New materials and processes will help with power distribution and thermal dissipation in advanced packages.
Interposers and substrates are undergoing a profound transformation from intermediaries to engineered platforms responsible for power distribution, thermal management, high-density interconnects, and signal integrity in the most advanced computing systems.
This shift is being driven by AI, high-performance computing (HPC), and next-generation communications, where the need for heterogeneous integration is pushing the limits of packaging technologies. While transistor dimensions have shrunk to the single-digit nanometer range, conventional PCB technology remains constrained to line widths of 20 to 30μm — a gap spanning three orders of magnitude.
The inability of traditional packaging to keep up with silicon scaling has created a critical bottleneck in performance and integration density. As a result, interposers and advanced substrates are evolving rapidly to enable:
As the industry moves toward larger chiplet-based architectures, silicon interposers are increasingly being replaced with organic interposers, which allow for greater package sizes and higher-density interconnects. At the same time, glass substrates are emerging as a scalable alternative to organic materials, providing mechanical stability and ultra-fine RDL capabilities.
“Advanced packaging is evolving rapidly, forcing us to constantly adapt our processes,” says Dick Otte, CEO of Promex. “Whether it’s new substrate materials or novel bonding methods, being able to quickly adjust and optimize processes is now a critical capability.”
Bridging the interconnect gap
The semiconductor industry has long relied on redistribution layers (RDLs) to route signals between dies and external interfaces. But as packaging demands increase, RDL technology is pushing the limits of traditional materials and manufacturing methods. New substrate materials and process innovations are now essential to achieving the interconnect density required for AI, high-performance computing, and 5G applications.
A key part of this transition is the move away from silicon interposers to organic and glass-based solutions. Organic interposers use glass carriers for structural support, providing a scalable alternative to silicon interposers, which require through-silicon vias (TSVs) and depth etching processes. As chiplet-based architectures expand, organic interposers enable larger package sizes, while maintaining fine-pitch interconnects.
At the same time, glass-core substrates and glass interposers are emerging as an alternative to organic materials, offering mechanical stability, lower dielectric constants, and finer redistribution layers (RDLs). However, manufacturing and handling challenges remain, particularly in warpage control, plating uniformity, and defect inspection.
“The challenge with interposers today isn’t just about scaling line widths,” says Chee Ping Lee, managing director for advanced packaging at Lam Research. “It’s about making sure we have stable, manufacturable substrates that can support these finer features. As we transition from silicon to organic interposers, and from organic substrates to glass, we have to solve new issues with plating, warpage, and handling.”
Another major development in glass interposers is the push for rectangular glass carriers, which improve handling and processing efficiency compared to traditional circular wafer-shaped carriers. “Today, glass carriers are circular,” says Lee. “But the industry is moving quickly toward rectangular glass carriers for interposers.”
Meanwhile, redistribution layer (RDL) technology is evolving to support 1μm line/space resolutions, approaching the dimensions required for chip-to-chip interconnects. Brewer Science, in collaboration with imec, has demonstrated how semi-additive processes (SAP) can achieve line/space resolutions of 2μm/2μm in production environments, with advanced research pushing 1μm/1μm capability in controlled settings.
In a white paper presented by Alice Guerrero of Brewer Science and imec, researchers demonstrated how “a collective die-to-wafer bonding flow extended beyond the N=2 tier to the N=3 and N=4 tier by collectively bonding multiple layers of dies on top of a target wafer.” That showed the feasibility of advanced stacking techniques with “die-to-target wafer alignment with the majority of dies below +/-2µm.”
Fig. 1: Simplified flow for N=2, N=3 and N=4 collective die-to-wafer transfer. Source: imec
This is particularly critical for fan-out panel-level packaging (FOPLP), which enables cost-effective, high-density integration at scale. However, manufacturing at the panel level presents new yield and process control challenges.
“FOPLP offers potential cost advantages over other approaches by packaging more chips in a large panel format,” says Lee. “However, there are challenges that can negate the potential cost savings of FOPLP for certain applications, including initial equipment costs, limited supply chain, and processing yield issues due to the large format.”
