The last three decades have witnessed the development and maturity of many printed wiring board (PWB) fabrication processes. Despite new forms of multilayer, HDI, and SBU constructions, using new dielectric materials, the core electroless copper plated through-hole (PTH) technology has remained the predominant choice, especially for multilayer technology. But changes are inevitable. The escalating global environmental pressures are now challenging many fundamentals, and no region is exempt. Following its RoHs initiative, Europe is now confronting the REACH challenge. China’s global supply engine is facing severe water and effluent regulations, both threatening the PWB supply chain. In simple terms, the inability of fab shops to meet the rapidly tightening regulations will limit the expansion of plating facilities, the permitting of new facilities, and will challenge the broader supply capacity. Despite this mounting pressure, the performance and reliability of the copper metallization system is even more critical, especially for the newer electronic devices sweeping the industry. So is there a conflict in these demands? Are there alternative green choices that will not only meet the OEM and EMS supply needs, but which will provide better performance and enable higher capability for the industry? The article, which was also given as a presentation at the 2008 HKPCA in December, looks at one alternative PTH approach, using a conductive polymer system, which has the proven ability to deliver these goals and economically justify the necessary cost of qualification.

Scarcity of resources challenges growth

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The increasing capability, complexity, and applications of electronic devices all continue to drive the global computer, communications, and consumer markets. As a result, the global PWB industry has been undergoing extremely strong growth for the past six years. A large part of this global expansion has been centered in Asia, especially in China, where the industry has grown in high double-digit terms over the past few years. Having cooled from the annual expansion rates of 20-25 percent per seen in the mid 2000s, the China PWB output still continues to grow at around 12 percent CAAGR in “board area” terms (i.e. the total area of boards produced measured in square meters). This is seen in Table 1 and Chart 2. These manufactured board areas directly drive the amount of materials or utilities used in the fabrication process, including but not limited to: the laminate (dielectric) materials; the specialty and commodity chemistry; the water usage; the power usage; and waste generation. In such periods of sustained high growth, the increased consumption of all these materials has a significant impact not only on the environment but also on the ability of the material and utility providers to keep pace. The concern is clearly to avoid constraints that may threaten the electronics supply chain itself and any subsequent price escalation and supply limitation.

Enlarge this picture Chart 2 Recent Years’ Sales of Printed Circuit Fabrication by Producing World Region (USD—Based on 2006 Exchange Rates)

Many of the influencing conditions are now more threatening than they have ever been in recent years and could, if viewed very pessimistically, be gathering for almost a perfect storm. An excellent example of adverse material constraints has been seen with the current global supply of crude oil where prices hovered in the USD20-30/barrel range over the period 1986 – 2004. Following the acceleration of global growth and demand, heavily influenced by China, oil prices peaked at some USD147/barrel in July 2008, precipitating some very major changes seen, especially in the USA. Central to these changes has been the acute fall-off in demand for the favored, but heavy gas consuming, SUVs and trucks. This has now been followed by a scramble to re-tool manufacturing plants, led by the somewhat disadvantaged U.S. manufacturers,* in order to produce more fuel-efficient vehicles. (*Disadvantaged because of their past manufacturing strength and focus on producing larger, higher gas consuming utility vehicles). In concert with this, there has been an explosion of activity to produce cars with alternative energy sources, and for energy companies to tap alternative sources of energy. In short, there has been a paradigm shift in action to conserve resources. But, as always, success will be delivered to the most reliable, best performing, and most economical solutions, whether it is achieved by the energy consumers or by the energy providers.

Critical PWB resources

Enlarge this picture Diagram 3 The Advantage of Selectivity

So, back to the PWB business. The increased global demand and relative scarcity of some resources has also driven recent price escalations of materials used in the industry. These include: copper, tin, nickel, and palladium, all of which have seen similar peaks to oil. Water and power utilities also share this stage where China, as the global PWB fabrication engine, has growing issues and challenges on water supply and waste handling. The ongoing and frequent power outages experienced by S. China manufacturing facilities have been widely observed. This is one issue that drives the demand for the maximum efficiency in manufacturing output per operating hour, thus favoring the small-footprint, high-output processes within the printed circuit producers. Perhaps the most concerning issue is that of water availability. PWB fabrication shops use large volumes of water to feed the many wet chemical processes employed. Back in the early 2000s, the mounting issue of water availability influenced the decisions of some Taiwan fabricators to re-locate manufacturing plants in China. However, China has its own mounting pressures to address water conservation.

