Printed Wiring Boards PWBs

By far the most important commercial application of electroless copper plating has been the PTH process for fabricating printed wiring boards (PWBs). Prior to the mid-1950s, mounting components onto PWBs relied on mechanical interlocking using inserted eyelets. The through-holes could not be electroplated directly because the base material of the circuit board (paper-phenolic, epoxy-glass, or other dielectric) did not provide the needed electrical conductivity. Development of electroless copper plating solutions (Ref 22), made practical by concurrent development of suitable selectively adsorbing catalytic materials (see the section "Processes" in this article), led to a dramatic improvement in the reliability of printed circuit boards and prompted the rapid growth of the PWB market during the 1960s through 1980s.

The vast majority of PWBs are made by a subtractive process using base material, typically epoxy-glass laminate, clad with electrolytic copper foil. The substrate is drilled, after which electroless copper is plated through the holes (and incidentally over the foil surface). Then electrolytic copper buildup and surface copper imaging (using dry film) take place; the order of these processes defines the fabrication process as either panel plating or pattern plating. In pattern plating, the plated areas are next overplated with a dissimilar metal (typically Sn/Pb), the dry film is removed, and all exposed copper is etched. In panel plating, no dissimilar metal plating is required; etching takes places immediately after imaging. The term subtractive processing arises from the fact that the circuitry is ultimately formed by removing base copper from the circuit board.

Although the basic overall process flow for fabricating a PWB has remained essentially constant for decades, the technology associated with every subprocess has undergone great improvement over that time period, and the electroless copper process is no exception. Improvements in electroless copper formulations, as discussed earlier, have greatly improved control of PWB processing and end product reliability. However, developments in PTH pretreatment have probably contributed even more greatly to end board reliability over that period. New hole preparation methods for multilayer boards provided such substantial benefits in terms of key criteria like copper coverage, adhesion (to laminate) and solderability (Ref 23) (see the section "Performance Criteria" in this article) that two-sided boards are now also frequently processed through these additional steps.

Careful integration of the steps immediately preceding catalyzation has also ensured that the optimal uniform, yet minimal, catalyst film is deposited in preparation for plating. This has provided the key to achieving interconnect bonds sufficiently strong (Ref 24) to withstand multiple thermal shocks, thus assuring very high end-product reliability (see the section "Performance Criteria" in this article). At present, the PWB market is under considerable environmental and cost pressure to replace electroless copper with a direct metallization process (see the section "Environmental and Safety Issues" in this article). Recent quality improvements, such as those described, present a formidable target for replacement technologies.

Additive processes for PWBs rely, as the name implies, on the buildup of the circuit copper over bare laminate. Variations on the fully additive process include semiadditive and partly additive processes. Description of these is documented in many published reports, which have been recently reviewed (Ref 25). Fully additive processes have been in use since 1964 and offer a number of advantages over conventional subtractive processing. Despite this, growth in the additive market has been slow; worldwide production in 1992 has been estimated at 2 to 5% of the world PWB production (Ref 26, 27). This slow growth is due partly to the more difficult adhesion and selectivity requirements and partly to the lack of adequate physical properties in the full-build electroless copper that is an integral component of the process. However, in recent years the quality of full-build electroless has become satisfactory, and this, coupled with improved process control and new automatic bath controllers, provides support for future growth of the additive PWB market (see the section "Equipment" in this article).

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