A, the composition of conductive adhesive
(1) The matrix of conductive adhesive includes prepolymer, curing agent (crosslinking agent), diluent and other additives (plasticizer, coupling agent, defoaming agent, etc.).
Commonly used polymer substrates include epoxy resin, phenolic resin, polyimine acid, polyurethane cool and so on. Compared with other resins, epoxy resin has the advantages of good stability, corrosion resistance, low shrinkage, high bonding strength, wide bonding surface and good machinability. Therefore, epoxy resin is the most studied and widely used matrix material at present. However, epoxy resin has moisture absorption and poor heat resistance, so the modification of epoxy resin, through the adjustment of the main ripple structure and substituents of epoxy resin, to obtain a modified resin with higher comprehensive performance is under development.
Diluents are divided into active diluents and inactive diluents. Active diluents contain active end groups, which can participate in cross-linking reactions. They do not need to be removed before curing, and become part of the system after curing. Inactive diluents do not participate in crosslinking and only play a regulating role and need to be removed before curing. Prepolymer, crosslinking agent and diluent are the main factors affecting the volume change in curing process.
(2) Conductive packing Conductive packing is usually carbon, metal, metal oxide three categories.
The commonly used fillers are Au, Ag, Cu, Ni and other metal powders with low resistivity. Au powder has excellent electrical conductivity and chemical stability, is the most ideal conductive filler, but the price is expensive, generally only used in the case of high requirements. Ag powder price is relatively low, good conductivity, and in the air is not easy to oxidize, but in the humid environment will occur electromigration phenomenon, which makes the conductive property of the adhesive decline. Cu powder and Ni powder have good electrical conductivity and low cost, but they are easy to oxidize in the air, which makes the electrical conductivity worse. Therefore, the conductive filler is generally selected Ag or cu.
Two, the advantages of conductive adhesive
As a substitute for Pb/Sn solder, conductive adhesive emerged. Compared with Pb /Sn solder, it has five advantages:
(2) The coating process is simple and the connection steps are few;
(4) Good thermal mechanical properties, toughness is better than alloy solder, high contact fatigue resistance;
Three, the classification of conductive adhesive
(2) According to the conduction mechanism, it is divided into intrinsic conductive adhesive and composite conductive adhesive. Intrinsic conductive adhesive refers to the conjugate polymer whose molecular structure itself has conductive function. This kind of material has high resistivity, poor conductive stability and repeatability, and high cost, so it is rarely studied. Composite conductive adhesive refers to the addition of conductive filler in the organic polymer matrix, so as to make it have similar conductive properties as metal. Current research mainly focuses on this piece.
(4) According to the different curing system, conductive adhesive can be divided into room temperature curing conductive adhesive, medium temperature curing conductive adhesive, high temperature curing conductive adhesive and UV curing conductive adhesive. Curing at room temperature takes too long, generally several hours to several days, and the volume resistivity is prone to change when stored at room temperature, so it is rarely used in industry. The medium temperature curing conductive adhesive has excellent mechanical properties, and the curing temperature is generally lower than 150℃. This temperature range can better match the use temperature and temperature resistance of electronic components, so it is widely used at present. When the conductive adhesive is cured at high temperature, the metal particles are easy to be oxidized and the curing speed is fast. The conductive adhesive requires a shorter curing time, so it is also used less. Uv curing conductive adhesive mainly depends on UV irradiation to cause curing reaction of resin matrix, curing speed is fast, resin matrix can be preserved for a long time under the condition of avoiding light, it is a new curing method. This new curing method combines UV curing technology with a conductive adhesive, giving the adhesive new properties. At present, the research on this aspect is also the focus of attention.
Four, the factors affecting the performance of conductive adhesive
(2) Resin system and curing process Resin system as the main source of mechanical properties and bonding properties of conductive adhesive, its selection is very important. Different resin systems are selected according to the needs of different mechanical properties and applications. The commonly used resin system is epoxy resin, which has good adhesion, low viscosity and moderate curing temperature, and is suitable for the preparation of conductive adhesive. It is generally used for normal temperature curing or medium temperature curing conductive adhesive. According to the requirements of electronic components, when high temperature curing is required, polyphthalimide resin can be used as the matrix resin or the traditional epoxy resin can be added with high temperature resistant substances such as bismalephthalimide resin to improve the heat resistance. For photosensitive curing conductive adhesive resin system choose acrylic acid epoxy photosensitive substance. The curing process has certain influence on the conductivity of the conductive adhesive. When heating curing, the time before the gel point should be shortened as far as possible, because a long gel time will cause the adhesive to adequately cover the surface of the conductive particles and reduce the conductivity. Therefore, when heating curing, it is generally placed directly at the curing temperature to cure, in order to reduce the adverse effects of wetting coating. Curing temperature and time not only affect the conductivity of the adhesive, but also its mechanical properties. For room temperature curing copper powder conductive adhesive, extending the curing time will reduce the shear strength, and extending the curing time of medium and high temperature curing conductive adhesive will improve the mechanical properties.
