Estimated reading time: 3 minutes

Trouble in Your Tank: Processes to Support IC Substrates and Advanced Packaging, Part 5

Direct metallization systems based on conductive graphite or carbon dispersion are quickly gaining acceptance throughout the world. Indeed, the environmental and productivity gains one can achieve with this process are outstanding. In today’s highly competitive and litigious environment, direct metallization reduces costs associated with compliance, waste treatment, and legal issues related to chemical exposure. What makes these processes leaders in the direct metallization space? This is detailed below.

The Carbon-based Systems (Graphite and Carbon Black)

While both are carbon-based materials, graphite and carbon black have a few differences. The graphite process is based on a very fine and stable aqueous dispersion of synthetic crystalline graphite. The graphite particle, by virtue of its crystalline structure, is highly conductive. Carbon black is an amorphous material with the ability to conduct current. Both materials are well-represented in the global market for printed circuit board fabrication. So, what makes these two carbon-based processes ideal for thin material metallization and plated through-holes?

Both processes are quite versatile in their ability to deposit the carbon or graphite on non-conductive materials. These carbon-based systems can be likened to a coating technology; surface topography is not an issue to adhere to the resin materials.

This fact is especially important today as the industry’s material suppliers push the envelope to produce higher-performance resins and laminate composites. With each incremental enhancement in materials properties, such as coefficient of thermal expansion (CTE), temperature of decomposition (Td), signal integrity, and glass transition temperature (Tg), these materials become more difficult to process. These higher-performance materials are highly cross-linked and are more chemically resistant to processes such as alkaline permanganate desmear.  

In contrast to the carbon-based systems, conventional electroless copper requires a micro-roughened resin surface to effect sufficient adhesion of the copper to the resin. A precious metal catalyst (most commonly palladium) is required to bring about the oxidation of formaldehyde (the reducing agent most commonly used in electroless copper formulations). Essentially, electroless copper is composed of two half-cell reactions, with several process steps required to provide a void-free copper deposit. In addition, during the copper plating process, hydrogen gas has evolved. The production of hydrogen gas produces bubbles that can lodge in small diameter through-holes and blind vias. If the hydrogen gas bubbles are not efficiently evacuated from the vias, plating voids will result. The overall electroless copper reaction is shown below:

Overall Reaction: Cu(EDTA)2- + 2HCHO + 4OH- → Cu + H2 + H2O + 2CHOO- + EDTA4+

Second, the manufacturing cycle time to metallize a printed circuit board through a conventional electroless copper process is 45–55 minutes. CapEx requirements aside, direct metallization offers faster throughput and, in turn, reduces energy costs as well as greenhouse gas emissions. Certainly, sustainability should be on everyone’s list going forward. If one simply calculates the amount of energy required to heat process tanks and the time it takes to process a circuit board through any one process, it can be shown that processes that reduce production time and use less energy will reduce the carbon footprint and thus greenhouse emissions. More on this in a future column.

Ideal Applications for Direct Metallization

With more emphasis on HDI and ultra HDI, ease of use and speed are critical operational must-haves. Advanced packaging is driving higher densities for IC substrates, interposers, and product boards. This necessitates the increased complexity of these boards and substrates with ever finer lines and spaces, multiple sequential laminations, and smaller diameter blind vias. The carbon and graphite-based direct metallization systems are ideally suited for these challenges.

The level of complexity is depicted in Figure 1. In addition, the every layer interconnect (ELIC) process is also practiced in the industry (Figure 2).

With proper material selection, the constructions shown in Figures 1 and 2 will improve long-term reliability and withstand the multiple laminations required. The key here is to select materials with low CTE and higher temperatures of decomposition.

The direct metallization process described in this column will enable faster productivity through primary metallization in contrast with conventional electroless copper.

This column originally appeared in the July 2024 issue of PCB007 Magazine.