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Jun 22, 2020

A Comprehensive Introduction to Ceramic PCBs

With Surface Mount Technology (SMT) being the mainstay of the electronics industry, printed circuit boards {PCBs} are moving from the traditional organic laminates as substrate material to ones with exceptionally high reliability, density, and precision. This requirement is coming from the need for making electronic items thinner and smaller, and with more functionality than before.

As a new type of circuit material, ceramic for PCBs is receiving tremendous attention from the industry. Apart from being very effective in providing solutions for miniaturization for modern electronic products, ceramic PCBs offer many outstanding functional characteristics that make them suitable for use in many fields including LED lighting, semiconductor coolers, high power semiconductor modules, power control circuits, electronic heaters, intelligent power components, power mixing circuits, high frequency switching power supplies, automotive electronics, solid state relays, military electronics, aerospace, and communication.

Why Use Ceramic Technology for PCBs?

Ceramic technology offers substantial advantages over boards made of regular materials. Some of the major advantages are:

  • High Thermal Conductivity—core metal content is 96-98%
  • Low Coefficient of Thermal Expansion—higher reliability
  • High Surface Hardness—higher durability and longer service life
  • High Compression Strength
  • High Dielectric Strength
  • Capability of withstanding higher temperatures
  • High Breakdown Voltage
  • No Water Absorption
  • Low loss at High Frequencies
  • Resistant to Cosmic Rays
  • Free of Organic Constituents
  • High Density Assembly with Line/Spacing resolution of 20 µm

Fig 1: Difference Between Metal Core PCB and Ceramic PCB

High thermal conductivity offered by ceramic PCBs is the main reason why industries prefer to use them. FR-4 PCBs usually require thermal vias, metal planes on the inner layers, thermal landings, and active cooling devices such as fans to transport heat away from hot parts. Ceramic PCBs do not require any of these elements except in extreme cases, as the PCB can easily transport the heat to an active cooling element, thermal landing, or device packaging.

Although thermally conductive materials are usually good conductors of electricity as well, ceramic materials are different, and their electrical conductivity is low enough for manufacturers to use them as PCB substrates. Moreover, manufacturers also use doping to adjust the electrical conductivity of ceramic boards.

The other advantages of low coefficient of thermal expansion and high surface hardness add to the attractiveness of ceramic materials. Although ceramic PCBs are more expensive than are boards made of more traditional materials, the benefits ceramic PCBs offer far outweigh the additional expense. Most industries prefer using these expensive ceramic boards more out of necessity.

Ceramic PCB Technology

Ceramic PCBs primarily use metal cores. There is no single type of ceramic material. The term refers to a class of materials with similar physical properties and chemical structure. Aluminum Nitride boards provide the highest thermal conductivity, but the material is expensive. Aluminum Oxide boards are cheaper, but with a lower thermal conductivity. However, compared to regular metal core printed circuit boards, ceramic PCBs offer substantially better thermal performance, as they do not need the electrical insulation layer between the core and the circuit traces.

Manufacturers offer other options also for the metal base of ceramic PCBs, such as Boron Nitride, Beryllium Oxide, and Silicon Carbide. Although many manufacturers offer Copper or Gold, Silver is the usual material for traces in each layer for connection. The fabricator uses a layer-by-layer screen printing process to place the metal elements or substrates.

The bonding force between the ceramic and metal foil is high, providing high surface flatness. The fabricator uses medium or small power RF CO2 laser to drill via holes with high precision, speed, and efficiency. After transferring the circuit pattern on the metal foil with photo-lithographic techniques, the fabricator pre-plates the pattern with a layer of lead-tin resist, and chemically etches away the unprotected portion to form the circuit. Removing the tin-lead layer requires washing with a solution of nitric acid.

After printing and stacking the ceramic layers, the fabricator fires the entire stack in an oven, with the firing temperature for baking the ceramic board typically below 1000 ºC. The low firing temperature matches the sintering temperature of the metal traces, and this makes the low temperature baking process suitable for using Gold/Silver as metal traces on ceramic PCBs.

Unlike traditional boards, ceramic PCBs do not come with OSP, HASL, or any other traditional surface finish. However, it is possible to gold plate the exposed pads if there is a possibility of silver corrosion.

Types of Ceramic PCBs

Manufacturers offer three basic types of ceramic boards:

Thick Film — these boards have a conductor layer exceeding 10 microns in thickness, made of Silver or Gold Palladium. Manufacturers of thick film ceramic PCBs can put capacitors, resistors conductors, and semiconductors on the board, by printing and sintering them at high temperature. Laser trimming achieves the different resistor values.

Thin Film — these boards have conductors with thickness less than 10 microns thick. Thin film ceramic PCBs are suitable for circuits requiring high accuracy, stability, and performance. Microwave circuits use thin film ceramic PCBs extensively.

DCB or Direct Copper Bonded — these boards use a special technology of bonding copper foil on one or both sides of the metal core under high temperature and pressure. DCB technology offers good thermal conductivity, high mechanical strength, excellent electrical isolation, resistance to corrosion, good adhesion, and high reliability with excellent thermal cycling capabilities. It is possible to etch DCB ceramic boards like etching normal FR4 PCBs.

Multi-Layered Ceramic PCBs

Like regular PCBs, it is possible to manufacture single-, double-, and multi-layered ceramic PCBs. In multi-layered ceramic PCBs, the high thermal conductivity prevents any formation of hot spots in the inner circuit layers and on the surface, as the ceramic material helps to transport heat uniformly across the board.

Vias in multi-layered FR-4 boards are susceptible to fracture during thermal cycling because of mismatch in the coefficients of thermal expansion between FR-4 and copper. Designers must take special care to prevent via failure.

Coefficient of thermal expansion for ceramic circuit boards are close to the values of their conductor traces. This reduces the stress on these structures during thermal cycling. The high thermal conductivity of the board also helps in ensuring the thermal expansion is more uniform, thereby preventing any via from being subject to large stresses.

Fig 2: Multi-Layered Ceramic Board

The baking and sintering process for making multi-layered ceramic boards is also useful for integrating passive components directly within the inner layers of the board. Unlike in FR-4 boards, the designer can increase component and connection density on the inner layers of a ceramic PCB.

Conclusion

Industries requiring high frequency or high-speed boards that must also withstand harsh environments benefit from the use of ceramic PCBs. Switching from FR-4 to ceramic PCBs benefits heavy industrial equipment and aerospace industries in terms of vastly improved reliability. Although the primary drawback is cost, the gains are worth the extra expense.