Many people assume packaging only means putting a housing around a chip. In reality, packaging engineering is a tiered integration system spanning from the die itself to the end product. It consists of five technical levels, designated Level 0 through Level 4. Each level has a clear core mission, much like building a house: first lay the chip’s “foundation” (Level 0 / Level 1), build functional “modules” (Level 2), assemble them into “floors” (Level 3), and finally construct the complete “building” (Level 4). Here we break down the core logic, process details and application value of these five levels, and explain how packaging has evolved from merely “protecting the chip” to “enhancing system performance”.

Diagram labels from the original figure: Wafer — Level 0 Module — Level 1 Card — Level 2 Board — Level 3 System / Equipment — Level 4

1. Level 0

Level 0 is the most fundamental yet often overlooked stage: on-die interconnect fabrication. It refers to the process during wafer manufacturing where transistors, capacitors and other components on the die are interconnected via metal routing (e.g., copper, aluminum) to form complete circuit functions. Although this belongs to front-end wafer processing, it is classified as “Level 0 packaging” because it serves as the foundation for all subsequent packaging stages — if on-die interconnect fails, no precision packaging later can compensate.

The core requirement for Level 0 is fine patterning and reliability. As process nodes advance to 3 nm and 2 nm, metal line widths shrink below 10 nm, requiring mitigation of electromigration and crosstalk. For example, copper damascene processing is used for routing, paired with low-k dielectric materials to reduce signal delay and ensure efficient electrical signal transmission inside the die. Quality at this stage directly determines the baseline performance of the chip and is the prerequisite for interconnection across all higher packaging levels.

2. Level 1

Level 1 is the core first step of packaging engineering. Its primary task is converting a bare die into an assemblable module. It comprises three key processes: die attach, interconnection, and encapsulation.

• Die Attach: The bare die is fixed onto a package substrate or leadframe using materials such as epoxy adhesives or solders, delivering both mechanical fixation and thermal dissipation.

• Interconnection: Gold wire bonding, copper pillar interconnect or TSV (Through-Silicon Via) are used to connect the die I/O pads to the package substrate/leadframe, enabling signal and power transfer between the chip and the outside world. Copper pillar interconnect is commonly used in mobile phone chips, while 3D packaging relies on TSV for stacked-die interconnection.

• Encapsulation: The die is covered with molding compound, ceramic or metal to isolate it from moisture, dust and mechanical impact. Low-cost molding compounds are widely used in consumer electronics, whereas ceramic packaging is adopted for automotive electronics and aerospace applications due to its high reliability.

The final output of Level 1 is a packaged device, transforming the fragile bare die into a standardized component that can be handled and batch-assembled. The key challenge here is balancing miniaturization and reliability. For example, a CSP (Chip Scale Package) occupies only about 1.2 times the die area, yet must maintain stable operation for more than 5 years across the −40 °C to 85 °C temperature range.

3. Level 2

At Level 2, packaged devices from Level 1, together with passive components such as resistors, capacitors and inductors, are assembled onto a circuit card (PCB sub-card) to form a module with dedicated functions. Typical examples include smartphone RF modules and computer graphics cards.

The core process here is Surface Mount Technology (SMT): a mounter places packaged devices precisely onto PCB pads, followed by reflow soldering to form electrical joints. Key requirements are high-precision placement and interconnect reliability. As device dimensions shrink, placement accuracy must be controlled within ±5 μm; meanwhile, thermal stress during reflow must be prevented from causing package cracking or defective solder joints.

The value of Level 2 is functional integration: discrete components are combined into an independently functional module, simplifying downstream assembly and facilitating maintenance and replacement. For instance, an automotive radar module integrates the radar chip, signal processing chip and power management chip onto a single PCB at Level 2, and can be directly installed inside the vehicle bumper.

4. Level 3

Level 3 assembles multiple Level 2 functional modules onto a main printed circuit board to form a complete subsystem. Typical examples include smartphone motherboards and server motherboards.

The core challenge here is multi-module coordination: it requires handling signal interconnection between different modules, managing thermal dissipation, and ensuring mechanical stability to prevent module loosening during transportation and operation. On an AI server motherboard, high-power modules including CPU, GPU, HBM memory and I/O expansion modules must be integrated together. This demands high-speed interconnect standards such as PCIe 5.0 and DDR5, while liquid or air cooling keeps total power consumption at the kilowatt level. Quality at Level 3 directly determines subsystem performance, reliability and thermal efficiency.

5. Level 4

Level 4 is the final stage of packaging engineering: multiple Level 3 subsystems are integrated with mechanical structures, peripherals and power supplies into the final electronic product. Examples include smartphones, laptops and complete server systems.

The core requirement here is system compatibility: electrical matching, mechanical fit and thermal balance must be ensured across all subsystems. System debugging and reliability testing are also performed, such as drop tests for phones and long-duration stability tests for servers, to confirm the product meets operational requirements. Level 4 embodies the transition “from technology to product”, converting abstract semiconductor technology into end devices that meet user needs, fulfilling the ultimate mission of packaging engineering.

Flexible Reconfiguration of Packaging Levels

It should be noted that the division of packaging levels is not absolute. With the development of advanced packaging technologies, boundaries between levels are gradually blurring.

• Skipping Level 1: Chip-on-Board (COB) technology directly attaches bare dies to a Level 2 PCB, eliminating the Level 1 substrate and molding to achieve light, thin, compact form factors. It is commonly used for display driver ICs and low-cost consumer electronics.

• Level Fusion: Hybrid electronic circuits combine Level 1 and Level 2 techniques; resistors and capacitors are fabricated directly on a ceramic substrate before die attachment, delivering both high density and high reliability, used in aerospace electronics.

• Three-Dimensional Integration: 3D packaging and Chiplet technologies stack dies and interconnect chiplets, integrating multiple Level 1 dies inside a single package. This delivers multi-level integration functionality within a Level 1 package, greatly boosting system performance and integration density.

These flexible packaging forms essentially adapt to different product requirements: consumer electronics prioritize miniaturization and low cost; aerospace emphasizes high reliability and environmental robustness; advanced computing targets high performance and high density.

The five packaging levels represent a system integration logic “from micro to macro”: Level 0 solves on-die interconnection, Level 1 connects the die to the outside, Level 2 builds functional modules, Level 3 integrates subsystems, and Level 4 forms the end product. Each level links the prior and next stages, and none can be omitted. As semiconductors evolve toward advanced packaging and heterogeneous integration, the boundaries between levels will continue to blur, but the core logic of tiered integration remains unchanged. Packaging is no longer simple “protection and interconnection”; through optimized coordination across all levels, it becomes a critical factor in improving system performance, shrinking product size and reducing cost.

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