Electrolytic Plating: Filling Vias and Through-holes

Estimated reading time: 4 minutes

The electronics industry is further progressing in terms of smaller, faster, smarter, and more efficient electronic devices. This continuously evolving environment caused the development of various electrolytic copper processes for different applications over the past several decades.

This article describes the reasons for development and a roadmap of dimensions for copper-filled through-holes, microvias, and other copper-plated structures on PCBs. It will also discuss aspect ratios, dimensions, and results of plated through-holes used today in high-volume manufacturing for microvia and throughhole filling with electroplated copper. Furthermore, this article will also show feasibility studies of new electroplated structures for future applications, such as copper pillar plating on IC substrates.

Four Main Drivers

Four main drivers forced the chemical supply industry to introduce new electrolytic copper processes with the new feature of “filling” capability over the years. The first driver is the continuous miniaturization of electronics. The first blind microvias were introduced with HDI technology in the late 1980s and early 1990s. In 1996, the IC substrate market started to fill the microvias.

“Plugging” technologies were introduced to stack the microvias to save space or create via-in-pad structures. This plugging technology with conductive paste is very expensive because of the additional process steps required. In addition, this technology faced several disadvantages, such as “blow out,” outgassing, smear, and other quality concerns. To achieve the necessary miniaturization benefits, the industry has been leaning towards the completely copper filled blind microvia rather than a plugged microvia as the leading edge solution. Today, copper-filled microvias are the standard for almost all HDI PCB manufacturers.

The second driver is the thermal management on a substrate. One source said it this way:

“As the power and packing density of electronic components increase, the amount of waste heat generated in a small space also rises greatly. This results in dangerously high temperatures, and thus increases the failure risk of electronic devices. Today, 55% of electronic component failures are caused by increased temperatures alone.”

Solutions were needed to integrate features with high thermal conductivity to manage the heat transfer on the substrates from one side to the other to minimize hot spots on the electronic devices over a lifetime. Higher-performing chips tend to generate local hot spots, resulting in material degradation and premature field failure. Integration of thermal vias in high-performance electronics can minimize the occurrence of hot spots; therefore, their utilization in the industry has become more widespread.

In the beginning, thermal vias were nothing more than standard conformal vias, but the thermal conductivity was not good enough. Following that, plugging pastes were introduced to enhance the thermal conductivity of a standard through-hole. But in this case, similar disadvantages of plugging appeared. Meanwhile, complete copper-filled through-holes were realized in 2006 by bridge plating or X-plating technology.

Today, completely copper-filled through-hole structures are at the leading edge of technology for thermal via structures because copper has almost the best thermal conductivity—and it has to be plated, nonetheless.

The third driver is signal frequency. Electronic signal frequencies in an electronic package or inside of a PCB are increasing over time and continue to do so. Stacked microvias and fan-out vias are becoming more and more of a disadvantage for the transmission of high-frequency signals due to creating resistances at high frequencies. Thus, the push of high-frequency applications further increased the demand for technologies like copper-filled through holes. Right now, 5G infrastructure is already using the copper-filled through-hole technology in the field of smartphones.

Figure 1: Patented “SuperFilling” technology.

The fourth driver, especially for through-hole filling, is the quality-and-yield aspect. The alternatives for electroplated copper-filled through-holes require many additional process steps or new materials, such as plugging paste. Each of these additional process steps or materials introduces a variety of risks and manufacturing problems, resulting in a lower yield. Therefore, the “one-step” solution to fill through-holes with copper is the preferred solution without introducing new materials into the PCB.

Microvia Filling With Copper

The filling of microvias with copper was established as a standard in PCB HDI production more than 20 years ago. For example, there was the introduction of supervia filling technology with very low plated-copper thickness on the surface (Figure 1).

Figure 2: Plugged and capped microvia.

Meanwhile, the copper filling of microvias replaced many other filling technologies, such as plugging and capping realized by paste printing and overplating with copper (Figure 2). Both technologies—plugging and capping and copper- filled microvias—enable the so-called via-in-pad structure, which has the advantage for PCB designers. The advantages of the via-in-pad designs are also useful for high-speed designs.

Additionally, copper-filled microvias have significant advantages over plugging technology. For instance, the material inside the microvia is copper while other materials have the potential to outgas or introduce different CTE values. Moreover, voids in copper-filled microvias are far less common than with poorly controlled conventional plugging methods.

The development and introduction of copper-filled microvias opened the door to introduce any layer HDI technology, which enables copper-filled through-holes by stacking the copper-filled microvias. This kind of feature enables HDI board designers the flexibility to create complex signal paths through the PCB by just using copper filled microvias.

Today, almost all critical dimensions of microvias may be filled inclusion-free with copper.

To read the full article, which was published in the April 2019 issue of PCB007 Magazine, click here.