Saturday, May 24, 2008

Plating of Printed Wiring Board

When printed conductors are used on both sides of a printed wiring board, electrical connection between the two sides of the board are most reliable made via plated through holes in the board. Because copper is widely used as the base conductor metal in a printed circuit, it is widely used as the base conductor metal in a printed circuit; it is natural that copper also be used for plating the through holes. Copper pyrophosphate solution finds their primary application in this process because the product has consistently met engineering requirements and rigid quality demands. Board-thickness-to-hole-diameter ratios of 8:1 can be satisfactorily plated. This capability of pyrophosphate solutions has helped greatly in furthering the art of multilayer board fabrication.

The success of copper pyrophosphate solutions is attributed to the good throwing power attainable with these solutions. Relationship between throwing power as determined with a Haring cell and plating thickness on printed wiring boards. The effects of changes in bath composition and operating conditions on throwing power are summarized by Dini at al. a discussion of the electrochemical principles which make it possible to achieve good plate distribution in holes was presented.

Because of rigid quality and engineering requirements placed on printed wiring boards, some of the operating conditions are refined to provide optimum results. For example, many installations move the boards mechanically during plating in addition to agitation over the entire rack area. An informative through somewhat oversimplified explanation of the theoretical and practical reasons behind solution agitation for through hole plating. Preplating treatment of the board is also more critical; when pattern plating is used, a neutral or acid immersion cleaner should be used to avoid the resist breakdown resulting from use of an electrolytic cleaner.

The problems associated with the process include brittle deposits and occasionally, skip plating. The first is the biggest problem. Brittle deposits fail prematurely by cracking because the thermal expansion of the epoxy glass laminate in the z axis (thickness direction) is considerably higher than for copper. Actually, the corfficient of expansion for copper is 1.7 x 10-5 cm/(cm. oC) and for epoxy glass boards 6 x 10-5 cm/(cm.oC) up to 105 oC and above this temperature, it is approximately 40 x 10-5 cm/(cm.oC). For this reason, a ductile copper which will elongate plastically and accommodate this increase in thickness without fracturing is desired. An extensive analysis of cracking in plated through was published.

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