Milestones in horizontal plating: a brief look at the history of the technology reveals Asia as the willing innovator.

The origins of horizontal plating are "low profile," beginning in 1987 with the installation of the first horizontal plating line, for permanganate desmear, at a PCB maker in Germany. Horizontal plating has undergone its share of advances since. The basic concept, however, has never changed, and today there are more than 130 lines installed at PCB manufacturers around the world.

1991 saw the introduction of horizontal electroless copper systems. Based on the total installed capacity per country, Taiwan could be called the center of horizontal plating. Taiwanese PCB makers have installed about 50 lines, and horizontal plating is the technology of choice of almost all top Taiwanese PCB shops. Among the first customers was one leading Taiwanese PCB producer; since its first electroless copper installation, in 1991, it has added 10 lines, with 41 plating modules, at plants in Taiwan and China, making it the clear leader in horizontal plating capacity in the world.

The real breakthrough came with the introduction of copper platers, also in 1991. Compared to traditional vertical electroplating, horizontal plating combined permanganate desmear, electroless copper and electrolytic copper plating into a single integrated line, making a consistent metalization process available for the first time.

After just two installations in Europe, still in 1991, a leading Japanese IC substrate maker decided to install a complete line including the new copper platers. It opted for the new technology because at that time, Japanese companies had introduced capped vias to circuit design. This required plating of thin innerlayer sheets, a real issue for conventional vertical plating.

While the first generation of horizontal platers was equipped with soluble anodes, the second generation unveiled inert anodes and copper dissolution technology. The result: Clear quality improvements and a reduction in maintenance costs. In 1998, reverse pulse was added to the inert anode concept.

In 1994 and 1995, the two leading European suppliers of mobile phone boards started investing in horizontal technology. Today, all HDI PCBs for mobile phones produced in Europe are made on horizontal lines, and the same is true for any board made at their factories in China.

By 1991, Atotech's first system for horizontal conductive polymer direct metallization was installed. But the real breakthrough came in the mid '90s, when a huge number of European manufacturers decided to change from vertical electroless to horizontal direct plating. While leading German and British producers decided for palladium-based direct plating, manufacturers in France, Finland, Spain, Italy and Austria chose conductive polymer technology. Now, almost every larger Europcan manufacturer runs a horizontal line.

The latest innovation in direct plating is a new conductive polymer process. A horizontal line for this process was installed last year at the China factory of a leading Taiwanese producer.

In 1996, the first horizontal plating systems for tin and tin/lead plating were installed. There are 12 of these lines running today. Combined with desmear, PTH, copper platers and SES equipment, these platers can be used for tin tenting, an alternative production concept for HDI board manufacture.

Another breakthrough in the history of horizontal plating occurred in South Korea. In 1997, a leading captive shop there opted to "go horizontal," as it required a solution for BGA and module board production, including blind via capability (a new challenge which led to new solutions for fluid delivery). Since then, that company has installed four more lines, while two other large Korean companies have also invested in this technology. Today, three of the four leading Korean manufacturers run horizontal plating lines.

In 1998, a new product line arrived on the market, based on horizontal technologies like flood bar and AFD devices for fluid delivery, but targeted for other applications such as oxide replacement, surface preparation or immersion tin and silver finishes. The first system, a line for oxide replacement, was installed at PWC in Taiwan. Since then, a total of 70 lines have been installed around the world, with Taiwan having the largest installed base (10 million [m.sup.2] of annual capacity).

Whether for horizontal copper plating, surface finishing or surface preparation, new solutions are constantly required. Examples are the development of reliable transport systems for thin cores down to 50 pm and for single boards as small as 100 x 50 mm. Both developments were encouraged by the demands of Taiwanese HDI customers.

The advantages of horizontal plating for the demanding product group of organic IC substrates were realized first in Japan and shortly thereafter in South Korea and Taiwan.

RELATED ARTICLE: Horizontal plating considerations.

As PCB and chip carrier producers invest in horizontal plating systems, what considerations have driven their decisions? Here's an overview:

Handling. The modern plating line has to be capable of transporting very thin material. From desmear to solder-mask, horizontal lines can handle and transport materials down to 50 [micro]m thick.

Wetting. For plating HDI boards and backpanels, electrolytes must be able to penetrate high-aspect-ratio through-holes and laser-drilled blind microvias. Due to their promixity--only a few millimeters--we believe the flooding devices in a horizontal system can exert a mechanical impact unsurpassed by any vertical alignment. Special design considerations between the top and bottom flood bars, rollers and the panel facilitate multiple liquid passages through the vias by pressure and suction. The means of transportation ensures a symmetrical environment which enhances plating for all vias on a panel, from the front (leading) edge to the end. A balanced force in the x-axis, even at the panel edges, can be realized by adjusting nozzle sizes and flood bar widths.

Throwing power. "Throwing power" compares the thickness (or current density) of a metal layer plated on the panel surface to its thickness measured inside blind vias and through-holes. Whereas wettability relates to the initial, onetime penetration of vias, throwing power is associated with the multiple exchanges of electrolytes.

In addition to increasing the number of flooding elements, most important for enhancing the deposited metal layer thickness is the effective shape of the electrolyte stream when it hits the panel surface. Specially tailored nozzles of particular volume-pressure profiles are applied in active modules for multiple entries of liquids. With the proliferation of HDI, throwing power in electroless copper deposition became an imperative consideration. The reengineering of horizontal electroless copper modules resulted in a dramatic increase in thickness inside blind microvias (BMVs) and high-aspect-ratio holes. PCB and chip-carrier manufacturers profit from wider process windows, higher yields and shorter lines. Horizontal lines are capable of mass producing PTHs and BMVs with aspect ratios of 10:1 and 1:1, respectively, using 0.35 [micro]m electroless Cu.

Rinse efficiency. The ability to design flooding systems for effective exchange and replacement of electrolytes in very small vias is vital for an outstanding rinsing capability and high yield HDI manufacturing. The horizontal liquid delivery system, with its close vicinity to the panel surface and its tuneable pressure-volume characteristics, is optimal for rinse quality.

Floor space. Efficient work treatment results in short plating lines. Horizontal lines maximize space utilization and work-flow efficiency while minimizing floor weight.

--Dr. Jurgen Barthelmes, Atotech process team manager, and Bert Reents, manager, horizontal systems panel and pattern plating

REINHARD SCHNEIDER is vice president of electronics at Atotech (atotech.com). ANDY HUANG is managing director of Atotech Taiwan Ltd.