The full stack for electronics relies on PCBs and packaging, which in turn relies on metal deposition processes to form copper interconnects. A major point of defects and field failures in high-density PCBs and packaging is in the vias that form vertical interconnects, which has led to industry associations issuing reliability warnings for certain microvia structures. Since the original issuance of those warnings, the industry has worked hard to determine geometry and processing limits, as well as pioneer new processing capabilities for via formation.
As feature sizes in substrates and PCBs scale down, innovative manufacturers have relied on a better primary metallization process for via formation, known as direct metallization. The process can be used as a replacement to another deposition process known as electroless copper, giving higher reliability fabrication in certain structures. However, it’s fair to ask whether retooling of production lines or if new capital equipment investments should be allocated to this process.
Primary metallization processes
First, let’s compare the two main primary metallization processes: electroless copper and direct metallization processes. Electroless copper is a proven process that has been in use for decades; it involves deposition of a thin copper layer over a palladium seed layer, which is followed by standard electroplating to form the finished via structures. The fabricated film includes a copper-to-copper interface between the electroless copper and the electroplated copper.
Direct metallization is a new process that eliminates the palladium seed layer and the electroless copper layer entirely. The process proceeds by deposition of a carbon conductive seed layer followed by deposition of copper in a standard electroplating process directly (without any electroless layer). This eliminates the copper-to-copper interface in the electroless process, and the final electroplated copper layer will be homogeneous with higher mechanical strength.
By eliminating the electroless/electroplated copper interface in electroless copper metallization, the electroplated copper formed in direct metallization has much higher strength and adhesion to copper landing pads.
The table below summarizes the differences between direct metallization and electroless copper in terms of reliability/yield, environmental factors, and process by-products.
The major benefit for IC substrate fabrication at scale is the higher yield and reliability, followed by the environmental benefits relating to water usage. These are critical aspects that drive cost competitiveness, especially as the U.S. and Europe look seriously at onshoring packaging manufacturing capacity.
When to use direct metallization
Larger diameter via holes can be formed by either primary metallization process; the reliability concerns with electroless copper start to become important when forming laser-drilled microvias, and especially when forming stacked blind/buried microvias. Certain types of HDI PCB designs can greatly benefit from the use of direct metallization as the primary metallization process largely thanks to yield and cost concerns. The greater reliability of the direct metallization layer addresses the primary concerns of earlier IPC reliability warnings relating to stacked blind/buried via structures.
The other aspect to consider here is the environmental aspect, which may motivate investment in direct metallization processing for a new fabrication line, depending on production location. For example, as American and European industrial policies shift focus from semiconductor production to packaging, direct metallization will be the preferred process given the environmental regulations in these regions and the requirement for high yields to remain competitive. In addition, existing electroless fabrication lines can be quickly retooled to support a direct metallization process, which is expected to accelerate the adoption of direct metallization for advanced packaging and UHDI.
Should buyers or designers specify the use of direct metallization as a fabrication requirement? If any of the following points apply, then direct metallization could be considered a requirement:
- Small diameter laser-drilled microvias or mixed technology (with through-holes)
- Localized production (U.S. and Europe)
- Assemblies with stacked microvias and multiple reflow cycles
- Companies with environmental policies or sustainability goals
Access to direct metallization capacity
Direct metallization is clearly a superior process for forming more advanced substrates and high-density PCBs, both in terms of the reliability of fabricated microvias and the environmental benefits. However, not all fabrication houses will offer or use direct metallization in their processing, and buyers may not have any visibility into whether a particular manufacturer is using the process. Buyers can request processing information from their vendors, but the usage of the process is one of many factors to consider when determining where to locate production.
Today, buyers have access to fabrication capacity that uses direct metallization in Asia and in the U.S./Europe, although most of the capacity is located overseas. This uneven distribution of capacity is one of the significantly misunderstood aspects of the semiconductor crisis and has led to a push in the U.S. to localize production of substrates and UHDI PCBs. In a slowly deglobalizing world, companies can expect more access to this capacity closer to home, which will aid sustainable supply chain development initiatives.