Despite these challenges, FOPLP is emerging as a key enabler for high-volume, high-performance chips such as AI processors and HPC accelerators. As manufacturers continue refining panel-level processes, like electroplating uniformity, fine-line lithography, and warpage control, the adoption of standardized panel formats could accelerate FOPLP’s viability in mainstream semiconductor manufacturing.
Overcoming manufacturing challenges
As interposers and substrates become more complex, maintaining precision at nanoscale dimensions has become a significant challenge. The shift toward heterogeneous integration and fine-pitch interconnects demands extreme accuracy in die placement, material deposition, and bonding techniques. Even slight misalignments that used to be tolerable now can degrade electrical performance, reduce reliability, and impact thermal dissipation.
“The main challenge in substrate and interposer assembly is achieving co-planarity consistently,” says Otte. “Because these substrates are becoming so thin and delicate, maintaining precise alignment during die-to-substrate bonding is critical. Any slight misalignment or variation can have substantial impacts on performance.”
One of the most pressing concerns is the transition to panel-level processing (PLP). While wafer-level processing (WLP) has benefited from decades of standardization, PLP introduces new variables in handling, alignment, and yield management across large-format substrates. Variability in material expansion, warpage, and process uniformity presents a significant engineering challenge.
“Full panel uniformity is one of the most difficult aspects of FOPLP,” says Lee. “Multi-layer, fine-line redistribution layers (RDLs) require a high degree of uniformity, from lithography to electroplating. If not achieved, warpage and topography issues can disrupt subsequent layers and reduce yield.”
Glass-core substrates introduce another set of manufacturing and defect inspection challenges. Unlike silicon or organic interposers, glass interposers have circuits formed as grooves within the substrate. That increases the risk of circuit breakage caused by cracks.
“Since circuits in glass interposers are formed as grooves, cracks can lead to circuit breakage, increasing the risk of defective products,” says a representative from TASMIT. “More precise manufacturing and better defect inspection tools will be necessary before glass can see widespread adoption in advanced packaging.”
Another challenge is the transition to hybrid interposers, where different material sets — organic, silicon, and glass — are selectively combined for cost efficiency and electrical performance. However, thermal expansion mismatches between these materials introduce new mechanical reliability concerns.
“The combination of organic and inorganic interposers with small silicon bridges introduces completely different thermal expansion coefficients,” says Kelly Damalou, product manager for On-Chip Electromagnetic Simulation at Ansys. “These differences create significant mechanical challenges, often leading to reliability concerns in advanced packaging solutions.”
A particularly difficult aspect of scaling interposers is the electroplating process for high-aspect-ratio features, such as the tall, thin pillars surrounding embedded silicon bridges in organic interposers. Electroplating these structures uniformly without creating excessive process time is a significant challenge.
“To embed the silicon bridge, you need TSV-like structures surrounding it to connect the bottom to the top. These ‘megapillars’ are very tall (>100µm), slim, and hard to plate uniformly,” says Lee. “Slowing down the plating rate helps, but no one wants a process that takes 200 minutes per wafer.”
This need for speed and precision is forcing manufacturers to adopt AI-driven process control and real-time monitoring techniques. Statistical process control (SPC) is becoming critical in die placement, plating, and bonding to ensure consistency and yield in high-density interposer designs.
“Implementing rigorous statistical process control is crucial,” says Promex’s Otte. “With substrates and interposers becoming increasingly intricate, SPC enables real-time detection and correction of variations.”
To overcome these challenges, AI-driven metrology and adaptive process control are being integrated into manufacturing lines. By leveraging high-resolution imaging, machine learning algorithms, and real-time feedback loops, manufacturers can detect subtle misalignments or process variations before they propagate into defective products.
The challenge with manufacturing today requires more than just pushing to finer dimensions. It also requires process control to keep up, which is why AI-driven inspection and adaptive process tuning are becoming essential for keeping defect rates low as interposer designs scale up.
