In TSMC's 3D Fabric technology system, advanced packaging technologies including InFO, CoWoS, and SoIC have become the benchmark in the field. As chip size decreases and packaging complexity increases, strain and stress pose new challenges to the reliability, performance, and design of packaging. Exploring the core issues and solutions in advanced packaging is of great industrial significance.
Part 1
What is Advanced Packaging? Core Questions and Analysis
Advanced packaging is a key link in the modern integrated circuit manufacturing field. It breaks through the limitations of traditional packaging technology and aims to achieve higher chip integration, stronger performance, and better functional diversity.
Traditional packaging focuses primarily on the physical protection and basic electrical connections of chips, while advanced packaging uses innovative design concepts and process technologies to integrate multiple chips or chip modules in three dimensions, enabling high-speed interconnection, heterogeneous integration, and system-level optimization between chips.
In simple terms, advanced packaging technology integrates multiple chips into a single package module, achieving performance improvements, power optimization, and miniaturization. Through technologies such as fan-out packaging, silicon interposers, and redistribution layers (RDLs), it breaks through the limitations of traditional packaging, providing higher I/O density, lower latency, and higher signal integrity.
TSMC's InFO, CoWoS, and SoIC technologies embody the development direction of advanced packaging. For example, InFO achieves multi-chip integration through redistribution layer technology; CoWoS provides high-density interconnects through silicon interposers; and SoIC achieves true three-dimensional integration through wafer-to-wafer (WoW) stacking.
However, while advanced packaging brings performance and efficiency improvements, it also faces core challenges.
Core issues in advanced packaging:
● Thermal and mechanical stress challenges
◎ In heterogeneous chip components, thermal and mechanical stress issues are particularly prominent. As substrates become thinner to shorten signal transmission distances, the heat dissipation efficiency of silicon substrates decreases, and warping caused by lattice mismatch and uneven heating and cooling phenomena occur frequently.
This not only puts enormous stress on the interconnect structure, making it difficult to maintain stable contact between thousands of microbumps, leading to performance degradation and reduced yield, but also significantly increases the time and cost required to address all possible physical effects, dependencies, and interactions. For example, in multi-chip designs, differences in the coefficient of thermal expansion (CTE) of materials between different chips can induce stress when temperatures change, potentially causing reliability issues such as chip cracking, delamination, or interconnect failures.
◎ From the manufacturing process perspective, stress will be generated inside the device due to changes in temperature profiles and material CTE mismatch during reflow soldering and other processes.
This stress not only affects the stability of the mechanical structure, but also influences the electrical behavior of the transistor, changing macroscopic parameters such as the resistance of the wires and the threshold voltage of the transistor, thus causing unpredictable interference to the performance of the entire chip.
● Increased complexity in architecture design
◎ Chipsets face challenges in advanced packaging regarding trace density and architecture optimization. In 2.5D or 3D integrated devices, the trace density between chips differs significantly from that of traditional 2D chipsets, ranging from tens of nanometers to hundreds of micrometers.
This means that maintaining the routing density at the chipset boundary comes at a higher cost, including higher power consumption, larger footprint, and higher latency. For example, when designing data interfaces, these overheads need to be considered comprehensively at the architectural level to ensure a balance between performance and cost in specific application scenarios.
◎ In addition, the chip in advanced packaging is more closely integrated with the SoC itself, unlike the traditional PCIe interface which has full interoperability.
During chip decomposition and integration, data interfaces need to be carefully designed, and factors such as bus flow patterns, latency and throughput tolerance need to be thoroughly understood in order to achieve a high degree of adaptation to specific applications. This undoubtedly increases the complexity and difficulty of architecture design.
● Multiphysics Interactions and Modeling Requirements
◎ Thermal, mechanical, and electrical effects are increasingly interrelated and interdependent in advanced packaging, creating a complex multiphysics environment. For example, heat can cause stress, which in turn can cause bending and affect transistor behavior, thereby altering the electrical performance of the circuit.
This interaction makes it difficult for traditional single-physics analysis tools to meet design requirements, and the need for tools that can simultaneously simulate multi-physics effects is becoming increasingly urgent.
Engineers typically use finite element analysis (FEA) solvers to address stress and strain problems early in the packaging design process, but reliability issues in high-stress areas remain serious, with risks such as interconnect failures, chip cracking, or delamination still existing.
