Day 1: Cutting Edge Insights at the EIPC Summer Conference

Estimated reading time: 15 minutes

 Crystallization and Epitaxial Growth in Blind Microvias

Roger Massey, technical marketing manager at MKS Atotech, gave us a most interesting lesson in copper metallurgy and crystallography in his update on factors affecting crystallization and epitaxial growth in blind microvias. “What makes a microvia reliable?” he asked. “There is no silver bullet.”

He discussed a list of contributing factors, beginning with electroplated copper and examining its microstructure, epitaxy, nanovoid and inclusion density, and leveler/brightener co-deposition. Similarly, for the electroless copper layer, he looked at microstructure, epitaxy, nanovoid and inclusion density, and surface facets.

For the blind microvia wedge, he discussed aspect ratio, cleanliness, and electrical contact to the target pad, as well as the target pad microstructure, before and after laser drilling, its flatness and roughness, and the microvia shape, hole-wall, and general drilling quality.

He compared the characteristics of single-crystal and broken-crystal structures in electroplated copper via fill, asking, “How do microstructures form?” Immediately after plating, neither electroless nor electroplated copper has any significant microstructure; the microstructure develops over time due to crystallization.

Freezing samples in liquid Nitrogen directly after electrolytic plating makes it possible to study the crystallization process. After plating, the larger copper grains of the substrate grew and recrystallised the electroless and electrolytic layers by grain boundary diffusion and grain growth. Effectively, this was a “bottom-up” recrystallization. Massey said that interfaces need to be free of contaminants such as oxides, desmear residues, photo resist, and plating additives.

The characteristics of bottom-up recrystallization are that there are no visible interfaces or layers and no grain boundary alignment at the copper-copper-copper junction, resulting in high joint integrity. In comparison, top-down recrystallisation after electroplating and annealing for one hour at 140°C shows clear, visible interfaces and layers, grain boundary alignment at the copper-copper-copper junction, and low joint integrity.

Massey also discussed the impact that some additive packages in electroless copper baths can have on the copper layer and how it crystallizes. Regarding the crystallization of electrolytic copper, Massey noted that “annealing” included recrystallization and has a significant impact on its final properties. Annealing occurs at room temperature and is accelerated by heating.

Bottom-up recrystallization, or epitaxy, which occurs when there are clean interfaces between electroless and electrolytic copper deposits, is typically limited to the first few microns of plating, and plating additives become dominant in the bulk plated layer to create desired properties.

“So, in light of all this information, is the microstructure actually important?” Massey asked. A comparison exercise confirmed that, after six reflow cycles at 245°C, single-crystal structures consistently survive, whereas broken crystal structures consistently fail at the interface.

“Can the microstructure predict reliability?” Massey asked. He concluded that microstructure is an important indicator for relative joint reliability, and it can be inspected and quantified. However, blind microvia reliability is a complex subject involving many process interactions, not just the plating.

A Circular Economy Industry

The final technical session was “Towards a Circular Economy Industry: Materials and Manufacturing,” moderated by Alun Morgan.

His first speaker was Dr. Erdem Selver, representing Jiva Materials and the University of Portsmouth, with a presentation on the development of sustainable composites for PCB applications. He defined the problems of the moment: electronics is a growing climate threat, responsible for up to 4% of global carbon emissions; the PCB is the unsustainable backbone of the electronics industry; PCB manufacturing emits up to 20 kg (44 lbs) of CO₂ per square metre; and E-waste is a crisis built on PCBs. There were about 67 million tons (134,000 lbs) of E-waste in 2024, of which less than 23% was recycled.

Selver described the creation of a novel recyclable and biodegradable PCB substrate, incorporating biodegradable polymer and natural fibres for electronic devices in applications including computing, lighting, domestic appliances, aerospace, and automotive. The experimental methodology is based on biodegradable composite materials, including hydrophobically modified polyvinyl alcohol (PVA) with a halogen-free flame retardant (organic phosphonate). Jute, flax, or hemp fabrics are impregnated with PVA/flame retardant mixture using a belt press, the entire procedure taking around five minutes, including the heating and cooling of the composites. These prepregs are layered up and hot-pressed for various cycles depending on the final PCB manufacturing requirement.

