Ceramic substrate material and preparation process
The ceramic substrate mainly includes BeO, Al2O3 and AlN, Si3N4. Compared with other ceramic materials, Si3N4 ceramic substrate has high electrical insulation performance and chemical stability, good thermal stability, high mechanical strength, and can be used to manufacture high-integration large-scale integrated circuit boards.
In terms of structure and manufacturing process, ceramic substrates can be divided into HTCC, LTCC, TFC, DBC, DPC, etc.
High temperature co-fired multilayer ceramic substrate (HTCC)
HTCC, also known as high temperature co-fired multilayer ceramic substrate. In the preparation process, the ceramic powder (Al2O3 or AlN) is firstly added to the organic binder, and after mixing evenly, it becomes a paste-like slurry, then the slurry is scraped into a sheet shape by a scraper, and the sheet-like slurry is formed into a green body by a drying process; Then, according to the design of each layer, through holes are drilled, screen-printed metal paste is used for wiring and hole filling, and finally the green layers are stacked and sintered in a high-temperature furnace (1600°C).
Due to the high sintering temperature in this preparation process, the choice of metal conductor materials is limited (mainly metals such as tungsten, molybdenum, manganese, etc. with high melting point but poor conductivity), and the production cost is high, and the thermal conductivity is generally 20~200W / (m·°C).
Low temperature co-fired ceramic substrate (LTCC)
LTCC, also known as low temperature co-fired ceramic substrate, its preparation process is similar to HTCC, except that the Al2O3 powder is mixed with a low melting point glass frit with a mass fraction of 30%~50%, so that the sintering temperature is reduced to 850~900 ℃, so the conductive Gold and silver with better rates are used as electrode materials and wiring materials.
Because LTCC uses screen printing technology to make metal lines, it is possible to cause alignment errors due to the problem of stretching the net; and there is also a difference in shrinkage ratio when multi-layer ceramics are laminated and sintered, which affects the yield. In order to improve the thermal conductivity of LTCC, thermal vias or conductive vias can be added in the patch area, but the cost increases.
Thick Film Ceramic Substrate (TFC)
Compared to LTCC and HTCC, TFC is a post-fired ceramic substrate. The metal paste is coated on the surface of the ceramic substrate by screen printing technology, and is prepared after drying and high temperature sintering (700~800℃). Metal paste is generally composed of metal powder, organic resin and glass. After high-temperature sintering, the resin binder is burned away, and the rest is almost pure metal, due to the vitreous bonding effect on the surface of the ceramic substrate. The thickness of the metal layer after sintering is 10~20μm, and the minimum line width is 0.3mm.
Direct Bonded Copper Ceramic Substrate (DBC)
It's formed by eutectic sintering of ceramic substrate and copper foil at high temperature (1065°C), and finally, according to wiring requirements, lines are formed by etching. Because copper foil has good electrical and thermal conductivity, and alumina can effectively control the expansion of the Cu-Al2O3-Cu composite, the DBC substrate has a thermal expansion coefficient similar to that of alumina.
DBC has the advantages of good thermal conductivity, strong insulation and high reliability, and has been widely used in IGBT, LD and CPV packaging. The disadvantage of DBC is that it utilizes the eutectic reaction between Cu and Al2O3 at high temperature, which requires high equipment and process control, and the cost of the substrate is high; because of the easy generation of micro pores between the Al2O3 and Cu layers, the thermal shock resistance of the product is reduced. ; Because the copper foil is easy to warp and deform at high temperature, the thickness of the copper foil on the DBC surface is generally greater than 100m; at the same time, due to the chemical corrosion process, the minimum line width of the DBC substrate pattern is generally greater than 100m.
Direct Copper Plating Ceramic Substrate (DPC)
In it's production, the ceramic substrate is pre-treated and cleaned, and a Ti/Cu layer is deposited on the surface of the substrate as a seed layer by vacuum sputtering. The plating method increases the thickness of the line, and the substrate fabrication is completed after the photoresist is removed.
DPC technology has the following advantages: low temperature process (below 300 ℃), completely avoids the adverse effect of high temperature on materials or circuit structure, and also reduces the cost of manufacturing process; the use of thin film and photolithography development technology makes the metal lines on the substrate more refined , so the DPC substrate is very suitable for electronic device packaging that requires high alignment accuracy. However, the DPC substrate also has some shortcomings: the thickness of the copper layer deposited by electroplating is large, and the pollution of the electroplating waste liquid is large; the bonding strength between the metal layer and the ceramic is low, and the reliability of the product is low.
