Fully Electroless Plating

Today, electrolessly deposited copper is considered as reliable as galvanically deposited copper, and fully electroless plating technology is finding its way to many applications. " In this chapter, we will discuss various methods of PWB fabrication by means of fully electroless copper-plating technology. 

Although an adhesive-coated surface provides adequate insulation characteristics for most applications, some boards, such as ones used for mainframe computers, require higher insulation resistance than adhesives can render. When IBM decided to use fully electroless plating to form fineline conductors on innerlayers of high-aspect-ratio MLBs in the mid-1970s, it took an approach different from the adhesive system. ... 

Perform fully electroless copper deposition (panel-plate). 

Figure 31.7 shows the plating racks filled with panels waiting to be placed in electroless plating tanks. In large-volume operations, the panel-racking operation is fully automated. 

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. 

Yullj Additive Method. No electrolytic plating step is used ia the fully additive process. The copper circuit is formed directly on the board without a continuous copper film. Heavy-build electroless coppers are used to iacrease the final thickness of the entire circuit. This process is much more difficult to control than the others. Additive processiag is becoming increasingly important ia high aspect ratio, very small diameter through-holes that caimot be easily electrolyticaHy plated. 

MOLDING/METALLIZATION. Molded thermoplastic circuit board substrates may be rendered selectively conductive by several additive process techniques including conductive polymeric thick film inks (PTF), and semi and fully additive electroless/electrolytic platings. Of the various chemical process methods developed to produce circuitry on a molded plastic substrate, one method practiced by Pathtek, A Kodak Company, combines both "catalytic" and "non-catalytic" resins in a highly automated commercialized two-shot molding/selective metallization process. 

The negative resist pattern is used to selectively build up the circuit by copper electroplating. When the required conductor thickness (typically 17 or 35 pm) is reached, it is plated with tin, tin-lead or gold as a positive resist. Removal of the resist is followed by rapid etching of the electroless copper layer. The remainder of the process is similar to that for subtractive methods. In the fully additive method, the full conductor thickness may be built up in a single, long (24 h) electroless deposition stage. 

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