EIPC’s 2018 Winter Conference in Lyon, Review of Day 1

Estimated reading time: 10 minutes

The first technical session was introduced and moderated by Martyn Gaudion, CEO of Polar Instruments, on the theme of trends and capabilities in PCB fabrication. First to present was Dr. Christian Klein, section manager for PCB development for automotive electronics, with Robert Bosch, who gave his perspective on future automotive requirements for PCBs.

As one of the world's largest automotive equipment suppliers, Bosch had developed an enormous variety of electronics solutions for applications. For example, connected mobility: connection from the vehicle to the internet and to other vehicles. Automated mobility: giving the driver various levels of assistance from partial to full automation, and powertrain systems for internal combustion, hybrid and full-electric vehicles. In all these applications, the over-riding focus was on reliability.

Dr. Klein made a comprehensive analysis of the possible failure modes associated with environmental stresses, including temperature cycling and storage, bending, vibration and humidity, separately and in combination, and assembly stresses on the PCB related to reflow soldering, selective soldering and press-fit technology.

Increasing environmental loads required adaptation of materials and concepts for automotive electronics, together with a very good understanding of cause-and-effect relationships. In particular, trends to plastic housings, longer operational times and hotter applications led to increased humidity load, increased temperature and temperature-cycle load and longer temperature-humidity-bias impact on the PCB.

New functional requirements, for example the trend to higher operating voltages in small enclosures, finer-pitch, higher I/O components, power electronics on organic substrates, and high-speed applications in radar and image processing, required new PCB concepts such as power PCBs and highly integrated logic PCBs.

The effects of humidity, both on the surface and within the structure of the PCBs were areas of critical concern, and the possible failure modes had been studied in great detail. Humidity on the surface, either as water vapour or condensed in the form of dew, in combination with ionic contamination and voltage bias, could lead to material degradation or material diffusion followed by electrochemical migration and dendrite growth. The mechanisms had been modelled so that electrochemical failure could be predicted, and all new components and materials were subject to standard qualification tests. Ongoing, the features of the test board were being adapted to reflect miniaturisation, and work was proceeding to correlate results with surface insulation resistance and ion chromatography measurements.

Humidity within the PCB led to a different class of failure modes resulting from electrochemical migration: conductive anodic filamentation and failures associated with hollow fibres and organic fibre contamination. Cracks in the resin could result from temperature degradation, high pressure, bending and mechanical load. The mechanism of CAF formation was understood and qualification methods for cracks were being developed to enable more effective material selection.

Finally, with the increase in high-speed applications, particularly in automotive radar systems, power integrity, signal integrity and electromagnetic compatibility, became significant considerations in PCB design rules and material selection, and inevitably the most cost-effective solution was sought.

As we experience the beginnings of Industry 4.0, the fourth industrial revolution, development continues in PCB materials and imaging processes. So it was perhaps logical that solder mask technology would enter its fourth generation: two-pack screen-printed, single-pack UV-cured, liquid photoimageable. What next? Andreas Dreher from Würth Elektronik introduced the concept of “solder mask 4.0” with his presentation on a technology branded “s.mask,” the outcome of a collaboration between two PCB manufacturers, Würth Elektronik and FELA, both from Baden-Württemberg in Germany, with the cooperation of solder mask manufacturer Taiyo, using digital 3D printing techniques.

Dreher described the inkjet printing process and how it had been adapted and optimised for applying solder mask exactly where it was wanted, at high resolution and precise registration and with the capability to vary the deposit thickness on different features of the PCB as required. A particular attribute was its ability to form solder dams and solder-mask-defined pads without solder mask on pad and without any undercut edges to trap process contaminants. It also avoided unwanted mask in via holes.

Würth Elektronik were conducting an extensive programme of compatibility and solderability testing, together with measurements of surface insulation resistance, ionic contamination, long-term temperature cycling and hot storage operational life testing. Initial results were all good and it was clear that s.mask met all the typical performance requirements for solder masks. And the exercise was a splendid example of PCB shops combining their resources to advance the technology.

A thermoplastic dielectric with some unique properties, liquid crystal polymer (LCP) had been employed as a flexible substrate and encapsulating material for a range of miniaturised hermetic modules for sensor integration, the subject of an informative presentation from Dr. Marc Hauer, R&D Manager and Engineering Manager at Dyconex in Switzerland.

He explained that liquid crystal polymer was a partially crystalline aromatic polyester that demonstrated a combination of excellent electrical, thermal and mechanical properties, and was fully compatible with PCB and thin film technologies. Being thermoplastic, it did not require any additional bonding material to make embedded or multilayered structures, and because of its extremely low water absorption and chemical inertness, it was ideal for biomedical applications. Compared with conventional flexible circuits, liquid crystal polymer structures had no interfaces; therefore, there was no possibility of diffusion along interfaces.

He showed examples of implantable devices in which plated or sputtered metals could be used as simple conductors or in combination as integrated resistors, thermocouples, thermistors or heaters, and thinned semiconductor components could be embedded for additional functionality.

The capability of liquid crystal polymer to encapsulate devices in harsh environments, was demonstrated by embedding moisture-sensitive silicon chips in LCP substrates and subjecting them to long-term immersion in phosphate buffered saline solution and concentrated sulphuric acid at elevated temperatures. No failures had been observed.

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