Advances in semiconductor packaging have enabled the development of flexible and stretchable systems for wearable and biomedical applications. Traditional rigid packaging is incompatible with devices that conform to dynamic surfaces like skin or tissue, necessitating innovations in materials and mechanical design. The focus here is on elastomeric substrates, stretchable interconnects, and reliability under repeated deformation, distinct from flexible electronics fabrication processes.

Elastomers serve as the foundation for flexible packaging due to their low modulus and high elongation at break. Polydimethylsiloxane (PDMS) is widely used, with a Young’s modulus ranging from 0.1 to 3 MPa and stretchability exceeding 100%. Its biocompatibility and chemical stability make it suitable for biomedical devices. Alternative elastomers include polyurethane (PU) and styrene-ethylene-butylene-styrene (SEBS), which offer tunable mechanical properties. For instance, SEBS can achieve elongations up to 600% while maintaining robust adhesion to embedded components. Thermoplastic elastomers like Ecoflex exhibit similar stretchability with improved tear resistance, critical for long-term wearability.

Stretchable interconnects must maintain electrical conductivity under strain. Metal-based solutions often employ serpentine or horseshoe geometries to accommodate deformation without fracture. Gold and copper thin films patterned in these designs can withstand strains of 50-100% before resistance increases significantly. However, metal fatigue remains a challenge under cyclic loading. Liquid metals like eutectic gallium-indium (EGaIn) offer an alternative, with inherent stretchability due to their fluidic nature. EGaIn interconnects maintain stable conductivity up to 500% strain but require encapsulation to prevent leakage.

Conductive composites combine elastomers with conductive fillers to achieve stretchable traces. Silver flakes or carbon nanotubes dispersed in PDMS or PU matrices provide percolation pathways for electron transport. These composites typically exhibit sheet resistances of 0.1-10 Ω/sq at strains below 50%. Recent developments include hybrid fillers, such as silver nanowires with graphene, which reduce resistance drift during cycling. A composite of silver nanowires in PU demonstrated less than 10% resistance change after 10,000 cycles at 30% strain.

Reliability under mechanical stress is a critical metric for wearable and biomedical packaging. Delamination at material interfaces is a common failure mode, addressed through surface treatments or adhesion promoters. Oxygen plasma treatment of PDMS improves bonding strength with metal layers by up to 300%. Silane coupling agents further enhance interfacial adhesion between dissimilar materials. Accelerated aging tests simulate long-term use, with parameters like strain rate, humidity, and temperature adjusted to mimic physiological conditions. Packages subjected to 10% cyclic strain at 1 Hz for 100,000 cycles must retain electrical functionality without cracks or delamination.

Encapsulation protects sensitive components from moisture and mechanical damage. Thin-film barriers of silicon oxide or nitride deposited via chemical vapor deposition offer water vapor transmission rates below 10^-4 g/m²/day. However, their brittleness limits compatibility with stretchable systems. Multilayer films alternating elastomers and inorganic layers improve flexibility while maintaining barrier performance. A stack of PDMS and Al₂O₃ achieved a WVTR of 5x10^-3 g/m²/day at 50% strain.

Thermal management is another consideration, as stretchable packaging must dissipate heat without compromising mechanical properties. Embedded heat spreaders using graphene or boron nitride sheets provide thermal conductivities of 5-20 W/mK while remaining flexible. Phase-change materials like paraffin wax can absorb transient heat loads in high-power applications.

Biocompatibility is essential for implantable devices. ISO 10993 standards govern material selection, requiring cytotoxicity, sensitization, and irritation testing. PDMS and certain polyurethanes meet these requirements, but additives like conductive fillers must also be evaluated. Long-term stability in physiological environments involves testing in phosphate-buffered saline at 37°C for weeks or months. Corrosion-resistant coatings like Parylene-C are often applied to metal interconnects in such applications.

Integration with existing semiconductor components requires careful design. Chip-scale packaging techniques adapt rigid ICs for stretchable systems by isolating strain concentrations. Island-bridge architectures place rigid islands containing chips on stretchable bridges housing interconnects. This approach localizes deformation to the bridges, protecting the components. Anisotropic conductive films enable electrical connections between islands and bridges without solder brittleness.

Testing protocols validate performance under realistic conditions. Uniaxial, biaxial, and torsional strain tests quantify electrical and mechanical stability. Environmental chambers combine mechanical cycling with temperature and humidity control. For biomedical devices, simulated body fluid immersion tests assess corrosion resistance. Accelerated lifetime models extrapolate from these tests to predict years of use.

Emerging trends include self-healing materials that repair cracks autonomously. Polymers with dynamic covalent bonds or microencapsulated healing agents can recover conductivity after damage. Another development is wireless power transfer integration, eliminating the need for stretchable power interconnects. Near-field communication or inductive coupling enables energy transmission through the packaging.

Future challenges include improving the density of stretchable interconnects to support complex circuits while maintaining reliability. Multilayer stretchable printed circuit boards are under exploration, though layer-to-layer alignment during stretching remains difficult. Another area is enhancing thermal dissipation in high-power flexible devices without adding rigidity.

The convergence of material science and mechanical engineering continues to push the boundaries of flexible and stretchable packaging. Innovations in elastomers, conductive composites, and encapsulation techniques enable robust integration of electronics into wearable and biomedical systems. Reliability under mechanical stress remains a central focus, driving advancements in testing and design methodologies. As these technologies mature, they will unlock new applications in health monitoring, therapeutics, and human-machine interfaces.