The key technology for TPC substrates is the preparation of high performance metallic pastes. Metal pastes are mainly composed of metal powder, organic carrier and glass powder. The conductor metals available in the pastes are Au, Ag, Ni, Cu and Al. Silver-based conductive pastes are widely used because of their high conductivity, thermal conductivity and relatively low price (accounting for more than 80% of the metal paste market share). Studies have shown that the silver particle size particle size, morphology, etc. on the conductive layer properties have a great impact. For example, Park et al. reduced the resistivity of silver paste by adding appropriate amount of silver nanoparticles: Zhou et al. pointed out that the resistivity of the metal layer decreases with the size of spherical silver particles, and the resistivity of the metal paste prepared from sheet silver powder (size 6m) is much smaller than that of the paste prepared from the same size spherical silver powder.
Direct bonded ceramic substrate (DBC) process and technical characteristics
DBC ceramic substrate is prepared by first introducing oxygen between copper foil (Cu) and ceramic substrate (Al2O3 or AN), then forming CuO eutectic phase at 1065°C (the melting point of metallic copper is 1083°C), and then reacting with ceramic substrate and copper foil to form CuAO2 or Cu(AO2)2 to realize eutectic bonding between copper foil and ceramic. The preparation process and products are shown in Figure 9 and Figure 10, respectively. Due to the good thermal conductivity of ceramic and copper, and the high strength of eutectic bonding between copper foil and ceramic, DBC substrates have high thermal stability and have been widely used in insulated gate bipolar diodes (GBT), lasers (LD) and focused photovoltaic (CPV) devices for heat dissipation.
DBC substrate copper foil thickness is large (generally 100μm-600μm), can meet the high temperature, high current and other extreme environment device packaging applications (to reduce substrate stress and bending, a ship using the C1-A1O2C sandwich structure. The thickness of the upper and lower copper layers are the same on the day of the event)
Although the DBC substrate has many advantages in practical applications, but in the preparation process to strictly control the eutectic temperature and oxygen content, the equipment and process control requirements are high, and the production cost is also higher.
In addition, due to the limitation of thick copper etching, high precision line layers cannot be prepared In the process of DBC substrate preparation, oxidation time and oxidation temperature are the two most important parameters. After the copper foil is pre-oxidized, the bonding interface can form enough CuxOy phase to wet the Al2O3 ceramic and copper foil with high bonding strength; if the copper foil is not pre-oxidized, the CuxOy wettability is poor and a large number of voids and defects will remain at the bonding interface, reducing the bonding strength and thermal conductivity. For the preparation of DBC substrates using AlN ceramics, the ceramic substrates also need to be pre-oxidized to form Al2O3 films before eutectic reaction with copper foil. Xie et al. prepared Cu/Al2O3 and Cu/AlN ceramic substrates by DBC technology, and the bond strength between copper foil and AlN ceramic exceeded 8 N/mm. A transition layer with a thickness of 2 μm existed between copper foil and AlN, and its composition was mainly Al2O3, CuAlO2 and Cu2O.
Currently, the preparation of reactive solder is a key technology for AMB substrate preparation. The first report of reactive solder was in 1947 when Bondley used TiH2 reactive metal method to connect ceramics to metal, based on which Bender et al. proposed Ag-Cu-Ti reactive soldering method. The reactive solders are mainly divided into high-temperature reactive solders (Ti, V, and Mo, etc., soldering temperature 1000°C ~ 1250°C), medium-temperature reactive solders (Ag-Cu-Ti, 700°C ~ 800°C, shielding gas or vacuum), and low-temperature reactive solders (Ce, Ga, and Re, 200°C ~ 300°C). 300°C). Naka et al. used Cu60Ti34 reactive solder for Si3N4 ceramics and NiTi50 reactive solder for SiC, and the former achieved an interfacial shear strength of 313.8 MPa at room temperature, while the latter achieved interfacial shear strengths at room temperature, 300°C and 700°C. At 700°C, the weld interface shear strengths were 158 MPa, 316 MPa and 260 MPa, respectively.
Due to the high temperature of the DBC ceramic substrate preparation process and the high stress at the metal-ceramic interface, the AMB technology has received increasing attention from the industry, especially using low-temperature reactive solders. For example, Chang et al. used Sn3.5Ag4Ti(Ce, Ga) reactive solder to weld ZnS-SiO2, ITO ceramics and Al2O3 ceramics with Cu layer at 250°C, and Tsao et al. used Sn3.5Ag4Ti(Ce) reactive solder to weld Al with micro subarc alumina (MAO-Al).
Direct Plating Ceramic Substrate (DPC) Technology Process and Characteristics
The DPC ceramic substrate preparation process is shown in Figure 13. Firstly, laser is used to prepare through holes (generally 60 μm ~ 120 μm) on the ceramic substrate, followed by ultrasonic cleaning of the ceramic substrate; magnetron sputtering is used to deposit metal seed layer (Ti/Cu) on the surface of the ceramic substrate, followed by lithography and development to complete the line layer fabrication; electroplating is used to fill the holes and thicken the metal line layer, and surface treatment is used to improve the solderability and oxidation resistance of the substrate. Finally, the substrate is prepared by drying the film and etching the seed layer.
As can be seen from Figure 13, DPC ceramic substrate preparation uses semiconductor microfabrication technology (sputtering coating, lithography, development, etc.) at the front end and printed circuit board (PCB) preparation technology (graphic plating, hole filling, surface grinding, etching, surface treatment, etc.) at the back end, with obvious technical advantages. Specific features include: (1) the use of semiconductor micromachining technology, ceramic substrates on metal lines more fine (line width / line spacing can be as low as 30 μm ~ 50 μm, depending on the thickness of the line layer), so the DPC substrate is suitable for the alignment of high precision requirements of microelectronic device packaging; (2) the use of laser punching and plating filling technology, the ceramic substrate upper / lower surface vertical interconnection, can realize (2) the vertical interconnection of the upper and lower surfaces of ceramic substrates by laser punching and plating, which enables three-dimensional packaging and integration of electronic devices and reduces the size of devices, as shown in Figure 14 (b); (3) the use of plating growth to control the thickness of the line layer (generally 10 μm ~ 100 μm) and reduce the surface roughness of the line layer by grinding to meet the requirements of high-temperature, high-current device packaging; (4) the low-temperature preparation process (below 300°C) avoids the adverse effects of high temperature on the substrate material and metal line layer, and also reduces the production cost. It also reduces the production cost. In summary, the DPC substrate is a true ceramic circuit board with high graphic accuracy and vertical interconnectivity.
However, DPC substrates also have some shortcomings: (1) the metallic line layer is prepared by electroplating process, which has serious environmental pollution; (2) the plating growth rate is low and the line layer thickness is limited (generally controlled at 10 μm ~ 100 μm), which is difficult to meet the demand of high-current power device packaging. At present, DPC ceramic substrates are mainly used in high-power LED packaging, and manufacturers are mainly concentrated in Taiwan, but mass production has started in mainland China since 2015.
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