Thermal management
As semiconductor packaging evolves, thermal management has become one of the most critical barriers to scaling. Interposers and substrates, once passive elements in packaging, now play an active role in dissipating heat to maintain reliability in high-performance computing, AI accelerators, and multi-chip modules. Higher power densities, larger chiplet-based architectures, and finer interconnect pitches all amplify the need for efficient thermal solutions.
“When working with interposers that are four or five inches on a side, significant thermal gradients can form,” says Marc Swinnen, director of product marketing at Ansys. “These gradients lead to mechanical deformation and warpage, stressing thousands of micro-scale bonds. These stresses can feed back into the electrical performance of the silicon, affecting overall reliability.”
Effective thermal management in advanced interposer designs is now intertwined with electrical performance. As substrates become denser, removing heat efficiently while maintaining electrical integrity requires new approaches, particularly involving integrated thermal pathways.
The issue is particularly acute in multi-chip packages, where heat must be dissipated across increasingly dense interconnect structures. Traditional thermal solutions, such as heat spreaders and thermal interface materials, are proving insufficient as power levels rise. Instead, manufacturers are turning to new solutions that integrate thermal management directly into interposer and substrate designs.
To address these challenges, manufacturers are investigating embedded microfluidic cooling channels within interposers, phase-change materials that absorb heat during peak loads and release it gradually, and advanced thermal interface materials based on carbon nanotubes, which offer significantly lower thermal resistance than conventional pastes. Hybrid metal-organic heat spreaders also are being developed to improve heat dissipation while keeping costs and weight in check. The success of these new thermal management strategies will determine how well next-generation interposers can scale to meet the demands of AI and high-performance computing.
New material innovations
As semiconductor performance demands continue to increase, traditional organic substrates are reaching their fundamental limits. In response, manufacturers are turning to new materials such as glass-core composites, ceramics, and hybrid organic-inorganic structures to improve thermal performance, electrical properties, and mechanical stability.
Glass-core interposers have garnered significant attention due to their lower dielectric constant of approximately 4.0, which is significantly lower than silicon’s 11.7, reducing signal loss and making them well-suited for high-frequency applications such as 5G, 6G, and other millimeter-wave communications. Glass also provides better dimensional stability than organic substrates, reducing warpage and improving yield in panel-level packaging. Despite these advantages, manufacturing challenges remain, particularly in precision laser drilling for through-glass vias, via filling, and the inherent brittleness of glass materials.
“Glass substrates are really flat and mechanically strong, allowing us to expand the package beyond 120mm by 120mm,” says Lee. “This enables very fine-line RDL integration, critical for high-density interposers and substrates.”
Beyond glass, hybrid substrates that incorporate silicon bridges within organic interposers are gaining traction. These structures combine the cost efficiency of organic materials with the electrical performance benefits of silicon, creating a more versatile packaging solution.
“One of the key challenges in advanced packaging is managing warpage due to differences in the coefficient of thermal expansion (CTE) between these materials,” says Otte. “Even minor mismatches can create yield and reliability issues at these scales.”
Thermal expansion mismatches can lead to delamination, cracking, and warpage during thermal cycling, increasing the need for predictive modeling to anticipate these effects before they become manufacturing issues. As semiconductor packaging continues to push the limits of material integration, ensuring accurate material characterization and simulation is becoming a critical requirement.
“Material properties become critical as frequencies increase and substrates become hotter,” adds Swinnen. “Accurately modeling how these materials behave under real-world conditions is crucial. Mischaracterizing material behavior at the interposer and substrate level can drastically affect device reliability.”
Despite the promise of these new materials, significant manufacturing complexities remain. While glass and ceramic substrates offer superior electrical properties, they introduce processing difficulties, cost concerns, and supply chain limitations that must be addressed before they can fully replace traditional organic materials. Meanwhile, hybrid solutions provide a middle ground but require careful engineering to balance electrical, thermal, and mechanical tradeoffs.
