Day 2: More Cutting-edge Insights at the EIPC Summer Conference

Estimated reading time: 9 minutes

The European Institute for the PCB Community (EIPC) summer conference took place this year in Edinburgh, Scotland, June 3-4. This is the third of three articles on the conference. The other two cover Day 1’s sessions and the opening keynote speech. Below is a recap of the second day’s sessions.

High-density Integration

Delegates re-convened at Edinburgh’s Delta Hotel for the second day’s technical program, which opened with the session, “Enabling High-density Integration: From HDI to Medical and Chiplet Substrates,” moderated by Martyn Gaudion, CEO of Polar Instruments. Gaudion welcomed back Roger Massey, technical marketing manager at MKS Atotech, to comment on the capabilities and challenges in taking the step from HDI to advanced HDI.

He set out to answer questions like, “I want to move from 50/75-micron lines and spaces down to 25 microns. Do I need SAP?” and “Is that the “ultra HDI” thing I keep hearing about?” with expert guidance on changing and adopting advanced mSAP as a more realistic alternative technology to making the enormous investment involved in installing a true semi-additive process.

He considered the limit of subtractive HDI to be 40/45-micron line and space, whereas SAP was best suited for less than 20/20 micron. Ultra HDI, also referred to as mSAP or amSAP, was the best selection for achieving 30-40 micron line and space, with options to extend to 20-40 microns. The technology is in volume production and processes already exist, although operating costs are higher compared with traditional HDI processes, and there are opportunities to adopt existing infrastructure.

In-line horizontal plating is preferred, and requires new process chemistry to maximize blind microvia coverage with minimal surface copper, the objective being to minimize the amount of copper foil to be removed. Enhanced etch capabilities are required for optimal results.

Massey discussed pre- and post-treatment of copper surfaces through the laser drilling process, and the merits of horizontal in-line processing for electroless and flash-plating distribution. He commented on the throwing power and thickness distribution capability of the latest-generation electroless coppers, referring to his Day 1 presentation on copper crystallization phenomena, and advised how to avoid shorts arising from palladium activators. He explained how mSAP track geometry at the final-etch stage benefits from new proprietary etchant chemistries; almost square cross-sections can be achieved, with low side wall attack and minimal undercut.

He emphasized that experienced process suppliers are the best partners when moving to Ultra HDI technologies.

Substrates for Chiplet Technologies

Lars Böttcher, project manager at Fraunhofer IZM in Berlin, continued the theme of high density interconnect with his presentation on substrates for chiplet technologies, discussing an advanced semi-additive process for redistribution layers, bridging the gap between PCBs and wafer-based interposers on his chart depicting the landscape of advanced substrates.

With technologies advance to progressively finer pitches, he described features characteristic of mSAP and amSAP.

Like HDI, these use copper-clad laminate but with thinner base copper: 1.5–3 microns, and 1­–2 microns of electroless copper. Achievable line widths are in the range 50–15 microns.

SAP uses ABF film instead of copper-clad laminate, with 1–2 microns of electroless copper. Achievable line widths are in the range 25–8 microns.

Similarly, aSAP uses ABF film instead of copper-clad laminate, with a vapor-deposited seed layer of copper or titanium. Achievable line widths are in the range 10–5 microns and below.

Böttcher showed schematics of a process flow and a concept panel-level packaging line, and a process overview of an advanced semi-additive (aSAP) RDL process: ABF substrate, dielectric application, via formation, PVD seeding, photo resist application, lithography, electrolytic copper deposition, and differential etch.

He showed photographs and discussed three different via-formation techniques, comparing established 30-micron laser vias with 8-micron photo dielectric vias and 14 micron plasma vias, and indicated the relevant areas of research being undertaken, with notes on the impact of formation technique on yield.

He further discussed practical aspects of lithography, plating, and resist removal, and the differential etching of copper and titanium seed layers, together with notes on process control, electrical testing, and yield evaluation of RDL structures.

Looking to the future, glass core substrates offered an excellent opportunity in terms of outstanding dimensional stability. Initial approaches for this technology are in place.

Technologies for Medical Applications

David Capeder, application engineer at Dyconex in Switzerland, presented on semi-additive plating technology for medical applications. He believes that while lines and spaces of 18 microns are the limit of capability for subtractive processes, the semi-additive process (SAP) can achieve lines and spaces as fine as 8 microns.

He described the established SAP technology at Dyconex. Although the industry standard is a proven, cost-effective wet process with acceptable adhesion, Dyconex bases its process on sputtering, which is more flexible, gives superior adhesion, and supports alternative metals, even though it is more technically demanding and requires precise process control.

Copper is the company’s standard material for fine-line, high-precision SAP structures, but gold is ideal for biocompatible medical applications. Titanium and its alloys are used for precisely controlled resistive structures, and niobium has the potential for superconducting applications.

Capeder discussed some of the challenges they overcame in establishing reliable seed layer adhesion. Resist imaging is carried out using high-end LDI in ISO Class 5 clean rooms. Because SAP involves 30% more process steps per layer than subtractive technology, effective process control is critical to maintain process yield, and early defect detection is key. Dyconex uses high-resolution AOI for inspection, with automatic optical shaping (AOS) to remove shorts where necessary, providing the customer approves.

