Estimated reading time: 6 minutes

Trouble in Your Tank: Implementing Direct Metallization in Advanced Substrate Packaging

Direct metallization systems based on conductive graphite are gaining popularity throughout the world. The environmental and productivity gains achievable with this process are outstanding. Direct metallization reduces the costs of compliance, waste treatment, and legal issues related to chemical exposure. A graphite-based direct plate system has been devised to address these needs.

Why Use Graphite-based Direct Metallization?
Both carbon black and graphite-based chemistries are considered carbon-based, but there are significant differences between these two processes. 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 as compared to carbon black. A graphite-based system is quite versatile in depositing graphite on non-conductive materials, which is particularly important given that material suppliers push the envelope to produce higher-performance resins and laminate composites.

With each incremental enhancement in materials properties, such as CTE (coefficient of thermal expansion), Td (temperature of decomposition), signal integrity, and Tg (glass transition temperature), these base materials become more difficult to process. These higher-performance materials are highly cross-linked and more chemically resistant to processes, such as alkaline permanganate desmear. Conventional electroless copper, by contrast, requires a micro-roughened resin surface to effect sufficient adhesion of the copper to the resin.

On the other hand, carbon-based systems are like a coating technology. Surface topography is not an issue in adhering to the resin materials. With electroless copper, a precious metal catalyst (typically palladium) is necessary to oxidize the formaldehyde (the reducing agent most commonly used in electroless copper formulations). Electroless copper comprises 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 evolves. The production of hydrogen gas produces bubbles, which can lodge in small-diameter through-holes and blind vias. If the bubbles are not efficiently evacuated from the vias, plating voids will result.
This is a depiction of the overall electroless copper reaction:

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

Environmental Advantages of Direct Metallization
Usually, the manufacturing cycle time to metallize a PCB through a conventional electroless copper process is 45–55 minutes. CapEx requirements aside, direct metallization offers faster throughput, which reduces energy costs and greenhouse gas emissions. This is particularly important in light of ongoing sustainability concerns in the industry.

Water is an increasingly scarce commodity. The average electroless copper line uses 26–30 gallons of water per minute, versus four to six gallons per minute for a graphite-based process. Switching can generate an over 80% reduction in water consumption. In addition, the graphite process does not contain chelated copper, which is used in conventional electroless copper. This greatly simplifies waste generation concerns. Finally, the graphite process is free from formaldehyde, cyanide, and other toxic substances, satisfying the demand for safer, carcinogen-free chemicals.

Graphite-based Direct Metallization Is Ideally Suited to HDI and Flex Circuitry
With more emphasis on HDI and ultra HDI, ease of use and speed are critical. Advanced packaging is driving higher densities for both 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 graphite-based system from Technic is perfect for these tasks. The level of complexity is depicted in Figure 1.

In addition, the industry also practices the ELIC process (every layer interconnect). (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 is to select materials with low CTE and higher temperatures of decomposition. The graphite direct metallization process enables faster productivity through primary metallization in contrast with conventional electroless copper. 

The Science Behind Graphite Direct Plate Systems
The key element of the process is the development of a unique dispersion of highly crystalline graphite particles in a slightly alkaline medium. The specific particle size of the graphite is specifically formulated in a very narrow distribution range with a diameter of 0.4-0.6 microns.

Proprietary dispersion agents and organic binders enhance the stability of the graphite colloid. This, in turn, improves the adhesion and coverage of the graphite to a variety of materials, including PPO, PPE, liquid crystal polymer (LCP), flex, polyimide, PTFE, ceramic-filled materials, via fill pastes, etc. When discussing the consistency of colloidal graphite processes, the stability of the colloid is a major consideration. We define stability as:

• Maintaining a consistent particle size without aggregation
• Particles remain suspended in solution without settling

Each definition of stability involves different factors, which I will discuss separately. Our proprietary technology allows for a consistent graphite particle size and enhances both the stability of the dispersion and the excellent conductivity of the coating.

Consistent Particle Size
Colloidal graphite is a hydrophobic colloid, meaning it is not strongly attracted to water. Hydrophobic colloidal particles repel each other either sterically or electrostatically. Steric repulsion involves forming physical barriers to prevent the particles from aggregating. Electrostatic repulsion involves an electrostatic charge preventing the particles from coming into contact with each other and agglomerating. While both methods involve keeping colloidal graphite particles from aggregating, electrostatic repulsion is of most interest because of the influence of contaminants on the electrostatic charges.

As previously stated, the graphite particle has a net negative charge. This negative charge is further increased by the addition of a proprietary binder, which at the operating pH also has a negative charge because of the presence of carboxylate groups (R-COO-). Surrounding this particle is a cluster of counterions having a net positive charge. This unequal distribution of electrical charges sets up a potential across the interface and forms an electrical double layer. A schematic representation of the electrical double layer is shown in Figure 3.

A thicker electrical double layer causes the particles to be repelled at a greater distance. This thickness is inversely proportional to the concentration and valence of each ionic contaminant. Therefore, ionic concentration should be reduced as much as possible, especially when the ion is divalent or trivalent. Zeta potential is a practical measure to determine the effective charge on a colloidal particle. The more negative the zeta potential, the more stable the colloidal graphite dispersion. Zeta potential is also useful in evaluating the effect of contaminants on colloid stability. Our binder formulation helps minimize the effect of contaminants that can negatively influence the stability of the graphite dispersion and further negate the conductivity of the coating once the coating is on the hole walls.

Importance of the Cleaner/Conditioner
The cleaner/conditioner process step precedes the graphite step to ensure the negatively charged graphite particles find a “home” on the various materials, including glass fibers. Again, the science behind this is to match the positive charge of the cleaner/conditioner to the negative charge on the graphite. This allows the graphite to “floc” to the vias and form a thin layer of conductive graphite on the glass and resin. This is actually a polymerization action forming a thin, conductive film on the non-conductive areas of the circuit board or substrate.

In conclusion, graphite-based direct metallization offers several technical benefits compared to conventional electroless copper processes, eliminating potential issues:

• It improves the reliability essential for critical applications
•  It streamlines the process steps, leading to an overall reduction in costs
• It offers environmental benefits, being free from toxic substances such as formaldehyde
• It reduces water and energy usage with much shorter processing times