Due to its insulation, high temperature resistance, high thermal conductivity, good stability and high cost performance, alumina is an excellent raw material for electronic ceramic substrates. With the rapid development of the electronics industry, the demand for alumina ceramic substrates has been increasing year by year. However, the higher requirements of electronic ceramic substrate, the quality of raw materials and alumina performance indicators
  Directly related to the substrate product quality.
  This article mainly discusses the casting method of alumina ceramic substrate production performance indicators, combined with the casting process characteristics and the quality of ceramic substrate
  Volume requirements, the main technical indicators of alumina casting process and product quality impact, and presented suitable for casting ceramic substrate with the oxidation of raw materials
Aluminum requirements.
  Casting process overview
  Casting production of alumina ceramic substrate, there are two main process systems: organic casting system and water-based casting system. Organic casting solvents and more use of toluene and xylene and ketone organic solvents as a solvent, these solvents will be hazardous to the health of workers and the environment, the cost is higher, but the high quality of their products. Water-based casting system using water as solvent, adding a small amount of water-soluble dispersant, with pollution-free, green advantages, and the cost is relatively low, is a new type of ceramic substrate casting technology, but some of the key technologies can not be broken, Quality is more difficult to control.
  Casting process basic process is as follows: a slurry preparation of a stencilled casting a drying a molding a skim a sintering.
  Slurry which mainly consists of ceramic powder, solvents, dispersants, adhesives, plasticizers and other additives. As the alumina ceramic substrate material generally requires alumina content above 95%, in addition to adding alumina powder, but also add a certain percentage of silicon, calcium and other powders. To meet the casting requirements, need to add binders, dispersants, plasticizers and other additives. The principles of ingredients are as follows: (1) to reduce the content of organic matter as much as possible; (2) to maximize the solid content; (3) to minimize the content of dispersant; (4) to have sufficient flexibility and Strength; (5) control the appropriate pH, to improve the dispersion of dispersant effect. Alumina Specifications Impact on Casting Processes and Products In general, electronic ceramic substrates are used in integrated circuits and therefore require products with a density (bulk density above 3.7 g / cm), small dimensional tolerances, high surface flatness , Good electrical properties, good thermal performance. Therefore, the requirements of the raw material alumina is also high. Alumina is usually divided into metallurgical grade alumina and non-metallurgical grade alumina, metallurgical grade alumina is used to produce primary aluminum electrolysis. Non-metallurgical grade alumina is a specialty-purpose alumina that is chemically synthesized or processed from metallurgical grade alumina. Metallurgical grade alumina is dominated by α phase, of which Na is usually above 0.3% by weight and can not be directly used to produce high quality ceramics. Alumina used in the production of ceramic substrates needs to be calcined at a temperature above 1400 ° C, after being de-sodiumized and fully ground to serve as a raw material for the ceramic substrate. Through long-term research and production applications, alumina purity, phase content, crystalline morphology, particle size distribution and other indicators of the casting process and substrate product quality greater impact.
  Effect of Alumina Purity
  In general, the impurities in the calcined alumina are mainly SiO, FeO ,, NaO, CaO, MgO, TiO: and so on, the total amount of impurities is generally not more than 0.5%, A12O3 content> 99.5%. Na2O content Na2O is one of the most important indicators of alumina, especially for the production of alumina ceramic. Na O at high temperature and A1 O, combined to form a stable sodium aluminate compound Na2O · 1 I Al2O3, also known as the β-phase alumina, its structure is a loose layered structure, Na2O in the rich A12O3, under the conditions of binding AI2O3 strong ability, Na2O: A12O3 mass ratio of 1: 18.096, even if a small amount of Na2O exist, but also a large number of β-phase A12O3, affecting the density of ceramics. And the high conductivity of this compound, at 300 ℃ up to 10 ~ S cnl ~, shadow II to the electrical properties of the ceramic substrate. Therefore, the alumina used in the ceramic substrate production must use low sodium alumina products, ie, alumina with Na2O content less than 0.1%.
