EIPC Winter Conference 2023: Day 1 Review

Estimated reading time: 18 minutes

Liquid Crystal Polymer
Carina Luchsinger, R&D Engineer with DYCONEX in Switzerland, discussed applications of liquid crystal polymer as a biomaterial for neural implants. 

She began by reviewing the types of electrodes used to interface the brain and the depths at which they operate. Electroencephalography records brain activity from the scalp, electrocorticography records electrical activity from the brain cortex, intracortical electrodes are inserted into the cerebral cortex, and depth electrodes are placed within the brain’s substance. Clinical applications include the treatment of tremor patients, Parkinson’s disease, cluster headaches, epilepsy, stroke, spinal cord injuries, and speechless patients. 

The basic requirements for neural interfaces are for thin film substrates that match the mechanical properties of brain tissue with high spatial and temporal resolution, biocompatibility, and long-term biostability. Further requirements are for miniaturization, with surface topography that maximise contact area, low impedance, and high charge storage capacity. Typical surface coatings of electrodes are platinum and platinum alloys or poly (3,4-ethylendioxythiopene) “PEDOT.” 

Electrodes are required to be capable of high resolution for recording and stimulation, and to be manufacturable at high quality and low cost.

Luchsinger reported that liquid crystal polymer has proven to be an appropriate material for neural implants. It is a flexible thermoplastic base material with a suitable elastic modulus, chemically inert under most conditions and temperature stable up to 190°C. It exhibits very low water absorption and diffusion rates and is suitable for high frequency applications up to 120 GHz. Multilayers can be fabricated from a single homogeneous material with no adhesive.

Liquid crystal polymer-based substrates with pure gold conductors have been used as neural interfaces, and the electrical characteristics of the electrodes have been matched to the requirements of the neurons in terms of impedance and charge storage capacity. Platinum alloys and PEDOT have been successfully used as surface coatings. 

She is confident that liquid crystal polymer thin film technology will advance neuroscience research and enable high resolution electrodes with optimised surfaces.

Board-level Reliability
With the increasing complexity of electronic systems, board level reliability becomes a critical issue. Dr. Anna Graf, marketing manager, PWB Materials with Resonac Europe in Germany, explained the principal failure mechanism and discussed how reliability may be improved using a new prepreg designed to absorb thermal stress.

Traditionally in FR-4 PCBs, the focus of attention regarding thermal reliability has been expansion mismatch in the Z-direction between the glass-resin composite and copper in plated through-holes. Graf pointed out that in soldered assemblies, an equally critical consideration is expansion mismatch in the XY-direction between FR-4 substrate and components mounted on the surface of the PCB, leading to stress cracking of solder joints—particularly with lead-free alloys. 

Customers’ reliability requirements are increasing, especially in the automotive sector, which is very cost-conscious. So, the option of making PCBs on low-expansion substrates is not economically viable. 

Graf introduced the concept of a stress-absorbing prepreg to form the surface layer of the PCB, which will offer a cost-effective solution. Resonac’s new material is softer than standard FR-4 with a lower elastic modulus and low XY thermal expansion. It is available in a range of glass styles and resin contents giving pressed thicknesses from 0.069 mm to 0.208 mm.

Extensive thermal cycling testing, both in-house and independently by third parties, have demonstrated that board level reliability is significantly improved, and solder joint cracking minimised by simply replacing outer layer prepreg with the new material. 

PCB Material Technology Trends
Yonghyon Kim from Doosan Corporation Electro Materials Europe in Germany gave an encyclopaedic overview of high-speed PCB material technology trends, covering all aspects of glass fabrics, resins, filler systems, and copper foils, and their effects on signal loss.

Doosan offers a full range of materials for 5G applications, and their latest development is a Tier 6 product with a loss tangent between 0.003 and 0.002 at 10GHz for 800 gigabit Ethernet applications. They also have a comprehensive selection of specialist materials for HDI and automotive applications and for the manufacture of IC substrates. Kim discussed the technology roadmaps for allthe main product categories.

Solder Mask Restrictions
The final session of the first day was a roundtable discussion on the consequences of current formulation restrictions imposed on solder mask suppliers by the SVHCs defined in upcoming REACH regulations, particularly regarding key photoinitiators. Ink manufacturers Agfa Gevaert, Electra Polymers, and Taiyo America were represented, together with inkjet equipment manufacturers Notion Systems and SUSS MicroTec. 

Conference Day 1 Conclusion
The convivial conclusion to an active and productive conference day began with a bus ride downtown to enjoy Lyonnaise cuisine and networking at Restaurant Le Cintra, and certain technical debates continued for a while afterwards in the hotel bar. 

(The second day of the conference will be reviewed separately, stay tuned.)

As ever, I am grateful to Alun Morgan for allowing me to use his excellent photographs.

• < Previous Page