ACF
Anisotropic Conductive Adhesive Film
1 Introduction
As electronic products become lighter, thinner, shorter and smaller With rapid development, almost all kinds of portable electronic products have LCDs as display panels. Especially in products such as camcorders, notebook computers, mobile terminals or personal digital processors, LCDs have become an important component. . In addition to the LCD panel, the LCD monitor must be connected to a driver chip on its periphery for control of display signals. Generally speaking, the interface connection technologies between LCD panels and drive IC systems can be roughly divided into the following types: Tape Automated Bonding (TAB), Chip on Glass; COG), die-soft board bonding technology (Chip on Flex; COF).
2 Anisotropic Conductive Film (ACF)
2.1 What is anisotropic conductive adhesive: It is characterized by the difference between the Z-axis electrical conduction direction and the XY insulation plane The resistance characteristics are significantly different. When the difference between the Z-axis on-resistance value and the XY plane insulation resistance value exceeds a certain ratio, it can be called good conductive anisotropy.
2.2 Principle of conduction: Use conductive particles to connect the electrodes between the IC chip and the substrate to make them conductive. At the same time, it can avoid conduction and short circuit between two adjacent electrodes, and achieve only Z The purpose of axial conduction.
2.3 Product classification: 1. Anisotropic conductive paste. 2. Anisotropic conductive film. Anisotropic conductive film (ACF) has the characteristics of continuous processing (Tape-on-Reel) with extremely low material loss, so it has become a more commonly used product form currently.
2.4 Main components: Mainly including resin adhesive and conductive particles. In addition to moisture-proof, heat-resistant and insulating functions, the resin adhesive mainly fixes the relative position of the electrodes between the IC chip and the substrate, and provides a pressing force to maintain the contact area between the electrodes and conductive particles.
Generally, resins are divided into two categories: thermoplastic resins and thermosetting resins. Thermoplastic materials mainly have the advantages of low-temperature bonding, fast assembly and easy rework, but they also have the disadvantages of high thermal expansion and high moisture absorption, making them prone to deterioration at high temperatures and unable to meet the requirements for reliability and reliability. Thermosetting resins such as epoxy resin (Epoxy), Polyimide, etc. have the advantages of high temperature stability and low thermal expansion and hygroscopicity. However, they have the disadvantages of high processing temperature and difficulty in reworking, but their advantages of high reliability are still The most widely used material at present.
In terms of conductive particles, the anisotropic conductive properties mainly depend on the filling rate of conductive particles. Although the conductivity of anisotropic conductive adhesives will increase as the filling rate of conductive particles increases, it will also increase the probability of short circuits caused by contact between conductive particles.
In addition, the particle size distribution and distribution uniformity of conductive particles will also affect the anisotropic conductive properties. Generally, conductive particles must have good particle size uniformity and true roundness to ensure that the contact area between the electrode and the conductive particles is consistent, maintain the same on-resistance, and at the same time avoid that part of the electrode is not in contact with the conductive particles, resulting in an open circuit. situation occurs. The common particle size range is between 3 and 5 μm. Conductive particles that are too large will reduce the number of particles in contact with each electrode, and may also easily cause the conductive particles of adjacent electrodes to contact and short circuit; conductive particles that are too small will easily cause The problem of particle aggregation causes uneven particle distribution density. In terms of types of conductive particles, metal powders and polymer plastic balls coated with metal are currently the main ones. Commonly used metal powders include nickel (Ni), gold (Au), gold plating on nickel, silver and tin alloy, etc.