Enlarge this picture Chart 4 Basic Process Comparison Electroless Copper and Conductive Polymer

The country’s annual per capita water supply is only 2,200 cubic meters, 25 percent of the global average, according to the World Bank. The government says that by 2030, the water supply is expected to fall below 1,700 cubic meters per person, which the World Bank calls dangerously low. During the same period, water demand is expected to more than triple, from 120 billion tons a year to 400 billion tons. Using uncharacteristically strong language, the World Bank warned that the situation “will soon become unmanageable, with catastrophic consequences for future generations.” As one example of a call to action, provincial water resource authorities in S. China announced, in November 2007, that water drawn from the Dongjiang river would be capped at 10.66 billion cubic meters each year for the cities of Shenzhen, Heyuan, Huizhou, Dongguan, Guangzhou, and Hong Kong to protect the river, one of the three major water systems in the Pearl River Delta region. The Dongjiang River can supply a total of 32 billion cubic meters of water a year.

Increasing regulation of harmful materials

However significant, these supply restrictions show only one aspect of the problem, and a whole other dimension of constraint stems from the global environmental initiatives to eliminate toxic and harmful materials. A significant number of such materials are used in PWB fabrication and assembly. At the forefront of the drivers for change are the RoHs and REACH initiatives, firmly established by the EEC, but driving global standards. The electronics industry is still wrestling with (if not reeling from) the inordinate number of material and process changes driven by the elimination of lead from solder. So the impact of current and future directives is both daunting and challenging, not only to the material suppliers, but also to the fabricators who are also being asked for increasing process performance and greater reliability.

The OEMs have an increasingly vested interest in the resolution of these issues within the supply chain. Not only do they need products that can be safely disposed at the end-of-life, but under mounting pressure from Greenpeace and other environmental watchdogs, they also need to embrace and support increasingly green manufacture. Satisfying both of these initiatives is essential; bearing in mind the relatively short product life cycles endured by many of today’s consumer electronics gadgets. However, there is a fundamental need to provide even greater performance and reliability, despite the change of technologies necessary to meet the increasing miniaturization of the designs. So the escalating environmental issues have broader technological ramifications. The bottom line is that the new processes evolving at the fabrication stages must not only be green, but also must ultimately perform as well or better than the current standards. It is encouraging, and is becoming noticeable, that many OEMs are more ready to embrace the necessary cost of change and are more amenable to validate newer, environmentally friendly technologies that meet the criteria.

Considerations of the core PWB metallization process

At this point, we must now consider the metallization technology used to produce the core construction of the PWB. This comprises: the desmear process; the PTH process (typically electroless copper or direct metallization) to make the holes conductive; and finally, the electroplating processes to build the copper thicknesses of the circuit traces, the through-holes, and the blind microvias (and, in some case cases, to fill them).

Challenges for electroless copper

Enlarge this picture Chart 5 Typical Discharge Levels From Electroless Copper and Conductive Polymer

The electroless copper PTH process has been the industry standard for the last thirty years. Although not the easiest process to operate, its management, performance, and reliability is well understood and documented by all parties. Variants of the process have evolved over the years, from low-build (0.5µ deposition) to high-build thickness (2.5µ) coppers. Traditionally operated in vertical dip equipment, there has been a trend towards horizontal equipment over recent years, driven by the need for improved microvia plating and generally better environmental containment and automation. The electroless plating of microvias has become very significant as much more effective chemical solution transfer into the confined spaces is critical for plating the minimum copper thickness. This necessitates very high multiples of solution replenishment within the microvias immersed in the copper bath. As microvia sizes reduce down to 50 micron diameters, about 200 - 250 solution exchanges are necessary to achieve full deposition. This calls for critical equipment design to deliver the required fluid dynamics. The electroless process is characterized by six main process chemical steps, each followed by a rinse, which means that the system uses relatively high volumes of water. From an environmental aspect, this is not the only issue as most electroless copper baths are based on a formaldehyde reducing system, incorporating a high degree of chelation using EDTA or other complexants. The copper micro-etching step also significantly contributes to the total copper entering the discharge stream from the process, all of which increasingly taxes the waste treatment. The extremely harmful (carcinogenic) nature of the formaldehyde also means increasingly stringent health and safety management and control, to a point where consideration is being made by the EEC to ban its use.

