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Design and Application of PVD in DPC Manufacturing Process

Monday, July 29th, 2024

With the increasing demand of electronic devices working in high temperature environment, DPC (Direct-Plated Copper) ceramic PCB has been widely used as a heat dissipation material with strong high temperature stability and high thermal conductivity. This paper discusses the design and application of DPC ceramic substrate from the aspects of material, structure and technology.

Material Selection of DPC Ceramic PCB

It is very important to select ceramic substrate with good high temperature stability and high thermal conductivity. Commonly used ceramic materials are Al2O3, AlN, Si3N4 and so on. Taking Al2O3 as an example, its thermal conductivity is as high as 24~30W/(m·K), and the coefficient of thermal expansion is about 8.2×10^-6/℃, which is similar to the coefficient of thermal expansion of most chips. The ceramic layer on the surface of the substrate should be smooth and uniform, and have good adhesion, oxidation resistance, wear resistance and corrosion resistance.

Design and Application of PVD in DPC Manufacturing Process

The heat dissipation performance of DPC ceramic substrate is one of the key factors affecting its work in high temperature environment. In order to test the heat dissipation performance of DPC ceramic substrate, thermocouple temperature measurement, infrared temperature measurement, thermal imager and other testing methods can be used. The experimental results show that the DPC ceramic substrate has good heat dissipation performance and can effectively heat dissipation and reduce the influence of temperature on electronic devices.

DPC Ceramic PCB Structure Design

The structure design of DPC ceramic substrate includes the thickness of copper foil, the size of aperture, the layout of copper foil and the thickness of ceramic substrate. The choice of copper foil thickness should be determined according to heat dissipation requirements and chip power density and other parameters.

There are two layout methods of copper foil, one is fully plated surface; The other is local copper, that is, only the copper foil is laid in the area around the chip. The local copper laying method can reduce the copper foil area and reduce the cost, but the adhesion between the copper foil and the substrate needs to be higher. The thickness of the ceramic substrate also needs to be selected according to the specific application requirements.

PVD Process of DPC Ceramic PCB

Direct Plating Copper (DPC) is a ceramic circuit processing technology developed on the basis of ceramic film processing. In this process, the copper metal composite layer is sputtered on the ceramic substrate by vacuum coating, and then the circuit is made by re-exposure, development, etching and film removal with the photoresistance of yellow light microshadow. Finally, the thickness of the circuit is increased by electroplating/electroless deposition, and the metallized circuit is made after the photoresistance is removed.

The process is the process of vaporizing metal materials (titanium and copper) into molecules and then forming a metal film on the surface of a ceramic substrate. The main process includes cleaning the surface of the substrate, heat treatment of the ceramic substrate, preparation of the target of metal materials, DPC of the target, measurement of film thickness and surface treatment. Among them, heat treatment can improve the thermal stability and adhesion of the ceramic substrate, and ensure that the covered copper foil will not peel off in high temperature environment.

DPC ceramic substrate small size, precision structure, high reliability requirements, complex process flow, fine production process, belongs to technology-intensive industries, with high technical barriers. Its production process mainly involves drilling, magnetron sputtering, chemical copper deposition, copper plating, solder resistance printing, chemical silver/chemical gold deposition and other main processes

Application of DPC Ceramic PCB

a. VCSEL package

The power density of VCSEL is very high, and the DPC ceramic circuit board has a high thermal expansion coefficient matching with VCSEL, so as to solve the stress problem caused by the mismatch of thermal expansion between chip and substrate. The DPC ceramic circuit board makes the metal edge closely combined with the ceramic substrate, avoiding the additional paste process, coordination accuracy and other problems in the later assembly process, as well as the reliability problems caused by the aging of the glue. The ceramic circuit board of DPC film technology almost meets the packaging requirements of VCSEL.

b. IGBT package

The insulated gate bipolar transistor has become the mainstream of the development of power semiconductor devices due to its high input impedance, fast switching speed, low on-state current and high blocking voltage. Its applications are as small as frequency conversion air conditioning, silent refrigerator, washing machine, induction cooker, microwave oven and other household appliances, and as large as electric locomotive traction system. Due to the high output power and high heat output of IGBT, heat dissipation is the key for IGBT packaging. At present, the DPC ceramic substrate is mainly used for IGBT packaging, because the DPC ceramic substrate has the characteristics of large metal layer thickness, high bonding strength (good thermal impact) and so on.

c. LED package

Throughout the development of LED technology, the power density continues to improve, and the requirements for bracket heat dissipation are becoming higher and higher. DPC ceramic substrate has high insulation, high thermal conductivity and heat resistance, low expansion and other characteristics, especially the use of through hole interconnection technology, well meet the LED flip, eutectic, COB (chip on board), CSP (chip scale package), WLP (wafer package) packaging needs, suitable for medium and high power LED packaging.

In conclusion, the design and application of DPC copper-coated ceramic substrate need to consider many factors, and a lot of experiments and data analysis are required. The performance and reliability of DPC ceramic substrate at high temperature can be improved by selecting suitable materials and designing reasonable structure and process, and the high temperature application of microelectronic devices can be guaranteed.

The advent of DPC ceramic substrate products opens the development of heat dissipation application industry. Due to the heat dissipation characteristics of the ceramic substrate, it has the advantages of high heat dissipation, low thermal resistance, long life and voltage resistance. With the improvement of production technology and equipment, the rationalization of product prices is accelerated, and the application field of LED industry is expanded.

