The high bandwidth requirements of AI workloads are pushing semiconductor manufacturers toward 2.5D and 3D packages, in which multiple dies or chiplets are assembled within a small area and at high interconnect density. However, increasing the number of interconnects does not solve the performance problem on its own; instead, it increases the difficulty of managing heat, mechanical stress, and electromigration within the package.
The analysis published in Semiconductor Engineering on August 20, 2026, focuses on the role of the interposer in addressing these challenges. It reviews research findings on material expansion, package deformation, and damage to redistribution layers, in addition to the effect of copper microstructure on the lifetime of interconnect lines.
Why Is the Industry Moving Toward 2.5D?
3D packages achieve the highest possible interconnect density by stacking dies vertically using hybrid bonding technology and through-silicon vias (TSVs). However, this close integration creates major challenges in alignment, testing, and reliability. Heat dissipation also becomes more difficult because the size of active devices increases faster than the area available for removing heat. Rising temperatures can cause TSVs to expand, generating stresses that lead to cracking or delamination between layers.
By contrast, 2.5D packages place dies on a passive interposer that redistributes signals among multiple dies or between the dies and the package substrate. This interposer typically contains wiring, and possibly capacitors and resistors, but it does not include transistors or other switches. It is therefore simpler than active circuits in terms of manufacturing and modeling, while avoiding some of the reliability problems associated with 3D packages.
However, the fact that the interposer is a passive component does not mean that its design is easy. It must provide interconnects and mechanical support and contribute to heat dissipation, while the package consists of materials that differ substantially in their coefficients of thermal expansion (CTE). Silicon and copper have relatively low expansion coefficients, whereas the package substrate, molding compound, and organic underfill materials expand to a greater degree.
Thermal Deformation Does Not Produce a Simple Shape
When the package is heated and cooled during manufacturing, or when resistive heat is generated during operation, the layers expand and contract at different rates. In a package containing a single die, it may be relatively easier to model this behavior, but heat during operation is not distributed evenly; it depends on the signal paths and operating patterns within the die.
The complexity increases when multiple dies are integrated. Two adjacent dies may generate opposing stresses on the interposer and the substrate beneath them, producing a saddle-shaped deformation rather than a simple concave or convex bend. Strengthening one area to prevent buckling may also transfer stress to another area instead of eliminating it. The result may be failure of the interconnect between the interposer and the substrate or delamination of the redistribution layers (RDLs).
Hakjun Kim and colleagues at Seoul National University found that thermomechanical failure represents one of the most difficult reliability challenges in 2.5D packages. Ming-Sheng Luo and his team at South China University of Technology also identified the spacing between dies and the interconnect between the interposer and the molding compound as areas vulnerable to failure in a model containing two dies and three redistribution layers. For larger packages, the researchers proposed using dummy dies, which are bare silicon chips, to balance stresses.
The problem is not limited to heat. Moisture absorption can cause materials to expand at different rates, and a material's moisture-related behavior does not necessarily match its thermal behavior. This adds another load to reliability models, particularly in Fan-Out Wafer-Level Package structures.
Electromigration in Copper Lines
As current density within the package increases, electromigration becomes a more significant cause of interconnect failure. This phenomenon occurs when high current gradually moves metal atoms, potentially producing voids or damage in the lines.
Yi-Quan Lin and colleagues at National Yang Ming Chiao Tung University studied the effect of the crystal structure of copper lines fabricated using electroplating processes. The results showed that copper with a nanocrystalline structure may increase susceptibility to electromigration, apparently because grain boundaries facilitate diffusion. By contrast, nanotwinned copper, particularly after heat treatment, had a strong (111) crystal texture and a high proportion of coherent twin boundaries, which helped limit the formation of oxides and voids.
Another study associated with ASE Group indicated that lines with smaller grains achieved lifetimes approximately seven times those of lines with larger grains in RDL structures examined during operation. This suggests that controlling microstructure may be a way to improve reliability without relying solely on alloying additions or dielectric layers inside the trenches, solutions that may increase circuit resistance or manufacturing complexity.
What Changes in Practice?
Evaluating an interposer design solely from the perspective of signal density or path length is not sufficient. Material selection, die distribution, layer thickness, copper structure, and interconnect locations all affect heat paths, stresses, and package lifetime. A solution that reduces deformation in one area may increase the risk of delamination in another.
For this reason, researchers are turning to deep-learning-based surrogate models. Finite element simulation enables detailed study of individual regions, but applying it to all regions and layers in a complex package becomes computationally expensive. Initial results indicate that multiscale deep-learning models can reduce the need to rerun finite element analysis for every case and provide a practical way to predict deformation and nonlinear behaviors on a larger scale. These models do not mean that physical simulation will be eliminated; rather, they can serve as surrogate models that accelerate design-space exploration and identify cases requiring more detailed analysis.