Direct metallization alternatives

There are several alternatives to electroless copper for making the holes conductive in the first stage of the PTH process. These are broadly covered by a group of direct metallization (DM) processes, which use alternative chemical approaches to facilitate direct electrolytic plating. Included here are a variety of different systems based on palladium, carbon, graphite, and conductive polymer technology. The hallmark of these processes is that they are essentially shorter and/or potentially faster systems, which do not involve the chemical deposition of chemical copper. These DM processes are applied prior to the second PTH step (that of electrolytic copper deposition) to build the working thickness of copper that becomes the central interconnection network for the PWB itself. Within this group, the conductive polymer process is extremely novel, as this system is the only one that forms its unique conductive film entirely selectively on the non conductive material areas. These areas (formed in the drilled holes and microvias), between the pre-existing copper layers within the PWB blank, are the ones that need be metallized and connected to the copper cores. While offering some environmental benefits, the other DM processes all require a post treatment micro-etch of the copper to remove the superfluous conductive material that coats all of the pre-existing surface copper as well as the dielectric. This micro-etch is applied for two main reasons: 1) to prevent a copper to copper:copper adhesion loss to the inner layer junctions (in the PTH holes) and also to the outer surface copper, during final electroplating; and 2) to clear out any material obstructing the capture pads in the blind microvias.

This micro-etch results in a small loss of copper from the existing layers, in and around each hole, potentially leaving an annular non conductive ring that could become an interconnection defect (ICD), especially in very small holes. This retroactive micro-etch problem is completely eliminated with the conductive polymer system. The advantage of this selectivity is shown in Diagram 3.

Looking more specifically at the comparison between the electroless copper process and the conductive polymer process, the latter has several other immediate benefits. Firstly, from the keynote environmental perspective, the polymer process only requires three rinses relative to the five or six needed for electroless copper. The process basics are compared in Chart 4. So right off the bat there is 40-50 percent water saving assuming similar levels of rinsing efficiency, and a similar reduction in waste discharge volumes from the line.

As the three steps in the conductive polymer process are all very short, the total process time in horizontal mode is less than six minutes, compared to about 13-15 minutes for electroless copper. This means well over double the productivity in the same footprint area, or a cost saving of several hundred thousand U.S. dollars on a larger 2-3 m/minute horizontal installation. The benefits don’t stop here and, from a technological perspective, they get better as the complexity of the boards increases.

The conductive polymer only requires about 100-150 nanometers of thickness to reach its full conductivity, and this is achieved by only one or two solution exchanges in each hole. Having the optimum equipment design still remains very important, but as the hole aspect ratios increase, with higher layer counts and smaller microvias, the ability to render them completely and reliably conductive remains the same with minimal application of chemistry.

Water usage and waste generation comparisons

As previously indicated, in comparing the two systems, conductive polymer and electroless copper, there are substantial differences in the amount of water consumed and chemical waste generated per square meter of board processed. Chart 5 shows the amount of waste material generated from a horizontal production line processing 400,000 square meters of board per year. The differences are striking, especially considering the large amounts of metal, chelator, and formaldehyde produced only from the electroless process. Not only does the conductive polymer process save the significant cost attached to dealing with this additional chemical waste from the electroless process, but there is also a large additional saving in water consumption. This cost saving can be measured not only in terms of the water used and waste treatment, but also in the productivity increase in the production unit and its ability to grow.

Enlarge this picture Chart 6 Water Usage and Waste Treatment Cost Comparison

A plant processing multilayers with an average layer count of eight (layers) can effectively increase its total production capacity (all processes) by 9 percent just by switching from electroless copper to conductive polymer, without using any additional water. Given the higher added value of multilayer boards, this can provide a major revenue increase of many millions of dollars. Better still, a plant producing only double-sided PWBs could capture a significant revenue increase of approx 21 percent, if limited by water usage. The potential water savings are dramatic, and of the order of 19,000,000 liters per year for each million of board square feet processed.