Capeder shared examples of high-end IC substrates enabled by SAP technology, featuring up to 20 layers with lines and spaces as small as 10 microns, measurement structures, precision passive components, and devices for ultrasound neuro-stimulation.

Thermal Management and Wastewater Treatment

Emma Hudson moderated the session on insights into thermal management and wastewater treatment. Ian Stewart, director of circuit formation, circuitry solutions with MacDermid Alpha Electronic Solutions, spoke about overcoming thermal management challenges in high-performance devices, highlighting heat, which Stewart referred to as,“The climate change of PCBs” when discussing increasing thermal loads. PCBs can handle a degree of heat, but excess temperatures cause issues, including loss of structural integrity and incompatible material expansion rates, which can induce failures in the PCB, components, and the finished device. The more power, the greater the thermal management issues.

After discussing, in general terms, the thermal management of PCBs at system, board, and component levels, Stewart commented on the widespread use of copper coins in PCBs and explained some of the benefits of using sintered copper paste to manage heat and conduct power. There are no restrictions on feature sizes, shapes, and geometries; sintered paste used in place of copper coins opens up a range of design possibilities.

 Stewart explained the mechanism of sintering as a thermal process of converting loose, fine particles into a solid, coherent mass by heat and/or pressure without fully heating the particles to the point of melting. Molecules in the sintered material diffuse across the boundaries of the particles, fusing the particles and creating a densely packed homogeneous structure with no voids or cavities. The actual sintering can be carried out by the PCB fabricator using a laminating press.

He described the characteristics of a sintered copper coin and how its physical properties (porosity, thermal conductivity, and coefficient of thermal expansion) are measured. Thermal conductivity is around 120-130 W/mK, although newer versions have conductivities approaching that of pure copper.

Sintered copper paste has also been used in power-trench applications, both for conducting electrical power and for thermal management, and Stewart explained how the hardness and porosity of the sintered deposits are evaluated.

He concluded that sintered copper provides an alternative to traditional copper coin technology, giving a greater degree of design flexibility in the PCB. It can also deliver power to geometries that are currently either difficult or impossible to achieve with traditional plating processes.

The Importance of Water Recycling

Carlo Enrico Martini, marketing manager at SAITA in Italy, explained why water recycling matters and describing recycling solutions in the PCB industry. His opening slide summarized the realities: PCB production is water-intensive and generates wastewater with high pollutant loads; water reuse improves sustainability and operational resilience, and there is increasing regulatory pressure on water discharge. There are clear economic and environmental benefits from closed-loop systems.

His approach to water recycling projects is to separate and group wastewater streams by pollutant concentration, because “to mix creates waste.” The objective is to characterize individual streams and define the best available technology for each stream type. Wastewater can be classified as exhausted process baths containing concentrated chemical solutions, which require special waste handling or targeted recovery, or rinse waters with contaminant concentration levels depending on whether they are “first,” “intermediate,” or “final.” The key treatment technologies are: ion exchange resins, membrane filtration, chemical-physical, evaporation technologies, and zero-liquid-discharge.

DI water from final rinses can be recycled through ion exchange resins on a closed loop with a high flow rate. Semi-permeable membranes are ideal for producing high-quality water, but generate a concentrated stream requiring further treatment.

Reverse-osmosis membranes can be used downstream of water treatment plants to recover pure water, but generate a reject brine that must be handled with other treatment processes.

Chemical-physical treatment can be used in continuous or batch operation for efficient precipitation and separation of dissolved metals, or can eliminate suspended solids by coagulation or flocculation.

Vacuum evaporation can be used to treat small volumes of highly polluted wastewater. The distillate is reusable in the manufacturing process, and the concentrate is sent for disposal or the recovery of raw materials.

Finally, zero liquid discharge maximizes water recovery and minimizes waste output. It involves advanced evaporation and filtration and is critical for full sustainability in PCB manufacturing.

Martini described two case histories. The first is a new PCB project in Europe with a water treatment process designed to recycle more than 50% of water. It has taken an integrated approach involving cooperation between the end user, the chemical suppliers, the machinery suppliers, and the water treatment plant supplier.

The second is the evaporation of exhausted bath solutions from a dry film stripping process, which uses a heat pump vacuum evaporator and produces high-quality distillate water and a distillate: concentrate ratio of 20:1.

He finished by noting the key design factors for a water recycling solution: deep analysis and an integrated approach to production, chemistry, and water, accurate separation of wastewater streams, based on contaminant concentration and type.

Suface Finishing Processes

The final session was moderated by EIPC Technical Director Tarja Rapala-Virtanen on the topic of training on surface finishing processes. Kunal Shah of LiloTree gave a comprehensive tutorial on key issues, root causes, selection criteria, and best practices comparing the relative merits of legacy finishes like ENIG and ENEPIG with his novel nano-engineered “Ni-less ENIG-Premium” product.

Alun Morgan closed the conference and thanked the sponsors for their support, Tarja Rapala-Virtanen for curating a first-rate conference program, the moderators for managing the sessions, the speakers for their excellent presentations, and Kirsten Smit-Westenberg and Carol Pelzers for their professional organization and management of the event. 

May I add my thanks to the EIPC team for making me welcome, and Alun Morgan for sharing his photographs.