Direct Plating Copper (DPC) is a ceramic circuit board developed on the basis of ceramic film processing. Aluminum nitride/alumina ceramic is used as the substrate of the circuit, and the composite metal layer on the surface of the substrate is sputtered, and the circuit is formed by electroplating and lithography. Its basic processes are:

Incoming Quality Control (IQC)

The process begins with the inspection of raw materials to ensure they meet strict quality standards. This step involves checking ceramic substrates for physical defects such as cracks, chips, or surface irregularities, and verifying material properties like thermal conductivity and dielectric strength. Advanced equipment such as microscopes and X-ray fluorescence (XRF) analyzers are used for thorough inspection, ensuring only the best materials proceed to the next stage.

Laser Drilling

Before laser drilling, a water-soluble, food-grade substrate pigment is brushed onto the ceramic substrate to reduce reflectivity and enhance laser drilling effectiveness. After drying in an oven, a laser drilling machine is used to drill through the substrate, creating pathways for connections between the upper and lower surfaces. Depending on the ceramic material, different laser wavelengths such as infrared, green light, ultraviolet, or CO2 are used to burn away the material with each laser pulse.

How does a DPC Ceramic PCB Manufactured?

Laser Marking

Laser marking involves using a laser marking machine to engrave product QR codes onto the ceramic substrate. This step ensures precise and permanent identification marks on the PCB.

Ultrasonic Cleaning

After laser drilling and marking, the substrates are cleaned to remove any attached particles or residues. This involves coarse and fine debris removal using ultrasonic cleaning, followed by water washing to eliminate any remaining particles. The cleaned substrates undergo micro-etching to roughen the surface, enhancing the effectiveness of subsequent magnetron sputtering, and are then dried to remove surface moisture.

Magnetron Sputtering

In the magnetron sputtering process, a high-vacuum chamber is used to ionize argon gas, producing an ion stream that bombards a target cathode. This causes atoms of the target material to be ejected and deposited as a thin film on the ceramic substrate. Pre-sputtering treatments such as dust removal, degreasing, and slow pulling are conducted to ensure optimal results.

Chemical Copper Plating

Chemical copper plating thickens the copper layer, improving the conductivity of the vias and ensuring better adhesion with the sputtered copper layer. This catalytic redox reaction involves pre-treatment steps like degreasing, micro-etching, pre-immersion, activation, and acceleration to ensure proper copper deposition.

Full-Panel Electroplating

Full-panel electroplating increases the thickness of the copper layer. This involves processes such as degreasing, micro-etching, acid washing, copper plating, and stripping excess copper from the fixtures. The copper plating uses copper balls as anodes and an electrolyte solution of CuSO4 and H2SO4, with the primary reaction being Cu2+ + 2e- → Cu.

Grinding, Laminating, Exposure, and Developing

Post electroplating, the copper layer is prepared for patterning. This includes acid washing to remove oxides, grinding to roughen and clean the surface, laminating with a photoresist dry film, UV exposure through a photomask to create the desired pattern, and developing to dissolve unexposed areas, leaving behind the patterned photoresist.

Pattern Plating (Electroplating Copper)

The exposed and developed areas of the substrate where the circuit pattern is defined undergo further copper electroplating to thicken the circuit traces.

Etching and Stripping

Unwanted copper and dry film are removed through etching, stripping away excess copper deposited in previous steps. This includes rough grinding, film stripping, copper etching, and titanium etching, ensuring only the desired circuit pattern remains.

Annealing

The ceramic boards are annealed in a furnace to relieve stresses introduced during electroplating, enhancing the ductility and toughness of the copper layer and ensuring dense copper grain packing.

Belt Grinding

Post-annealing, the substrate surface may have an oxide layer and rough texture. Belt grinding is used to remove these oxides and smoothen the surface, ensuring the quality of subsequent gold or silver surface treatments.

Flying Probe Testing

High-speed flying probe testers check for continuity and shorts in the vias and circuit traces, ensuring electrical integrity.

Sandblasting Before Solder Mask

Prior to solder mask application, the substrate undergoes sandblasting to roughen and clean the surface, removing oxides and contaminants. This includes acid washing, sandblasting, and micro-etching.

Solder Mask Printing

Solder mask is printed on areas of the PCB that do not require soldering, protecting the circuits during soldering and assembly. Screen printing applies solder mask ink, which is then UV-cured and developed to remove unexposed areas.

Sandblasting After Solder Mask

After solder mask application, exposed areas that need surface treatments undergo sandblasting to roughen the surface and remove oxides, preparing for gold or silver plating.

Surface Treatment

Surface treatments like electroless or electrolytic plating of gold, silver, or other metals are applied to the solder pads, enhancing solderability and preventing oxidation.

Laser Cutting

Laser cutting equipment precisely cuts the finished PCBs from the manufacturing panel, ensuring accurate dimensions and clean edges.

Testing

Various testing equipment, including thickness gauges, Automated Optical Inspection (AOI) machines, and ultrasonic scanning microscopes, are used to inspect the PCBs’ performance and appearance, ensuring they meet quality standards.

Packaging and Shipping

Finally, the PCBs are vacuum-packed using packaging machines to protect them during transport. They are then stored and shipped to customers, ensuring they arrive in perfect condition.

Above is the all the processes of DPC muanufacturing. If you are interested in process of other PCBs, welcome to contact us.