Electroless copper plating Additive process

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 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. 

The first commercial boards usiag electroless plating for connection between the sides were made by Motorola ia 1953 (41). An unclad plastic board was coated with electroless silver, a reverse resist appHed, and copper electroplated to the required circuit thickness. The resist and excess silver were removed. This semi-additive process is stiU used, with copper replacing the silver. 

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. 

Plated-through holes can be formed by additive (electroless) copper deposition, of which there are three basic methods fnlly additive, semiadditive, and partially additive. Of these, semiadditive involves pattern electroplating for PTHs with very thin snrface copper, bnt the other two form PTHs solely by electroless copper deposition. The additive process has various advantages over the subtractive process in forming fineline condnctors and PTHs of high aspect ratio. A detailed account of the additive process is given in Chap. 31. 

Plating. Plating is an additive process where the copper layer is first etched.Then the layer is catalyzed for electroless plating, and photoresist is applied. Resistor locations are imaged into the resist and the pattern developed. Exposed catalyst in the proper areas then initiates plating of the resistors. The resist is stripped, background catalyst is removed, and the resistors are in the proper locations. 

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 "folly additive method, the full conductor thickness may be built up in a single, long (24 h) electroless deposition stage. 

There are more process steps involved in laser subtractive structuring (LSS) than in the additive or semiadditive processes (Fig. 3.2). Injection molding is followed by short surface activation to permit electroless copper or nickel plating. This chemical premetallization is followed by a galvanic process to build up the plating to target thickness. The next step is structuring as such, by application of an activatable etch resist. 

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