  Fe content
  For the Fe content in alumina, on the one hand refers to the combination of chemical Fe, on the other hand refers to the inclusion of fine powder in the free iron (also known as mechanical iron). For the chemical Fe, it is usually present as Fe2O3. For the raw material A12O3 entrained Fe particles in the ceramic sintering process, the ceramic surface or inside the color, red or black spots, not only the appearance of the ceramic substrate impact, but also affect the insulation performance, so a. The appearance of spots will lead to product scrap, reduce the yield. At the same time, the content of Fe2O3 is too high, the color of the ceramic substrate can also be yellowed, which is closely related to the color reaction of Al ions in Fe-substituted corundum. Moisture content
In general, the ceramic substrate material A12O3 smaller particle size, finely ground powder, high activity, during processing or storage will absorb moisture, due to
  This is a trace of moisture. For water-based casting l art, where the content of H2O does not affect the process. However, organic casting has a great influence on organic casting. Usually organic solvent (such as PVB) used in organic casting is insoluble. As long as there is a small amount of water, it will cause flocculation of slurry and gelatinous agglomeration, which will not only cause casting difficulties , But also lead to a decline in yield. Therefore, alumina used in organic casting should be tightly controlled for moisture content.
Other impurities (pollutants)
  In addition to the impurities mentioned above, the ceramic substrate belongs to the category of fine electronic ceramics, which requires high purity of the raw material alumina powder. The particle size is usually below 3,000 meshes. However, when the visible impurities or contaminants are added, Cattle production technology and product quality caused a great impact. Therefore, the raw material of aluminum cyanide is required to avoid the contamination of alumina by external impurities and improve the cleanliness of the product in the process of J [j].
  Effect of crystal morphology
  Alumina calcination in the high temperature, due to the addition of different mineralizing agent, the crystalline morphology is also different, spherical, flaky, worm-like micro-morphology]. Casting process requires high concentration of slurry and has good fluidity, casting film requires high uniformity. Therefore, the better the dispersibility of the alumina particles in the organic solvent, the better the production. When the crystal of alumina is incomplete and the sphericity is not high, the surface activity of the particles increases, the surface energy is large, and the oil absorption rate is high, resulting in the increase of the viscosity of the slurry, the deterioration of fluidity and the decrease of the uniformity of the cast film, Cast film cracking, local wrapped porcelain blisters, thereby reducing the yield. Practice has proved that when the higher the spherical shape of the alumina, the lower the viscosity of the prepared slurry, the better the flow, the better casting performance, the higher the yield of the product.
  Effect of phase conversion rate
  Casting production of ceramic substrates of small thickness, size is small, and requires high density and high flatness. Therefore, it is required that the raw material has high sintering activity and can not produce large deformation amount. For calcined αAL2O3, the higher α-phase conversion, the more complete the crystallization, the higher the true density, the lower the sintering activity. The lower the α-phase conversion, the more crystalline defects, the lower the true density, the higher the sintering activity, the greater the shrinkage rate, too large a shrinkage rate will cause large substrate deformation, large warpage. However, when the α-phase conversion is too high, the sintering activity is low. Although the shrinkage is small, the compactness is relatively low, and a higher sintering temperature is required, which is not good for controlling the production cost. Therefore, the α-alumina used to prepare the ceramic substrate not only controls the proper α-phase conversion, but also ensures stable conversion, which is advantageous for the production of high-quality ceramic substrates.
The influence of powder agglomeration
  Alumina During the preparation from aluminum hydroxide, each aluminum hydroxide particle becomes a number of fine particle agglomerates after dehydration and remains agglomerated in the latter calcination. After thorough grinding, the agglomerates are dispersed in a single Particles of fine particles, in the alpha alumina, grinding process, if the agglomeration particles can not be completely grinding and dispersing, there will be a part of agglomeration particles, these are not grinded small agglomeration in the casting process will form a void, thereby reducing the ceramic Density and strength of ceramic substrate. Therefore, the alumina powder used for the cast ceramic substrate must be a fully ground powder and can not agglomerate.
  In summary, the alumina used in casting ceramic substrate impurity content, crystalline morphology. Phase conversion rate, particle size distribution of the casting process and the quality of ceramic substrates have a greater impact, so the general requirements:
  (1) Na2O content below 0.1%, Fe and Fe2O H2O content as low as possible;
  (2) the morphology of the spherical shape is better;
  (3) the α-phase conversion of the raw material alumina should be controlled appropriately and remain stable;
  (4) Alumina should be thoroughly ground to reduce agglomeration particles.