Conductive polymer performance and reliability

Chart 6 presents a direct comparison case history of water usage at a company using vertical electroless copper and then switching to conductive polymer. Based on production volumes of five million board sq. ft. (465,000 sq m) per year, the projected water reduction is some 95,000,000 liters per year. Taking into consideration the significant reductions in sludge, waste treatment, and discharges from the more environmentally acceptable chemistry, the total savings were calculated to be USD407,000, or approximately USD0.80 per sq. m. (USD0.08 per sq ft) of circuit board processed.

All round, the environmental and financial benefits of switching are very compelling and are beginning to be exploited by the circuit board industry. Europe has led the way with substantial changes away from electroless copper toward direct metallization systems, certainly supported by the increasingly stringent environmental regulations over recent years. Of the DM systems employed in Europe, conductive polymer processes account for well over 45 percent of the volume. From this progression, a solid track record of reliability and performance has been established to meet the industry demands from the fabricators, EMS providers, and the OEMs. As indicated earlier, electroless copper has been at the core of PWB manufacturing from many years, and has been the automatic conservative choice for many fabricators. This has been mainly due to the known reliability, where the industry has evolved an approach of using low-build copper often followed by flash electroplate for more critical higher technology/higher reliability applications. However, this approach also favors the conductive polymer because flash coppers give very rapid and effective electrolytic coverage of the treated holes in either horizontal or vertical application. The a conductive polymer has the lowest conductivity compared to other direct metallization systems. As a result, the acid copper coverage from the flash process is extremely fast, as seen from standard backlight tests, resulting in very high metallization integrity, see Picture 7.

Enlarge this picture Picture 7

The absence of any post treatment etch ensures that the inner layer and outer layer junctions are not eroded, thus preventing any voiding in high aspect ratio through-holes and blind microvias. However, a conductive polymer system is used frequently for direct imaging applications with extremely consistent and reliable results. The conductive polymer, which is dependent on the generation of the manganese dioxide (MnO2), forms equally well** on all non conducting dielectrics and provides excellent hole wall adhesion and thermal reliability. (**The MnO2 formation, which is generated by a neutral permanganate initiator, only occurs on the dielectric and glass materials.) The process meets all standard thermal cycling and solder shock testing requirements, even on the most advanced constructions comprising very high layer counts. Many of the other process attributes have already been described including the blind via hole formation benefits. One recent area of investigation has focused on the impact of the conductive polymer process, compared the electroless copper approach to hole metallization, on Conductive Anodic Filament growth (CAF). Unlike electroless copper, the conductive polymer system does not use or apply any copper ions to the hole wall. The danger here is that metallic ions can be entrapped or potentially absorbed in cavities or within the glass bundles and provide the base fuel for the development of Conductive Anode Filament growth, and potential shorting. The conductive polymer process lays down only an organic conductive polymer layer on the hole-wall, completely replacing the deposited MnO2 in the final process step. This shields the hole wall from the acid copper electroplating solution and initial testing has shown that this can result in 20-30 percent slower CAF growth with some dielectrics relative to electroless copper. Hence there is better reliability potential in this approach and further work is continuing to better quantify this benefit.

Conclusions – Is there a pathway forward?

The conductive polymer system is not a new process, and has been increasingly applied by the industry over the past 12-15 years. During this time a great deal of technical progress has been achieved with process refinements, leading up to the simple three-step system seen today. Arguably, the conductive polymer system was perhaps ahead of its time, but with the hindsight of its demonstrated performance in the growing number of global volume and high technology installations, it is a proven and viable option. The winds of change are now blowing and, for the fabricators, the tightening waste and hazardous material regulations, the escalating water shortages, and discharge regulations are now really impacting their business. The OEMs are clearly looking to embrace a number of key environmental initiatives that can not only enhance their business model, but can also help prevent future growth restrictions on bare boards. Performance and reliability of course are key needs, as is the enablement of new HDI designs and constructions. The conductive polymer metallization has both the capability and track record to address all these combined requirements, and bring major cost of ownership savings to the market.