(4) The influence of other additives Other additives are mainly based on the needs of the conductive adhesive, adding some substances to improve the performance of the conductive adhesive. Generally, coupling agents are added in conductive adhesive to reduce the interface surface energy between metal particles and adhesive layer, such as silane coupling agent, titanate coupling agent, etc.
With the improvement of the conductive mechanism of conductive adhesive, the current research on conductive adhesive mainly focuses on the preparation of high-performance conductive adhesive, which mainly includes the following directions
(2) Composite conductive adhesive composite conductive polymer materials have developed into a new type of functional materials, in the antistatic, electromagnetic shielding, conductive automatic control and positive temperature coefficient materials have broad application prospects, its market demand is increasing. The active point was formed on the powder of epoxy resin (EP) by the palladium free activation process, and a new type of silver-coated copper /EP composite conductive particle was successfully prepared by electroless plating method. Its resistivity was 4.5*10-3Ω·cm, which can be used as the conductive filler of anisotropic conductive adhesive (instead of the pure metal conductive filler). A new low melting point anisotropic conductive adhesive was prepared by Eom et al. The results show that the resistance of the adhesive is less than 10 mΩ, while that of the conventional adhesive is less than 1000 mΩ. The conductive adhesive can be used under the condition of current density of 10000 A/cm2; The resistance and current density of the adhesive did not change before and after the high pressure cooking test, but the shear strength changed at a rate of 23%.
In recent years, a kind of NCA bonding technology (lead-free non-conductive particle interconnection technology) has attracted much attention. This interconnection has good bonding strength and low cost. The connection material used is NCA polymer, which is usually not filled with any conductive fillers. In order to realize the connection, the NCA can form a direct physical connection on the surface of the chip bonding site by applying pressure to the IC chip and substrate under certain temperature conditions. The technique eliminates the need to add conductive fillers to the colloid, eliminating the cost of filling the metal, and creating a simple, efficient and inexpensive interconnect where the metal is directly in contact.
In recent years, the development of non-conductive particle conductive adhesive is very rapid, there are new conductive products (similar to anisotropic conductive adhesive) in the Z-axis direction, and the gap size in the connecting material reaches the nanoscale. Conductive adhesive is a kind of adhesive with both conductive properties and bonding properties, it can connect a variety of conductive materials together, so that the connection between the materials to form an electrical pathway. Since its introduction in 1966, conductive adhesives have played an increasingly important role in electronic technology. At present, conductive adhesive has been widely used in the packaging and bonding of printed circuit board components, light-emitting diodes, liquid crystal display, smart cards, ceramic capacitors, integrated circuit chips and other electronic components. However, Pb/Sn solder is still widely used in electronic surface packaging technology. Although conductive adhesive has many advantages, it still cannot completely replace Pb/Sn solder because of its own existing problems to be solved.
The main problems of conductive adhesive are as follows:
(2) The bonding effect is greatly affected by the type of components and PCB(printed circuit board);
(4) The bond strength is relatively low. In the connection with small pitch, the bonding strength directly affects the impact resistance of the component.
Vi. Market conditions of conductive adhesive
At present, conductive adhesives used in some highly sophisticated sectors of the domestic market are mainly imported: the IC and LED sectors are almost completely occupied by Ablistick and 3M of the US, and Sumitomo of Japan and Yihua of Taiwan are also involved in these sectors. Japan's Three-Bond controls the entire application of conductive adhesives in quartz crystal resonators. Domestic conductive adhesive is mainly used in some medium and low grade products, this market is mainly occupied by the metal research institute. TeamChem Company series of conductive adhesives are mainly applied to LED, high-power LED, LED digital tube, LCD, TR, IC, COB, PCBA, lattice block, display, crystal oscillator, resonator, solar cells, photovoltaic cells, buzzer, ceramic capacitor, semiconductor discrete devices and other electronic components and components packaging and bonding. The application scope covers electronic components, electronic components, circuit board assembly, display and lighting industry, communications, automotive electronics, smart card, radio frequency identification and other fields.
Vii. Summary
In order to greatly improve the comprehensive performance of domestic conductive adhesive, we must start from the following aspects:
(2) Develop new conductive particles. The important conditions for the preparation of conductive adhesive are the preparation of conductive filler mainly composed of nano particles, the coating alloy or low co-fusion alloy as conductive filler, and the activation treatment of conductive particle surface.
(3) Study new curing methods. Room temperature curing and high temperature resistant bonding materials are the future trend. Although the heat-curing conductive adhesive system is still dominant at present, its curing agent and coupling agent have environmental pollution and other problems, so light curing, electron beam curing and other technologies have been widely used in coating, ink, photoresist and medical adhesive fields. In addition, microwave curing technology, also achieved phased results; The development of dual curing system (UV curing + thermal curing) is also the future development direction.
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