EIPC Summer Conference 2023: The Keynote Session

Estimated reading time: 12 minutes

PCBs in Space
Stan Heltzel, materials engineer at the European Space Agency, responsible for qualification of printed circuit board manufacturers and technology development, described how Eurospace, the trade association of the European Space Industry, has set out to support the European PCB supply chain under the EU Chips Act, designed to strengthen Europe's semiconductor ecosystem. His initial comment was that the industry had so far not done a very good job explaining the realities to the decisionmakers. There was an urgent need to “reach out and make more noise.”

He began with an introduction to PCB technology and its drivers. Reviewing electronic packaging levels, he defined the following: Level 0: chip, bare semiconductor die; Level 1: electrical, electronic, and electromechanical (EEE) components and their back-end packaging; Level 2: assembled EEE components on the PCB; Level 3: modules integrated in a system; Level 4: the system. 

Using a series of paper-doll shapes to represent the anatomy of a spacecraft, he designated electronic assembly as a complex combination of materials and processes to provide a stable mechanical and thermal platform for the electrical interconnection of components. In his analogy, printed circuit board, electronic assembly, and packaging technologies become the nerves and veins of the spacecraft. PCB technology is driven by manufacturability, electrical performance, functionality, and reliability. Manufacturability and reliability depend on design, materials, capability, and processes.  

At the leading edge of Heltzel’s HDI roadmap of complexity vs. data rate was a construction with three-stacked microvias, spread glass weave, smooth copper foil, and a nickel-free finish. He reviewed some of the causes of microvia failure and introduced the EU IPC Microvia Working Group, whose objectives are to gather PCB experts from the European high-reliability electronics manufacturing industry, execute a specific first work proposal, identify future work proposals, and strengthen European industry presence in IPC microvia standardization. He illustrated their test panel design and described the relevant test methods. It was proposed to request test panels from the European PCB supply chain for the high-reliability industry and from global state-of-the-art manufacturers for benchmarking, and to validate test methodology as an input to setting common standards. The working group comprises an impressive list of high-profile participants and collaborators. 

The current space PCB market is characterised by a high level of activity and a corresponding high order volume, with lead times up to 30 weeks. It is necessary to make allowance for failure at outgoing inspection and to implement double sourcing for complex, non-qualified technology. Heltzel recognised that PCBs are too often treated as a commodity and that failures can occur even when all requirements are met. He believes that PCB manufacturers should be partners in the supply chain, since complex designs require early involvement of the PCB manufacturer, and technology development needs expertise and resources on both sides of the chain. Qualification and continuous improvement need support from supply chain, with problem areas addressed by supplier development programmes. 

Heltzel observed that the number of PCB facilities in Europe has been continuously declining for several decades. Although in recent years the revenue appears to have stabilised, over a longer time span this indicator also follows a declining trend. About 60% of PCBs in Europe are imported, mostly from China. This dependency is most prevalent in volume markets, although NewSpace equipment integrators occasionally source PCBs overseas, motivated by lower cost, shorter lead-time, and in some cases, a better capability. Qualification schemes and contractual conditions ensure local sourcing of PCBs in the high-reliability market segments of space and defence. 

For this low-volume, high-mix industry, the most critical impact is an indirect one: The weak European PCB ecosystem causes unavailability in its own supply chain. Raw materials are almost exclusively manufactured outside Europe, the workforce is difficult to maintain, educational institutes do not offer adequate training, and critical equipment and chemistry need to be imported, all of which hampers the availability of senior service personnel from these overseas companies. 

The end of a globalised market and the consequential reshoring has been initiated by the U.S.-Chinese trade conflict and further deepened by supply chain disruptions during the pandemic and the Ukrainian war. There is much excitement about the development of local chips technology and its ecosystem. The European Chips Act provides development opportunities from foundry to OSAT. But the act does not analyse the ecosystem beyond back-end packaging and is not inclusive of system level packaging, i.e., PCB technology and assembly. The global electronics industry struggles with the “hidden reliability treat” in microvias. Yet it remains difficult to obtain development funds.  

Heltzel asked, “How can the European PCB supply chain be improved for the benefit of all industry, not just space?” and referred to the Eurospace PCB supply chain white paper, “Supporting electronic assemblies and PCBs for a resilient and competitive EEE components supply chain in Europe,” which provides a review of the printed circuit board and advanced packaging supply chains in Europe, from the point of view of critical applications, such as for space and defence programmes, and from the standpoint of the main leading industry organisations, such as the IPC and the EIPC.  

It notes that against the key trends identified in electronics technologies, the European PCB supply chain exhibits a few weak spots which affect a broad range of industrial sectors, from consumer electronics to high-performance/high-reliability sectors like automotive, aerospace and defence, and requests support from the European Commission for the broad development of the PCB supply chain. 

Interconnect Reliability
Dr. Dongkai Shangguan, Fellow of the International Microelectronics Assembly and Packaging Society, was unable to attend the conference in person and gave an extremely comprehensive presentation on interconnect reliability in electronic systems via a video recording. 

He appraised the complete thread of interconnection, all the way from the chip to the system: die-to-package, package-to-termination, termination-to-PCBA and PCBA-to-system, including design and materials, then discussed reliability at each stage. Besides the primary issues of electrical interconnection for signal and power, and mechanical reliability under complex loading conditions and usage environments, he discussed thermal reliability in high density and high power situations, as well as manufacturability and sustainability issues.

Shangguan examined the thermomechanical reliability of solder interconnects in great detail, with comparisons of the behaviour of SAC alloys with tin-lead regarding crack initiation and propagation under cyclic and dynamic loading conditions, and listed the key requirements for high-reliability alloys with enhanced thermal cycling performance under a wide range of conditions. Certain proprietary SAC alloys containe additives which modify their precipitation and solution hardening mechanisms, stabilising the solder joint microstructure and morphology, and resulting in superior process control characteristics and improved reliability.

He made some interesting observations on the relationship between reliability and miniaturisation: Smaller solder interconnects are more susceptible to thermomechanical failures, higher current densities increase the incidence of electromigration, while tighter spacings encourage whiskering, and as the solder volume decreases, interfacial intermetallic compounds have a more significant effect.

In his experience, heterogeneous integration and the diversity of packaging methods within the same module has led to more complex, and often interactive, reliability failure modes and mechanisms. Miniaturisation additionally affects process yield. Electrical issues included signal integrity, crosstalk, and electromagnetic interference.

On the subject of thermal management, he discussed interfacial thermal resistance and emphasised the value of the thermal interface materials, describing the characteristics of various alternatives. 

It is important to ensure the thermal integrity of a semiconductor device during its development. Shangguan described the use of a thermal test chip, with either a uniform power distribution or configurable hot spots, as a “thermal twin” to simulate the thermal aspects of the real chip, to enable a thermal management solution to be developed concurrently.         

Following the presentations, a Q&A session moderated by Tarja Rapala-Virtanen brought many questions from the audience, the majority of which concerned the European Chips Act and its implications, with the urgent need to raise the awareness of the European Commission to the significance of the PCB and packaging industries. Alun Morgan and Stan Heltzel responded expertly and the session became a meaningful interactive discussion.

The conference proceeded with sessions on Smart manufacturing and special material solutions, which are reported separately.

References

1. "Pausing AI Developments Isn't Enough. We Need to Shut it All Down," by Eliezer Yudkowsky, March 29, 2023, Time Magazine.

All photographs courtesy of Alun Morgan, thanks Alun!

Links to other articles in this three-part series:

• Part 2: Day 1 Review
• Part 3: Day 2 Review

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