copper), and (c) electroless copper plated surface (xlOOO).

Fig. 14. Coulostatic potential decay curve for a copper electrode in an electroless copper plating bath (M. Suzuki et al., 1982 [22]).

A typical electroless copper plating bath contains divalent copper, EDTA as a com-plexing agent, NaOH, formaldehyde as a reducing agent, and (a) small amount(s) of additive(s) to stabilize the bath and to impart desired physical properties to the deposit. For thin metallization purposes, the bath may remain at room temperature, but the full-build bath is operated at elevated temperature. The composition and operation conditions of a typical full-build bath are shown in Table 11 [124]. It is well known that the two partial reactions are [3, 4] ... 

Fig. 23. Rate of copper deposition, determined with a quartz crystal microbalance, vs. potential in an electroless copper plating bath at various formaldehyde concentrations. A 0 mmol dm HCHO, 15 mmol dm , 30 mmol dm , 45 mmol dm", O 60 mmol dm (B. J. Feldmann et al., 1989 [131]).

Owing to the increasing technological interest of the electronics industry in electroless copper plating, a considerable amount of R D is still in progress in this field, and many publications continue to appear in the literature. It is impossible to review... 

Formaldehyde Oxidation during Electroless Copper Plating Studied... 

Electroless Copper Plating Using Co(II) Complexes as Reducing Agent.469... 

Presuming the electrochemical mechanism electroless copper plating [19], namely, the catalytic reduction of Cu(II) ions by formaldehyde, the partial reactions occurring at equal rates under open-circuit conditions could be written (using the deuterium tracer to specify the origin of hydrogen) in a simplified form as follows ... 

FORMALDEHYDE OXIDATION DURING ELECTROLESS COPPER PLATING STUDIED BY ONLINE ELECTROCHEMICAL MASS SPECTROMETRY... 

Similarly, partial reaction currents in electroless copper-plating solution can be extracted using electrochemical quartz crystal microgravimetry (EQCM) to in situ monitor the rate of copper deposition under open-circuit conditions and as a function of the electrode potential... 

FIGURE 19.9 Extraction of the partial reaction rates for formaldehyde oxidation (o) ((A) measured by DEMS, Figure 19.6A) and copper Cu(II) ion reduction (dotted line) ((B) measured by EQCM, Figure 19.8A) as the difference with the net current (solid line) in the positive-going scan in electroless copper-plating solution (for details see captions of Figures 19.6A and 19.8A). (From Pauliukaite, R. et al., J. Appl. Electrochem., 36, 1261, 2006.)... 

---