In the use of high power electronic devices, to achieve interconnection between chips and electronic components, ceramics, as packaging substrate materials, require surface metallization. Ceramic metallization has the following requirements: excellent sealing performance, low sheet resistance and resistivity of the metal conductive layer, strong adhesion to the ceramic substrate, and the ceramic must still maintain high thermal conductivity after metallization. Therefore, copper (Cu), with its excellent ductility, high thermal conductivity, and electrical conductivity, has become the most commonly used material in power electronic devices. Figure 1 shows a schematic diagram of a ceramic substrate coated with copper.
Although ceramics possess more excellent comprehensive performance compared to the other two types of packaging substrates, ceramic materials are strong covalent bond compounds. Their electron configuration is very stable, making them less likely to react with other materials and causing difficulty in wetting with common metals. The performance of the metallized ceramic substrate surface is closely related to the operational stability of power electronic devices, which is the reason restricting the widespread application of ceramic packaging substrates. Therefore, exploring ceramic surface metallization is of great significance. Currently, common methods for ceramic metallization mainly include electroless plating metallization, direct copper bonding metallization, thick film metallization, thin film metallization, etc.
Electroless Plating Metallization
Electroless plating metallization refers to a method where metal ions are reduced to metal by a reducing agent through a chemical reaction and deposited on the surface of the substrate material. The core lies in producing a metal layer through a controlled oxidation reduction reaction. Figure 3 shows a schematic diagram of the electroless plating process. Electroless copper plating involves reducing Cu²⁺ in solution to copper atoms and depositing them on a catalytically active substrate.
The reaction principle can be represented by the following formulas:
Step 1: Cu²⁺ is reduced to Cu atoms at the cathode, as shown in 1-1.
Step 2: Formaldehyde provides the electrons needed for the reaction at the anode, as shown in 1-2.
Step 3: The overall oxidation reduction equation for electroless copper plating, as shown in 1-3.
Direct Copper Bonding Metallization
Direct copper bonding metallization refers to a method of directly bonding copper foil to the ceramic surface using a copper-oxygen eutectic liquid in a high temperature, weakly oxidizing atmosphere. It is mainly used on Al₂O₃ and AlN ceramic surfaces. The principle is that Cu₂O and CuO, generated from the reaction of Cu with O₂, react with Al from the substrate within the temperature range of 1060-1083°C to form spinel substances like CuAlO₂ and CuAl₂O₄, promoting high bonding strength between ceramic and copper. When performing direct copper bonding metallization on AlN ceramic substrates, the AlN must first be oxidized to form an Al₂O₃ layer on its surface. Figure 4 shows a schematic flow diagram of the direct copper bonding process for AlN. The reaction formula is as follows:
Thick Film Metallization
Thick film metallization is a technique where metal paste is applied onto the ceramic surface via screen printing, followed by high temperature drying and heat treatment to form a metallized ceramic substrate. Figure 5 shows a schematic diagram of the screen printing process. The paste mainly consists of a functional phase, a binder, and an organic vehicle. The functional phase is the main body of the thick film paste, forming the metal film layer after coating the ceramic surface with metal powder and subsequent heat treatment. The binder, such as glass phase or oxide, sinters at high temperatures to enhance the adhesion between the metal film layer and the ceramic substrate. The organic vehicle comprises organic solvents or surfactants used to improve the surface activity of the organic paste, ensuring a more uniform mixture.
Thin Film Metallization
Thin film metallization is a process conducted under high vacuum conditions, using physical methods to ionize the surface layer of a solid material, which is then deposited as a thin film on the ceramic substrate surface via low pressure gas. This is known as Physical Vapor Deposition (PVD) technology, primarily including magnetron sputtering deposition, ion plating deposition, arc deposition, etc. Figure 6 shows the principle diagram of magnetron sputtering deposition. The core lies in Ar⁺ ions being accelerated by an electric field to bombard a target electrode made of the material to be sputtered. When the ion energy is appropriate, the Ar⁺ ions sputter atoms from the target surface, which then travel in a certain direction towards the substrate, achieving film deposition.
Each of the aforementioned ceramic substrate metallization methods has its advantages and disadvantages. Electroless plating metallization offers high production efficiency and enables mass production. However, the bonding force between the metal layer and the ceramic substrate is limited, failing to meet many specific application scenarios. Direct copper bonding metallization, i.e., the high temperature sintering method, satisfies production efficiency while providing a certain bonding strength between the metal layer and the ceramic substrate; it is a relatively common production process currently. However, because the metallization film formation is done via high temperature sintering, the application of many low melting point metals is restricted. Thick film metallization, i.e., screen printing, is simple to operate in production. However, it cannot achieve good control over the accuracy of metallization thickness, line width, and spacing, making it impossible to produce high precision fine circuits. Thin film metallization, i.e., magnetron sputtering, utilizes the principle of van der Waals forces, resulting in a strong bond between the metal layer and the ceramic substrate. However, its production efficiency is low, and it can only form very thin metal layers, typically on the nanometer scale.
QFPCB factory effectively utilizes a method combining several metallization processes. In the production process flow, first, a metal seed layer (Titanium layer 50-100nm, Copper layer 100-300nm) is formed on the ceramic substrate surface via magnetron sputtering (thin film metallization). The metal seed layer bonds to the ceramic substrate through van der Waals forces. Then, the metal thickness is increased on this metal seed layer through electroplating (electroless plating). In this way, the performance of the circuits produced on the ceramic substrate is significantly superior to those made by magnetron sputtering or electroless plating alone. On one hand, it effectively enhances the bonding strength between the metal layer and the ceramic substrate; on the other hand, it enables metallization production with various layer thicknesses (up to 1000μm).
After more than ten years of development, QFPCB factory has established long term, stable cooperative relationships with thousands of domestic and international enterprises in sectors such as semiconductors, chips, sensors, communications, RF devices, and high power lighting. We produce high performance precision circuit products on ceramic substrates that meet customer requirements for these companies, providing professional one-stop solutions and becoming an important strategic partner in our customers’ new product development, technological iteration and innovation, and company growth.
If you have any further questions, please feel free to leave a comment below or contact QFPCB by email ([email protected]).