Under the current trend of reliability and fine pitch, the anisotropic conductive adhesive used in COF and COG structures has the characteristics of conductive particles with multi-surface nickel-plated and gold-plated polymer plastic powder. The plastic core is compressible, so it can increase the contact area between the electrode and the conductive particles and reduce the on-resistance; at the same time, the thermal expansion of the plastic core and the basic resin raw material are relatively close, which can avoid thermal cycles and thermal shock environments. In high or low temperature environments, the difference in thermal expansion between the conductive particles and the resin base material reduces the contact area with the electrode, causing the on-resistance to increase and even open circuit failure to occur. 2.5 Lamination process: Normally, conductive particles are evenly distributed in the adhesive and do not contact each other. In addition, there is an insulating film. The ACF film is non-conductive. When the ACF film is pressurized and heated (generally pressurized and heated twice , the first time is temporarily attached to the product at 60 ℃ ~ 100 ℃, (3 ~ 10) × 104 Pa, 2 s ~ 10 s for shipment, the second time is about 150 ℃ ~ 200 ℃, (20 ~ 40） It is 3 to 4 times that of the original. Heating solidifies the adhesive and maintains the conductive state. Generally, the resistance of the conductive part is below 10 Ω, and the resistance between adjacent terminals of the non-conductive part is above 100MΩ.
3 Main ACF brands and differences
3.1 Sony ACF (Single Layer)
Sony has developed an advanced ACF technology called Microconnector, which is used in COF, COG Engage. This ACF material is mainly a breakthrough development in the production of conductive particles. In addition to plating the metal layer on the surface of the plastic core as usual, the conductive particles are also coated with a 10nm thick insulating layer on the surface of the metal layer. This insulating layer is composed of extremely fine resin particles.
The resin adhesive of the developed material can be a thermoplastic or thermosetting material, and then conductive particles are added to form a paste or film-like product. When this material is attached to the flexible substrate for hot pressing, the conductive particles, chip bumps and flexible substrate electrodes will simultaneously crush the insulation layer on the contact surface (i.e. the Z-axis direction), but not in the XY plane direction. The insulation layer will not be crushed and maintain its insulation. Therefore, Sony believes that the use of conductive particles coated with an insulating layer can increase the particle density of anisotropic conductive adhesives to meet the requirements of fine pitch and low on-resistance without causing short circuits.
3.2 Hitachi ACF (Double Layer)
In response to the requirements of fine pitch, Hitachi Chemical has proposed an ACF with a double layer structure. The upper layer of the double layer structure is an unknown layer. A resin layer to which conductive particles are added, while the lower layer contains an arrangement of a single layer of conductive particles. Compared with the traditional single-layer ACF, the double-layer structure can achieve a higher density of conductive particles without increasing the density of conductive particles. Because the local particle density of the lower layer is higher, the particle density within the unit contact area of ??the electrode is higher. At the same time, it is close to the chip bumps. area, the occurrence of short circuits is reduced due to the lower local particle density.
In terms of resin adhesives, for the sake of reliability, Hitachi Chemical chooses to use epoxy resin systems in its products to improve the material's adhesive strength, glass transition temperature and moisture resistance.
3.3 3M ACF
3.4 Toshiba ACF
4 Reliability requirements and testing
See national standards GB18910-2008, IEC61747.5
Storage method and usage period of ACF
1. Storage conditions of unopened ACF: -10~5℃, its usage period is six months after manufacture (date of manufacture and The validity period under the storage conditions will be indicated on the ACF trademark).
2. Storage conditions for opened products: -10~5℃, the shelf life is 15 days for SONY, 30 days for HITACH. Opened products, and exposed to the air, can be stored for only 7 days; unopened If the product is stored in a high temperature environment, its useful life will be shortened.
3. Accelerate the heat curing of ACF; if the expired product exceeds the use guarantee period, our company stipulates that unopened ACF can be used no more than one year from the date of leaving the factory, and will be scrapped after one year. , the opened ACF will be scrapped directly. (Shenzhen Jiecan Technology Co., Ltd.)
