The core of electronic packaging: material selection and performance comparison

The rapid development of the microelectronics industry has continuously improved the performance and complexity of chips. However, the production of chips is only the first step. In order to operate reliably in the real world, a key process is required: packaging.

The package not only protects the chip from the external environment, but also connects the chip to the external circuit to improve its heat dissipation performance and mechanical strength. With the advancement of technology, the requirements for the internal stress, thermal conductivity and electrical properties of the material are getting higher and higher. The selection of different materials directly affects the performance, cost and reliability of electronic products. So, what are the commonly used materials in microelectronic packaging? What performance do they have?

The necessity of electronic packaging

In the microelectronics industry, packaging is used to seal and connect integrated circuits to form electronic systems. It is an important part of ensuring chip performance and reliability. Its core functions include the following aspects:

① Protect the chip: The package provides physical protection for the chip, preventing the chip from being damaged by the external environment (such as moisture, dust, and chemicals), and avoiding mechanical stress, corrosion, and other damage, thereby extending the service life of the chip.
② Heat dissipation: The chip generates a lot of heat when working, and excessive temperature will affect its performance and even cause failure. The packaging material and structure help the chip maintain a suitable operating temperature range by effectively conducting and dissipating the heat, ensuring its stable operation.
③ Electrical connection: The package connects the chip to the external circuit through pins, solder balls, or other conductive paths to ensure the integrity and stability of signal transmission. In addition, the package can effectively reduce signal delay and crosstalk, and improve the overall performance of the system.
④ Mechanical support: The package provides mechanical support for the chip to prevent the chip from being damaged by vibration or pressure during assembly or operation, and ensure its reliability in various application environments.

Through these functions, packaging plays an indispensable role in the microelectronics industry, not only protecting the physical structure of the chip, but also improving the overall performance and reliability of the system.

Electronic packaging material classification and performance comparison

According to the packaging structure, electronic packaging materials are divided into substrates, wiring, frames, interlayer dielectrics and sealing materials. As the size of chips becomes smaller and lighter, and the heat dissipation and electrical properties become better and better, ideal electronic packaging materials must meet the following basic requirements:
(1) Low internal stress;
(2) Good thermal conductivity;
(3) Good chemical stability;
(4) High strength and rigidity to support and protect the chip;
(5) Good processing and welding performance, easy to process;
(6) To meet the demand for decreasing chip size and mass, the density of the material should be as small as possible.

The following are several common types of materials used in microelectronic packaging, such as plastics, ceramics, metals, and composite materials. Due to the development of lead-free packaging and the need for heat dissipation caused by high power, conductive adhesives and thermal interface materials have become popular materials that are being studied more in packaging.

1. Types of polymer packaging materials

Polymers are widely used in packaging. They can be used as adhesives to stick semiconductor chips to metal frames, as mold compounds for stacked packaged chips, as embedded capacitor materials for energy storage, and as electromagnetic interference shielding materials for electromagnetic waves that do not need to be attenuated. According to the type of polymer, packaging materials can be mainly divided into epoxy resins, acrylates, polyurethanes, polyimides, polyparaxylene, and silicones.

2.Conductive adhesive

Conductive adhesive plays an important role in packaging, especially in low-temperature welding and miniaturization applications. It combines conductive particles through the matrix resin to form a reliable conductive path and realize the connection of circuit components. Compared with traditional tin-lead welding, conductive adhesive is more environmentally friendly and has a lower curing temperature (such as epoxy resin curing at room temperature to 150°C), avoiding the damage that high-temperature welding may cause to the product. In addition, conductive adhesive also adapts to the miniaturization and high-density packaging requirements of electronic components, providing high-resolution conductive connections, and therefore occupies an important position in modern packaging technology.

