Saving Water Plating PCBs

Making Through-Holes Conductive

Direct metallization has been gaining attention in the printed wiring board (PWB) industry as an environmentally preferable alternative to electroless copper. The EPA has developed a guide that provides first-hand accounts of the problems, solutions, time and effort involved in implementing alternative technologies. The guide discusses carbon, graphite, palladium and conductive polymer technologies in detail. Highlights of each technology will be presented in this sidebar.

Carbon Method

In a carbon process, a conductive layer of carbon black particles is deposited onto the substrate surface and the through-holes. A typical carbon process has six chemical process steps: cleaner, carbon black, conditioner, carbon black, microetch and anti-tarnish.

For this guide, two facilities were interviewed about their experiences with the carbon method. Both process primarily multilayer boards, and both process boards with up to 16 layers and with aspect ratios of 8:1. The facilities switched to the carbon method to reduce cycle time, reduce waste treatment, decrease maintenance requirements and widen the process window.

The first facility, A, experienced few problems. Facility B, however, experienced several equipment-related problems, including difficulties with the pumps, cooling coils, chiller and air knives. Facility B had no problems with the chemistry.

Both facilities have noted benefits over electroless copper. For example, the electroless copper line at Facility B took 1.5 hours to get the first product through after running a load of dummies. After startup, the line could process approximately 60 panels per hour. The carbon system required only 6 minutes to get the first panel through, dummies were not required and product can be processed at 75 panels per hour.

Graphite Methods

Graphite methods disperse graphite (another form of carbon) onto the substrate. Similar to the carbon method, the conditioner solution creates a positive charge on the substrate surface, including the through-holes. Graphite particles are adsorbed onto the exposed surfaces. In contrast to the amorphous structure of the carbon black crystallites, graphite is a three-dimensional polymer that creates a conductive layer that covers both the copper and nonconductive surfaces of the outside layer and interconnects. A typical process has three or four steps: cleaner/conditioner, graphite, fixer (optional) and microetch.

Two PWB facilities were interviewed for this report. The facilities installed the system to eliminate formaldehyde, lower operating costs, improve worker safety, consume less water and reduce cycle time.

Again, equipment problems outweighed chemistry problems. Moreover, one facility said that it now spends more time on equipment maintenance with the graphite process. However, both companies report reduced cycle time and reduced water use.

Palladium Methods

Palladium systems use palladium particles to catalyze nonconductive surfaces of the through-holes. Palladium particles tend to cluster unless they are stabilized through the formation of a colloid that surrounds the individual palladium particles with a protective layer. The two main stabilizers are organic polymer and tin. A typical process has six chemical steps: cleaner/conditioner, microetch, predip, catalyst/conductor, accelerator/postdip and acid dip.

Because five companies were interviewed for the study, this sidebar will not go into any extensive detail. The reasons the facilities switched included elimination of formaldehyde, ease of conversion, reduced labor and material costs, decreased water use, ability to run a variety of substrates, quicker throughput, worker safety and ease of waste treatment.

The companies did report an increase in production throughput, some even tripling productivity. Time required for lab analysis also decreased at all the facilities.

Conductive Polymer Methods

This process deposits a conductive polymer layer on the substrate of the hole. A cleaner/conditioner step coats the glass and epoxy surfaces in the through-holes with a water-soluble organic film. A permanganate catalyst solution deposits manganese dioxide on the organic film. This only occurs on the film-coated glass and epoxy surfaces. Polymerization occurs when a conductive polymer solution containing the pyrrole monomer is applied to the surfaced coating with the manganese dioxide. The polymerization continues until all of the manganese oxidant is consumed, resulting in a layer of conductive polymer that coats the through-holes, which are then flash plated with copper. The process has six steps: microetch, cleaner/conditioner, catalyst, conductive polymer, microetch and copper flash.

This process was primarily introduced in Europe, and only one unit is operating in the U.S. It is available only as a conveyorized plating unit. The volatility of the conductive polymer precludes it use in an open system, because it would deposit a black coating on the surrounding area. Facilities using this process typically produce high volumes of FR-4 boards with 4, 6 or 8 layers for the consumer electronic and communications industries.

Lessons Learned

No matter what the technology, some common suggestions emerged from the experience for successfully implementing an alternative technology. High quality equipment for conveyorized systems is extremely important. Since there can be major differences between direct metallization and electroless copper processes, line operators need to be willing to accept changes and retraining. Facilities should take a "whole-process" view of the technology installation. Process changes upstream and/or down may be necessary to optimize the alternative process. Nevertheless, the most important factor may be a strong commitment from management and line operators to the new technology.