Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 367 of 707

Table IX. Electroless Palladium Bath Palladium chloride Rochelle salt Ethylenediamine Cool solution to 20° Sodium hypophosphite pH (adjusted with HCl) Temperature (PdCl2 C (68° F) and then add: 4.1 g/L 8.5 g/L 68-73° C(155-165° F) ) immersion for subsequent deposition of the electroless copper. Many pro- prietary activators are available in which these solutions can be used separately or together at room temperature. Palladium drag into the electroless copper bath can cause solution decomposition instantly. The pH of an electroless copper bath will influence the brightness of the copper deposit. Usually a value above 12.0 is preferred. A dark deposit may indi- cate low bath alkalinity and contain cuprous oxide. The plating rate is equally influenced by pH. In formaldehyde-reduced baths a value of 12.0-13.0 is gener- ally best. Stability of the bath and pH are critical. A high pH value (14.0) results in poor solution stability and reduces the bath life. Below 9.5, solution stability is good; however, deposition slows or ceases. The principal components of the elec- troless copper bath (copper, formaldehyde, and caustic) must be kept within spec- ification through replenishment. Other bath chemical components will remain within recommended ranges. Complexing agents and stabilizer levels occasion- ally need independent control. Other key operating parameters include tem- perature, air agitation, filtration, and circulation. Various common reducing agents have been suggested, however, the best known reducing agent for electroless copper baths is formaldehyde. The com- plexing agent (i.e., Rochelle salt) serves to complex the copper ion to prevent solu- tion precipitation and has an effect on deposition rates as well as the quality of the deposit. These conventional baths are stable, have plating rates of 1-5 µm or 0.04-0.2 mil/hr, and operate in an alkaline solution (pH 10.0-13.0). An example of a formaldehyde-reduced electroless copper bath is provided in Table VII. Recent formulations allow for alkanol amines such as quadrol-reduced baths. These high build [>10 µm/hr >0.4 mil/hr)] or heavy deposition baths operate at a lower pH without the use of formaldehyde. High build baths generally are more expensive and exhibit less stability but do not have harmful formaldehyde vapors given off during subsequent solution make up, heating, and deposition. These baths can deposit enough low stress copper to eliminate the need for an elec- trolytic flash. Quadrol is totally miscible with water and thus is resistant to Table X. Electroless Cobalt Bath Cobalt chloride Sodium hypophosphite Sodium citrate Ammonium chloride pH Temperature 366 30 g/L 20 g/L 35 g/L 50 g/L 9.5 95° C (203° F) 10 g/L 19 g/L 25.6 g/L

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