Currently, most of the conductive adhesives used in the market are filler-type. The resin matrix of filler-type conductive adhesives is generally used in thermosetting adhesives such as epoxy resins, silicone resins, polyimide resins, phenolic resins, polyurethanes and acrylic resins. Adhesive systems. These adhesives form the molecular skeleton structure of the conductive adhesive after curing, providing mechanical properties and bonding performance guarantees, and then conductive particles with good conductivity and suitable particle size are added to the conductive adhesive matrix to form a conductive path. Conductive fillers are mainly metal fillers such as gold, silver, copper, aluminum, zinc, iron, nickel, etc. and carbon-based materials such as graphite, graphene, carbon nanotubes and fullerenes, but can also include ceramic fillers such as aluminum nitride (AlN), boron nitride (BN), aluminum oxide (Al2O3) and silicon dioxide (SiO2), as well as composite materials such as polyimide-modified aluminum nitride fillers.

3.Thermal Interface Materials

Thermal interface material (TIM) is a general term for materials used to coat between heat-dissipating devices and heating devices to reduce the contact thermal resistance between them. An ideal thermal interface material generally needs to have high thermal conductivity, high flexibility and high insulation properties. Its application in computer heat dissipation and the schematic diagram of the two heat dissipation architectures are shown in the figure below. Since most polymers are poor thermal conductors, according to the phonon thermal conduction theory and free volume theory, improving the order of the polymer or adding nano-sized high thermal conductivity fillers to the polymer can significantly improve the thermal conductivity of the material. performance.

2.Conductive adhesive

The first is to improve the thermal conductivity of the insulating resin itself. Improve the order of the polymer chain so that phonons can be scattered more. The molecular chain structure has a rigid skeleton or strong forces between molecules (such as hydrogen bonds), which can inhibit internal rotation and achieve higher thermal conductivity. Heat is more easily transmitted along the chain, and the orientation of the molecular chain has a great influence on the condensed structure and thermal conductivity of the polymer. Liquid crystal epoxy (LCE) molecules have the ability to self-assemble, and have self-orientation characteristics and specific directions in the micro-region, as shown in the figure.

Commonly used thermal conductive fillers include metals (metal particles, metal fibers, etc.), oxides (Al2O3, BeO, MgO, etc.), nitrides (AlN, BN, Si3N4, etc.) and carbides (SiC, graphite, graphene and CNT, etc.). The type, loading, size, morphology and adhesion between the fillers and the matrix all affect the thermal conductivity of the material.

① Metal fillers: Metal fillers have high thermal conductivity, but metals are all conductive, which can easily lead to short circuits, limiting their application in thermally conductive insulating materials. In addition, metal fibers and particles have poor compatibility with polymers, which makes composite materials prone to microphase separation, resulting in low thermal conductivity of composite materials. The above problems can be overcome by polymer grafting modification of metal surfaces.

② Ceramic fillers: Ceramics themselves are commonly used insulating thermal conductive materials, and the use of ceramic fillers to improve thermal conductivity has been widely studied. Generally speaking, except for BeO, metal oxides (such as Al2O3, SiO2, etc.) have low thermal conductivity, while nitrides (AlN, BN, Si3N4, etc.) have high thermal conductivity.

③Carbon-based fillers:Carbon-based materials have higher intrinsic thermal conductivity than ceramic materials, so a small amount of carbon-based filler can significantly increase the thermal conductivity of the material. Among carbon-based materials, graphite has relatively low thermal conductivity, carbon fibers and carbon nanotubes are one-dimensional fillers with high thermal conductivity along the longitudinal direction, and graphene is a two-dimensional carbon material with a single-layer structure.

In short, in the packaging process, material selection plays a decisive role, not only affecting the heat dissipation efficiency, mechanical strength and electrical performance, but also directly related to the reliability and life of the chip. Appropriate material selection needs to find a balance between cost, environmental protection and processing difficulty to meet the needs of modern microelectronic products for high performance, miniaturization and sustainable development. At present, the research and development direction of packaging materials is focused on more efficient heat dissipation materials, more environmentally friendly packaging solutions, high reliability and low-cost materials, and multifunctional integrated materials. These innovations will further promote the development of packaging technology and support the continuous progress of the microelectronics industry.