In analog design, if the impact of strain on the electrical behavior of transistors is not considered in advance, it may lead to unpredictable deviations in circuit behavior, affecting the functional correctness and stability of the entire chip.
● To address the above issues, a series of optimization strategies were proposed for its technology platform.
◎ By utilizing InFO's high-density redistribution layer (RDL) and micro-bump technology, stress concentration caused by differences in thermal expansion coefficients is reduced;
◎ The deep trench capacitor structure using CoWoS silicon interposer board enhances signal integrity and power stability. SoIC reduces mechanical stress through precise alignment and bonding technology in wafer-to-wafer stacking.
Part 2
Some handling methods and core suggestions
Thermal analysis should be incorporated into architecture exploration and planning early in the design process. By performing thermal modeling and analysis on the entire multi-chip stack (including chips, interposers, packages, and PCBs), potential hot spots and thermal coupling issues can be predicted, allowing for early optimization of the power distribution network design to control heat generation and propagation, and avoid stress concentration and performance degradation caused by thermal problems.
In-depth research into the thermal coupling effect between chips, considering the heat dissipation characteristics and layout relationships of different chips, rationally planning the stacking order and spacing of chips, optimizing heat dissipation channels, ensuring that heat can be effectively dissipated, and reducing the impact of thermal stress on chips and interconnect structures.
Optimize material selection and process control, carefully select materials that are compatible with CTE, and reduce stress problems at the interfaces of different materials.
For example, when selecting silicon interposers, substrates, and chip bonding materials, their CTE compatibility should be fully considered to reduce stress differences caused by temperature variations. Close collaboration with OSATs and foundries is crucial to obtain accurate material data and gain a deep understanding of the impact of manufacturing processes on material properties and stress-strain. During manufacturing, strict control of process parameters, such as reflow soldering temperature profiles and bonding pressure, is essential to minimize additional stress introduced by the manufacturing process.
Improve multiphysics modeling and simulation, and develop and apply tools and workflows that can integrate thermal, mechanical, electrical and other multiphysics analysis.
Through multiphysics simulation, the interaction between various physical domains is fully captured, and the impact of stress and strain distribution on chip performance and reliability is accurately analyzed, providing an accurate basis for design optimization. Digital twin technology is used to fully simulate and verify the chip design, manufacturing and operation process in a virtual environment.
By establishing high-precision digital models, we can predict in advance the long-term interconnect behavior, stress and strain changes, and performance of chips under different environmental conditions, reducing reliance on expensive and time-consuming physical testing and improving design efficiency and product quality.
● Developing low-stress, high-reliability materials: Key materials in packaging, such as adhesives, silicon interposers, and the packaging substrate, require optimized thermal expansion coefficient (CTE) matching. Employing novel materials with low modulus and high thermal conductivity helps reduce packaging stress and improve heat dissipation.
● Enhanced EDA toolchain support: Develop dedicated, encapsulated EDA toolchains for multi-physics domain coupled designs. For example, a unified platform integrating stress simulation and electrothermal coupling analysis can improve design efficiency and reduce verification costs.
● Improved thermal management solutions: Introducing advanced heat dissipation technologies such as microfluidics, liquid metal thermal interface materials (TIM), and thermoelectric cooling modules (TEC) can significantly improve thermal management capabilities and meet the needs of high-performance chips.
● Improve manufacturing yield and process stability: In advanced packaging manufacturing, real-time monitoring and feedback systems are used to optimize process parameters, improve yield, and gradually expand process capabilities through small-batch pilot production to reduce the risks of new technologies.
With the continuous development of integrated circuit technology, advanced packaging technology is playing an increasingly crucial role in improving chip performance and functional density.
TSMC's advanced packaging technology is among the industry leaders, and its various packaging types under the 3D Fabric system provide strong support for many fields such as high-performance computing, artificial intelligence, and mobile devices. However, strain and stress issues are core challenges in advanced packaging and require high attention from all parties in the industry.
summary
Advanced packaging technology is driving the chip industry from functional integration to system-level optimization. The complexity of this technology also places higher demands on the design capabilities, manufacturing capabilities, and ecosystem collaboration across the industry chain. Faced with the challenges of strain and stress, optimizing materials, improving design tools, and strengthening ecosystem collaboration are inevitable trends for future development.
