The APPLAUSE project, funded by the German Federal Ministry of Education and Research (BMBF) and running from 1 May 2019 to 31 October 2022, brought together Würth Elektronik GmbH & Co. KG, Fraunhofer IMS, Osypka and other partners to develop advanced packaging solutions for photonics, optics and electronics aimed at low‑cost manufacturing in Europe. Würth Elektronik led the technical work on three application areas: low‑cost thermal imaging devices (Use Case 2), heart‑monitoring systems (Use Case 4) and minimally invasive implants (Use Case 5). The consortium’s overall goal was to create robust, scalable packaging techniques that could be integrated into medical and industrial products.
In Use Case 2 the team developed a hybrid panel‑level packaging process for a system‑in‑package (SiP) thermal imaging camera. Printed circuit boards served as the foundation for the assembly and interconnect architecture. Wire‑bonding experiments were carried out on a range of solder‑able surfaces, including conventional ENEPIG and ENIG, as well as novel optical‑grade surfaces such as black nickel‑oxide, EPIG (electroless palladium immersion gold) and ISIG (immersion silver immersion gold). Test PCBs were fabricated with these surfaces and used for both demonstration units and reliability trials. Gold and aluminium wires were bonded to the read‑out integrated circuit (ROIC) using ball‑wedge and wedge‑wedge techniques. Pull‑off and ball‑shear tests quantified bond strength. Results showed that nickel‑free ISIG and EPIG surfaces delivered the strongest adhesion, with ISIG and ENEPIG achieving the highest pull‑off forces in ball‑wedge bonding. Wedge‑wedge bonding produced lower values, and the black nickel‑oxide surface proved unsuitable, failing to provide reliable bonds. These findings guided the selection of surface finishes for future SiP assemblies.
The project also addressed the potting of large optical chips. A glob‑topping process was optimized to prevent potting material from covering the light‑emitting surface of components such as LEDs, which would otherwise reduce optical output. Three potting materials were evaluated; the first could not achieve full edge coverage, while a second material enabled a triple‑potting approach that first applied a transparent layer to the optical surface, followed by a thicker potting layer to seal the edges. This technique improved optical performance and reduced the risk of light loss.
For minimally invasive implants, Würth Elektronik investigated flexible PCB substrates suitable for catheter integration. Initial trials with polyimide and liquid‑crystal‑polymer (LCP) boards revealed mechanical failures: polyimide traces cracked under bending, and sharp polyimide edges damaged tissue during insertion. Consequently, a rigid‑flex PCB design was introduced, featuring a 600 µm rigid section that matches the inner diameter of the catheter, eliminating the need for bending. The flexible portion is a polyimide core covered on both sides with polyurethane, a skin‑friendly material that protects tissue. Side metallization provides direct electrical contact within the catheter, while a solder‑stop mask protects component placement. This architecture eliminates soldering and gluing steps, enhancing reliability and simplifying assembly.
Finally, the consortium produced a low‑cost heart implant prototype based on printed circuit board assembly (PCBA) technology. The device, intended for temporary observation and stimulation, was assembled and tested in collaboration with partner Osypka. Early results indicate that the PCBA approach can meet the functional and safety requirements of cardiac monitoring, and the team is optimistic that the design will be incorporated into future commercial products.
Overall, the APPLAUSE project demonstrated that careful selection of surface finishes, potting strategies, and flexible‑rigid PCB architectures can yield robust, low‑cost packaging solutions for high‑performance photonic and medical devices. The collaboration among industry, research institutes and academia, supported by BMBF funding, enabled the translation of these technical advances into prototype products that show promise for future market deployment.