PCBs are manufactured using basic industry-standard processes and subjected to comprehensive testing: thermal, mechanical, electrical, and environmental, which Selver described in detail. The recyclability of substrate materials is subsequently demonstrated by dissolving the PVA in hot water and re-manufacturing useable substrates from the recovered woven fabrics. Life cycle assessment indicates that the recyclable PCB substrate material emits approximately 68.5% less CO₂ than FR4.

Selver highlighted real examples of the material’s commercial applications with PCBs in a wall-charger power supply, a laptop track-pad, and an LED lighting assembly.

Future objectives are to manufacture multilayer PCBs, further reduce total CO₂ emissions, obtain a V0 flame rating, and reduce the cost of processing and raw materials.

Benjamin King, a University of Glasgow research associate in green large area electronics, said that sensors in electronics have become ubiquitous and the ongoing demand is for billions of devices, resulting at end-of-life in significant electronic waste, which would probably end up in landfill. For truly sustainable electronics manufacturing, he believes the focus should be on the three key pillars: materials, fabrication energy, and end-of-life considerations. Additionally, E-textiles offer opportunities for lightweight flexible sensors as well as power electronics and electronic components.

Considering deposition methods, electroless nickel immersion gold is well established and will work on rough textiles, but consumes a scarce precious metal. Direct-write printing is high-precision, but slow, and limited to small areas. Screen printing can cover large areas and is compatible with rough surfaces, even though it is limited in resolution.

King sees PEDOT/PSS as a sustainable alternative material, a blend of two distinct polymers: conductive poly(3,4-ethylenedioxythiophene) (PEDOT) and non-conductive polystyrene sulphonate (PSS). It is water-soluble, low-temperature processing, biocompatible, has metal-like conductivity, and is capable of being screen printed in multiple layers to achieve sheet resistance values of less than 5 ohms/square. It has the potential to facilitate the development of large-area fabrication techniques, and King demonstrated a procedure for the assembly of vertically-integrated E-textiles by screen printing PEDOT/PSS onto a non-woven fabric substrate. He also discussed the characteristics of high-frequency components based on sustainable PEDOT/PSS E-textiles, explained how through-hole interconnects are formed in a polyurethane dielectric, and spoke about the challenges in overcoming crosstalk in RF applications.

Besides the work of his own department in the University of Glasgow, there are two other nationally-leading EPSRC-funded strategic facilities, one working on the rapid prototyping and characterisation of a wide range of semiconductor devices, the other on RF and terahertz devices and systems. 

Enhanced Direct Metallization

To complete the first-day conference program, John Swanson of MacDermid Alpha Electronic Solutions spoke about the European implementation of an enhanced direct-metallization process for advanced PCB production. Comparing direct metallization with electroless copper in through-hole plating applications, he listed the macro trends impacting the global electronics OEM, including sustainability and environmental considerations as well as density, performance, and reliability. The objective is to make a quantifiable difference with more eco-friendly products, processes, and supply chains, creating advanced electronics that will do more, last longer, perform better, and in smaller spaces.

Direct metallization provides measurable reductions in water, power, and chemical consumption in the supply chain that could contribute directly to sustainability goals. Safer chemistries and processes reduce the likelihood of restrictions or bans. Furthermore, direct metallization is a low-risk, readily available technology with high performance and reliability.

The latest version of MacDermid Alpha’s graphite process has 50% fewer steps than electroless copper. From a sustainability viewpoint, it reduces power consumption by 57%, waste treatment by 69%, and water usage by 76%, together with significant reductions in CO₂ equivalent emissions and carbon footprint.

This direct-metallization product features improved colloid stability, resulting in simplified operations and a footprint half that of an equivalent electroless copper line. It improves hole-wall conductivity and copper plating propagation.

Swanson’s test results indicate increased microvia and plated-through-hole reliability in areas such as thermal cycling, simulated reflow testing, and OM thermal stress testing.

Following an intense day of technical presentations, delegates took a bus to the Port of Leith Distillery, Scotland's first vertical distillery, a remarkable new nine-story building in Edinburgh's traditional whiskey district, overlooking the historic Royal Yacht Britannia moored dockside. It was a fascinating guided tour of the distillery, concluded with a convivial and sociable conference dinner before the return bus journey to the hotel.The proceedings of the second conference day are reported separately.

Once again I am grateful to Alun Morgan for kindly sharing his photographs.

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