Activated carbon fiber (active carbon fiber)
Activated carbon fiber (ACF), also known as fibrous activated carbon, is a highly efficient activated carbon with better performance than activated carbon. Adsorbent materials and environmentally friendly engineering materials. More than 50% of its carbon atoms are located on the inner and outer surfaces, forming a unique adsorption structure, which is called a surface solid. It is made from fibrous precursors that are carbonized and activated through a certain procedure. The relatively developed specific surface area and narrow pore size distribution enable it to have faster adsorption and desorption speed and larger adsorption capacity, and because it can be easily processed into different shapes such as felt, cloth, paper, etc., it is also acid-resistant. Its alkali corrosion resistance has attracted widespread attention and in-depth research since its introduction. It has been widely used in environmental protection, catalysis, medicine, military industry and other fields.
Since the first U.S. patent in 1962 involving the use of activated carbon fibers to filter radioactive iodine radiation by ORNL in the United States, the research and application of different precursor organic fibers and their activated carbon fibers have developed rapidly. The United States, the United Kingdom, the former Soviet Union, and especially Japan are major countries in the research and use of ACF, with an annual output of nearly 1,000 tons. Domestic ACF research began in the late 1980s, and industrialized devices began to appear in the late 1990s. Most are in the laboratory research stage.
Manufacturing method: Depending on the precursor raw materials, the ACF production process and product structure are also obviously different. The production of ACF generally involves stabilizing organic precursor fibers at low temperatures of 200°C to 400°C, followed by (carbonization) activation. Commonly used activation methods mainly include: physical activation method using CO2 or water vapor and chemical activation method using ZnCI2, H3PO, H2PO4, KOH. The treatment temperature is between 700 ℃ and 1 000 ℃. Different treatment processes (time, temperature, Activator dose, etc.) corresponding products have different pore structures and properties. The organic fibers used as ACF precursors mainly include cellulose-based, PAN-based, phenolic-based, asphalt-based, polyvinyl alcohol-based, styrene/olefin polymers and lignin fibers. The first four types are mainly commercialized.
Structural characteristics: Activated carbon fiber is a typical microporous carbon (MPAC), which is considered to be "a combination of ultrafine particles, irregular surface structures and extremely small spaces" with a diameter of 10 μm ~ 30 μm. The pores are directly opened on the surface of the fiber, and the ultrafine particles are combined in various ways to form rich nanospaces. The size of these spaces is in the same order of magnitude as the ultrafine particles, thus creating a larger specific surface area. It contains many irregular structures - heterocyclic structures or microstructures containing surface functional groups, which have extremely high surface energy, and also create a strong molecular field that interacts with the molecules on the pore walls of the micropores, providing an adsorption High-pressure systems that undergo physical and chemical changes in molecular states. The diffusion path for adsorbates to reach the adsorption site is shorter than that of activated carbon, the driving force is larger, and the pore size distribution is concentrated. This is the main reason why ACF has a larger specific surface area, faster adsorption and desorption rate, and higher adsorption efficiency than activated carbon.
Functionalization method: Functionalization mainly meets the efficient adsorption and conversion of specific substances through pore structure control and surface chemical modification.
ACF is generally suitable for the adsorption of low molecular weight molecules (MW=300 or less) in the gas phase and liquid phase. When the pore size of the adsorbent is about twice the critical size of the adsorbate molecules, the adsorbate is easier to adsorb.
The purpose of pore size adjustment is to make the pores of ACF equivalent to the molecular size of the adsorbate. The following methods are usually used: 1) activation process or change of activation degree (to the nanometer level); 2) adding metal compounds or other substances to the fibrils for carbonization Activation, or using ACF to add metal compounds and then reactivation (mainly mesopores), the raw fiber has a pore size close to macropores (macropores) in advance; 3) Hydrocarbon pyrolysis is deposited on the pore walls, and high-temperature post-treatment (so that pore size becomes smaller).
Surface chemical modification mainly changes the surface acidity and alkalinity of ACF, and introduces or removes certain surface functional groups. Surface oxygen-containing groups can be removed (reduction) through high temperature or hydrogenation treatment; acidic surfaces can be obtained through gas phase oxidation and liquid phase oxidation. Modification needs to comprehensively consider the impact of physical structure and chemical structure
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