What is 2.5D Packaging?

To understand what 2.5 packaging is, it is important to know the evolution of semiconductor packages, beginning with the humble 2D packaging to the development of 2.5D and 3D advanced packaging.

2D, 2.5D, and 3D packaging are major types of integrated circuit (IC) packaging used in semiconductor devices. IC packaging is the process of enclosing a semiconductor die in a protective structure that ensures mechanical support, environmental protection, and reliable electrical and thermal performance. A well-designed package keeps out moisture and contaminants, manages heat dissipation, and enables interconnection through bond wires, bumps, or leadframes. For encapsulation, epoxy molding compounds (EMCs) and liquid encapsulants are the most widely used materials, with the choice depending on the specific IC package design.

2D packaging was first developed in the 1990s, which collectively referred to assembling integrated circuits either through wire bonding and flip-chip ball grid array (FCBGA). In 2D packaging, chips are arranged side-by-side on a single plane. 

With the continuous pursuit of innovating semiconductor packages, design limitations of 2D packaging paved the way for the development of more advanced IC packaging:

With the demand for more complex and more powerful ICs, 2.5D IC packaging was developed in response to these design challenges posed by its 2D predecessor. In this type of IC packaging, a silicon interposer sits between the Si chips and the mounting surface. This silicone interposer is etched with wiring patterns, which allows multiple connections between different chips mounted on its surface. Typically, two or more semiconductor chips are placed side-by-side on top of the interposer. Its design allows for combining heterogenous Si chips (i.e., central processing units and memory chips) within a package. 

Listed below are some key advantages of 2.5D IC packaging:

These advanced features of 2.5D IC packages make it suitable for applications requiring high computing power. This includes high power computing and integration of high bandwidth memories of high-end graphics processing units, field programmable gate arrays, switches and routers of data centers & 5G infrastructures, and artificial intelligence accelerators. In data center processors, heterogenous 2.5D IC packaging is used to connect CPUs with memory semiconductors.

Anatomy of a 2.5D Package

2.5D IC package is a type of packaging technology that allows multiple dies, or chips, to be placed side-by-side on a silicon interposer. This setup provides a high-bandwidth, high-density connection between the chips, and is almost a 3D package, where the dies are stacked vertically. Here are the main components of a typical 2.5D IC package:

Figure. Structure of a 2.5D Package

Table: Components of a 2.5D IC Package

Design Challenges in 2.5D IC Packaging

Manufacturing Complexities

Silicon interposers are expensive to fabricate. These interposers require delicate handling and the TSVs that connect the interposer to the package substrate must be precisely formed. Not to mention, the risk of interposer warpage is high due to mismatches between the coefficients of thermal expansion between the different materials it is exposed to. Warping makes it difficult to achieve precise alignment in die-to-die and interposer-to-substrate connections. 
Warpage is a major problem in semiconductor packaging, especially with 2.5D ICs. It happens when a package or wafer bends or distorts, often because different materials have mismatched CTEs. A 2.5D package is made up of various materials, like silicon dies, interposers, and molding compounds. These materials expand and contract at different rates when exposed to temperature changes during manufacturing processes, such as curing and thermal cycling. This can result in several issues, including:

Thermal Management

While silicon interposers are great for electrical connections, they pose a big challenge when it comes to managing heat. This is because their thermal conductivity is poor, which measures how well a material can transfer heat.

Bulk silicon, the material most chips use, has a high thermal conductivity of about 150 W/m·K. This means it effectively moves heat away from hot spots.

In contrast, silicon interposers have a much lower thermal conductivity of around 1.5 W/m·K. This happens because they are not just solid silicon; they contain thousands of tiny through-silicon vias (TSVs) filled with metal and complex wiring layers. This complicated structure disrupts the thermal path, causing the interposer to act more like an insulator than a conductor.

This low thermal conductivity creates a thermal bottleneck. It means that heat produced by the high-power dies on the interposer cannot easily dissipate downwards into the package. Instead, the heat gets trapped, leading to two major problems:

Interconnect Length and Signal Integrity

Integrating multiple dies on a passive interposer adds challenges in providing power and keeping signal quality intact.

CAPLINQ Solutions for 2.5D IC Packaging

Controlling Warpage

Henkel LOCTITE ECCOBOND LCM 1000AG-1 is an epoxy-amine based encapsulant designed for liquid compression molding (LCM) in advanced wafer-level packaging.

Liquid Compression Molding (LCM) is less stressful on delicate components than traditional transfer molding. In LCM, a precise amount of the liquid compound is dispensed onto the wafer, which is then pressed and cured under controlled conditions. This method:

• Applies lower pressure, which reduces mechanical stress on thin, fragile dies.  
• Allows for void-free filling of the fine gaps between dies, crucial for high-density packages.  
• Enables fast in-mold curing, increasing manufacturing throughput without compromising the material's integrity.

As an overmolding material, it is particularly valuable for 2.5D packaging, where it helps address key challenges such as warpage control, fine gap filling between closely spaced dies, and overall package reliability. The following are the key features of Henkel LOCTITE ECCOBOND LCM 1000AG-1:

More Information About LCM 1000AG-1

Thermal Management Solutions

Additionally, the close proximity of high-power components creates hotspots and increases the thermal design power, making it hard to dissipate heat effectively. The different materials in the package, like the silicon chips, interposer, and substrate, have different coefficients of thermal expansion. This mismatch can cause mechanical stress, warpage, and delamination, hurting the package's long-term reliability, especially during thermal cycling.

Thermal interface materials (TIMs) are designed to address these challenges by providing a highly efficient thermal pathway while accommodating mechanical stress. PTM7950 series offers a combination of high thermal performance and long-term reliability.

More Information About Thermal Interface Materials