Advanced bonding technology
As interposer and substrate designs become more complex, traditional micro-bump bonding is reaching its practical limits. With micro-bump pitches typically constrained to 40µm or more, they struggle to support the fine-pitch requirements of modern chiplet architectures. As a result, hybrid bonding has emerged as a promising alternative, enabling sub-10µm interconnect pitches by combining dielectric-to-dielectric and metal-to-metal bonding techniques. However, this shift introduces new manufacturing challenges, particularly in surface preparation, defect mitigation, and process uniformity.
“Hybrid bonding is central to the next generation of high-density interposer technology,” says a UMC spokesperson. “Achieving stable, uniform bonds at the wafer or substrate level is paramount. Variability in bond uniformity across large substrate surfaces is one of the biggest hurdles the industry faces right now.”
To ensure reliable hybrid bonding, manufacturers must create atomically smooth surfaces to prevent the formation of voids and electrical discontinuities. This requires precise surface activation techniques, such as plasma treatment and chemical functionalization, to enhance dielectric bonding strength. Achieving consistent metal-to-metal contact also demands tight control over material removal rates, particularly in direct copper-to-copper bonding, where oxidation and interface contamination can degrade bond reliability.
Beyond hybrid bonding, direct copper interconnects are being explored as an alternative to micro-bumps, eliminating the need for solder and further reducing electrical resistance. By removing intermediate materials, direct copper bonding improves both signal integrity and thermal performance, making it particularly well-suited for high-speed AI and HPC applications. However, this process presents its own set of challenges, including preventing oxidation during bonding and managing the high pressures required to form reliable interconnections.
The move to finer-pitch bonding techniques is placing new demands on modeling and simulation tools, which must keep pace with increasingly complex interposer and substrate architectures. As hybrid bonding and direct copper interconnects scale, ensuring accurate process modeling and defect prediction will be required for high yield.
“Capacity in simulation tools becomes a significant challenge with increasingly complex substrates and interposer assemblies,” says Lang Lin, principal product manager at Ansys. “Deciding where to place high-fidelity models and where approximations are acceptable is key. AI-driven adaptive meshing helps manage this complexity, focusing computational resources on critical hotspots.”
Scaling hybrid bonding and copper interconnects for high-volume manufacturing remains an industry-wide challenge.
“The challenge with manufacturing today isn’t just about pushing to finer dimensions,” says Lam Research’s Lee. “It’s about ensuring that process control keeps up. AI-driven inspection and adaptive process tuning are becoming critical for keeping defect rates low as interposer designs scale up.”
Enhancing reliability at the nanoscale
As interposer and substrate technologies grow more complex, ensuring long-term reliability requires a shift from traditional rule-based design approaches to AI-driven predictive modeling. High-density interconnects and hybrid material integration are introducing new failure mechanisms that must be anticipated and mitigated early in the design process. Advanced simulation tools now integrate multi-physics analysis, allowing engineers to predict issues such as electromigration, thermal gradients, and mechanical stress before a design reaches fabrication.
“Using predictive simulation and AI-driven analysis has become essential,” says the UMC spokesperson. “Simulation allows us to anticipate electromigration and thermal effects on interposers, which directly impacts long-term reliability. This capability is essential as substrate complexity increases.”
The accuracy of these models, however, depends on the quality of input data, particularly for novel materials that lack extensive empirical testing. As interposers move beyond organic substrates into hybrid and glass-based designs, precise characterization of material properties becomes critical. Any mischaracterization of thermal expansion coefficients, dielectric constants, or mechanical stresses can have significant downstream effects on device reliability.
“Material properties become critical as frequencies increase and substrates become hotter,” says Ansys’ Swinnen. “Accurately modeling how these materials behave under real-world conditions is crucial. Mischaracterizing material behavior at the interposer and substrate level can drastically affect device reliability.”
