In the electronic packaging process, the substrate mainly plays the role of mechanical support protection and electrical interconnection (insulation). With the gradual development of electronic packaging technology towards miniaturization, high density, multi-function and high reliability, the power density of electronic systems increases and the heat dissipation problem becomes more and more serious. Good device heat dissipation depends on the optimized thermal structure design, packaging material selection (thermal interface material and thermal substrate) and packaging manufacturing process. Among them, the selection of substrate material is the key link, which directly affects the device cost, performance and reliability.
Commonly used substrate materials mainly include plastic substrates, metal substrates, ceramic substrates and composite substrates four categories. The good thermal conductivity, heat resistance, insulation, low coefficient of thermal expansion and the continuous reduction of cost of ceramic substrates are increasingly used in electronic packaging, especially in power electronic devices such as IGBT (insulated gate bipolar transistor), LD (laser diode), high power LED (light emitting diode), CPV (focused photovoltaic) packaging.
Electroplated ceramic substrate preparation process includes the advantages and disadvantages and applicable requirements of using semiconductor micromachining technology, laser punching and electroplating hole-filling technology, electroplating growth control line and grinding, low-temperature preparation process, etc. The bonding strength of the metal line layer to the ceramic substrate and the electroplated hole filling are the key to the reliability of the ceramic substrate as well as the key technology for the preparation of ceramic substrates.
Preparation and characteristics of electroplated ceramic substrates
In terms of structure and fabrication process, ceramic substrates can be further divided into HTCC, LTCC, TFC, DBC, DPC, etc. Electroplated ceramic substrate (DPC) is made on ceramic substrate using thin film process, its chemical stability, high thermal conductivity, fine line, coefficient of thermal expansion (CTE) and chip material match, so it has become an important development direction for high-power LED packaging thermal substrate.
The DPC ceramic substrate preparation process is shown in Figure 1. Firstly, laser is used to prepare through-hole on ceramic substrate (aperture diameter is generally 60-120μm), followed by ultrasonic cleaning of ceramic substrate; magnetron sputtering technology is used to deposit metal seed layer (Ti/Cu) on the surface of ceramic substrate, followed by lithography and development to complete line layer fabrication; plating is used to fill holes and thicken metal line layer, and surface treatment is used to improve substrate solderability and oxidation resistance, and finally The substrate is prepared by removing the dry film and etching the seed layer.
From the above figure, it can be seen that the front end of DPC ceramic substrate preparation adopts semiconductor microprocessing technology (sputtering coating, lithography, development, etc.) and the back end adopts printed circuit board (PCB) preparation technology (graphic plating, hole filling, surface grinding, etching, surface treatment, etc.), with obvious technical advantages.
Specific features include: 1) the use of semiconductor microprocessing technology, metal lines on ceramic substrates are more delicate, so DPC substrates are ideal for microelectronic device packaging with high alignment accuracy requirements; 2) the use of laser punching and electroplating hole-filling technology to achieve vertical interconnection of the upper/lower surface of ceramic substrates, which can realize three-dimensional packaging and integration of electronic devices and reduce device volume, as shown in Figure 2; 3) the use of electroplating growth to control the thickness of the line layer (generally 10-100μm) and reduce the surface roughness of the line layer by grinding to meet the demand for high-temperature, high-current device packaging; 4) low-temperature preparation process (below 300°C), avoiding the adverse effects of high temperature on the substrate material and metal line layer, and also reducing production costs.
In summary, DPC substrates have high graphic accuracy, can be vertically interconnected and other characteristics, is a true ceramic circuit board. However, DPC substrates also have some shortcomings: 1) the metal 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 10-100μm), which is difficult to meet the demand for high-current power device packaging. At present, DPC ceramic substrate is mainly used in high-power LED packaging, manufacturers are mainly concentrated in Taiwan, China, but from 2015 onwards the mainland region has begun to achieve mass production.
Key to the preparation of electroplated ceramic substrates
The bonding strength between the metal line layer and the ceramic substrate is the key to the reliability of the DPC ceramic substrate. Due to the large difference of thermal expansion coefficient between metal and ceramic, in order to reduce the interfacial stress, it is necessary to add a transition layer between copper layer and ceramic, so as to improve the interfacial bonding strength. Since the bonding strength between the transition layer and the ceramic is mainly based on diffusion adhesion and chemical bonding, metals with higher activity and good diffusivity such as Ti, Cr and Ni are often selected as the transition layer (also as the seed layer for plating). Plasma cleaning of ceramic substrates can greatly improve the bonding strength between metal films, mainly because 1) the ion beam removes contaminants from the surface of ceramic substrates; 2) ceramic substrates produce suspended bonds due to bombardment by the ion beam and bond more tightly with metal atoms.
Plating and filling holes is also a key technology for the preparation of DPC ceramic substrates. At present, most of the DPC substrate plating hole-filling uses pulsed power supply, whose technical advantages include: 1) easy to fill through-hole and reduce the plating defects in the hole; 2) dense surface plating structure and uniform thickness; 3) higher current density can be used for plating to improve the deposition efficiency. Due to the high cost of pulse plating, new DC plating has gained renewed attention in recent years to achieve efficient filling of blind or through holes by optimizing the plating solution formulation (including leveling agents, inhibitors, etc.).
Ceramic Substrate Application and Analysis
01 IGBT Package
Insulated Gate Bipolar Transistors (IGBT) have become the mainstream of power semiconductor devices with high input impedance, fast switching speed, low on-state voltage and high blocking voltage. Its applications range from small household appliances such as inverter air conditioners, silent refrigerators, washing machines, induction cookers, microwave ovens, etc. to large electric locomotive traction systems. Due to the high output power and high heat generation of IGBT, heat dissipation is the key for IGBT package. At present, IGBT package mainly adopts DBC ceramic substrate, because DBC has the characteristics of large thickness of metal layer and high bonding strength (good thermal shock).
02 LD package
Laser diode (LD), also known as semiconductor laser, is an optoelectronic device based on the principle of excited radiation of semiconductor materials, with small size, long life, easy pumping and integration. It is widely used in laser communication, optical storage, optical gyroscope, laser printing, ranging, and radar. Temperature has a large relationship with the output power of semiconductor lasers. Heat dissipation is the key to LD packaging. Due to the high current density and high heat flow density of LD devices, ceramic substrate becomes the preferred heat sink material for LD packaging.
03 LED packaging
Throughout the development of LED technology, the power density has been increasing, and the requirements for heat dissipation have become higher and higher. Due to the high insulation, high thermal conductivity and heat resistance, low expansion and other characteristics of ceramics, especially the use of through-hole interconnection technology, can effectively meet the LED flip-chip, eutectic, COB (chip on board), CSP (chip scale packaging), WLP (wafer package) packaging needs, suitable for medium and high power LED packaging.
04 Photovoltaic (PV) module packaging
Photovoltaic power generation is based on the principle of photovoltaic effect, using solar cells to convert sunlight directly into electricity. As the focusing effect leads to an increase in sunlight density, the chip temperature rises and ceramic substrates must be used to strengthen heat dissipation. In practical application, the metal layer on the surface of ceramic substrate is connected to the chip and heat sink respectively through thermal interface material (TIM), and heat is quickly conducted to the metal heat sink through ceramic substrate, which effectively improves the system photoelectric conversion efficiency and reliability.
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