Beyond simulation, defect detection methodologies must evolve to keep pace with next-generation packaging complexity. Traditional optical and electrical test methods often fail to catch subtle defects at the substrate level, necessitating AI-driven inspection techniques. Machine learning algorithms are being deployed to analyze high-resolution imaging data, identifying defects that would be missed by conventional inspection processes.
“Machine learning has become essential for detecting subtle substrate-level defects before final assembly,” says Lesly Endrinal, failure analysis lead at Google. “Traditional inspection methods miss these defects, but AI-based image processing and analytics significantly improve defect detection, improving overall interposer reliability.”
To further improve reliability, manufacturers are integrating design-for-test (DFT) and embedded sensing technologies directly into interposer and substrate architectures. These advancements enable real-time monitoring of critical performance parameters during the manufacturing process, allowing defects to be detected and addressed early.
“Testing substrates and interposers for subtle electrical defects has become significantly more challenging,” says Jack Lewis, CEO of Modus Test. “High-density interconnects complicate traditional testing methods, demanding new approaches, especially on the wafer and panel level.”
By embedding diagnostic capabilities within interposer structures, manufacturers can detect and address potential failures much earlier in the production cycle. This proactive approach is particularly valuable for large-format substrates used in panel-level packaging, where yield optimization is critical.
These AI-enhanced testing and simulation strategies improve first-pass yield and reduce the long-term failure rates of advanced packaging architectures. As interposers and substrates become more active components in the computing stack, ensuring their predictability and reliability will be key to scaling chiplet-based architectures and high-performance computing applications.
Active interposers and intelligent substrates
As interposers and substrates evolve from passive routing layers to intelligent system components, researchers and manufacturers have begun exploring active interposer designs that embed transistors, power management circuits, and even optical interconnects directly into the interposer layer. This shift represents a fundamental transformation in semiconductor packaging, enabling smarter signal routing, adaptive power management, and localized processing.
“As AI workloads grow, reducing power consumption in data movement is critical,” says Lee. “Active interposers can integrate embedded circuits for on-interposer signal conditioning and power regulation, improving efficiency at scale.”
One of the most significant advancements in this area is the integration of optical interconnects into interposers. Traditional copper interconnects face increasing challenges at higher data rates, particularly in AI and HPC applications where minimizing power loss and maximizing bandwidth are critical.
Silicon photonics-based interposers are emerging as a solution, enabling chiplet-to-chiplet optical communication without conversion to the electrical domain. Recent demonstrations have achieved data rates exceeding 200 Gbps per channel, signaling a potential shift away from traditional electrical interconnects.
“Eventually, all packaging direction will be focusing on optical interconnect,” says Lee. “The industry is already moving toward silicon photonics as the ultimate solution for high-speed data transmission.”
Still, the adoption of active interposers introduces new thermal challenges. Higher power densities and embedded transistors generate additional heat that must be dissipated efficiently. Researchers are developing phase-change materials for transient thermal buffering, embedded microfluidic cooling channels, and high-conductivity thermal interface materials to mitigate these challenges. These solutions are designed to smooth temperature fluctuations, dissipate localized heat, and improve long-term reliability in high-power AI and HPC applications.
Despite these promising advancements, the commercial viability of active interposers depends on overcoming several manufacturing challenges. Ensuring high-yield fabrication of embedded transistors and optical components is a major hurdle. So is the development of cost-effective process flows to scale up production. Additionally, the industry must establish robust design and verification methodologies for interposer-integrated circuitry to ensure long-term reliability.
To push active interposers into mainstream will require better co-design methodologies between packaging, chip architects, and system designers. That transition will involve more than just manufacturing breakthroughs. It’s a complete paradigm shift in how the industry thinks about integration.
Conclusion
The semiconductor industry is entering a new era where interposers and substrates are no longer just passive structural elements but essential enablers of advanced computing architectures. As Moore’s Law slows and chiplet-based integration becomes the industry standard, these components are evolving into sophisticated, functionalized platforms that directly impact performance, power efficiency, and reliability.
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