Introduction to Ceramics

Ceramics have been the traditional material not only for electrical applications for the past 100 years, but have also been especially useful for highly reliable electronic applications. For instance, in the 19^th century, ceramics were the standards for isolators and light bulb sockets. Moreover, radio tubes, early pacemakers, and military electronics extensively used ceramics in the 1930s. Since then, manufacturing technology has enhanced the material class amazingly from plain materials through new mixtures and nanotechnology to the level of today’s technical ceramics.

Properties and Materials

Compared to the plain ceramic materials earlier, new technical ceramics have improved on their durability, inertness, and chemical characteristics. Even their physical properties have undergone a sea of change, for instance, they do not shatter as easily. In most application cases, specifically for applications in space, it is much more than a single reason for using ceramic as the appropriate material system. However, ceramic materials are only a category and not the technology or a specific chemistry. Ceramic is usually a large group of technical materials providing good opportunities for enabling advanced requirements.

The greatest advantage of ceramic materials is their thermal mechanical behavior. Among thermal characteristics is included the coefficient of expansion, thermal conductivity, thermal capacity, aging under the influence of thermal cycling, and the ability to withstand higher temperatures.

Individually, as well as combinations of the above characteristics, are of advantage to the electronic applications, especially for space. For instance, unlike polymers and epoxies, ceramic materials do not show decomposition, and their chemical bonding does not break down from heat and UV radiation as it happens with organics. Moreover, ceramics do not soak or absorb humidity in a significant scale, and do not outgas in the extreme vacuum of deep space.

Function

In comparison with FR type of PCBs, ceramic materials need structuring for electronic functionalities. This requires different technologies and use of other materials. For instance, PCBs made of ceramic and copper may use alumina or aluminum nitride covered by copper foils using epoxy adhesives, but this would not help in thermal applications. This and other restrictions have led to product solutions such as DBC or direct bonded copper, including comparable covering techniques for AlN, which is widely used for power chips such as IGBTs.

Aerospace Application

Aerospace applications usually do not have miniaturization as their main target, and use ceramic PCBs mainly as a base for power dominated technology. To benefit definitely from this group of materials, engineers and designers must understand the limits and restrictions these materials possess, and interact with necessary process conditions in combination with calculations and balancing of the pros and cons.

Some advantageous characteristics of ceramic materials for electronics in aerospace applications are:

• Coefficient of thermal expansion CTE very close to silicon and far below that of most usual metals
• Excellent electrical isolation (even in elevated temperatures and over lifetime)
• Good thermal conductivity as an isolator (useful for heat spreading and distribution)
• Stable dielectric properties and low losses at high frequencies
• Chemical stability against many chemicals, moisture, solvents, and consumables
• Very slow aging due to consistency of substance
• Compatibility to noble metal paste sintering technology, resulting in highly reliable conductors
• High processing temperatures, far removed from normal operating range
• Thermal resistance, showing no classic melting, decomposition, or softening
• Mechanical stiffness, allowing rigid carriers, hardness, and wear resistance for sensors working in vacuum, fluids, and in industrial pollution
• Resistance to EUV, plasma and ion bombardment as well as practically no outgassing in high vacuum, ideal for sensors for EUV semiconductor equipment.

Conclusion

At PCB Global, we have the technology capabilities not only to fabricate ceramic PCB’s, but to also assist you with any design specifications you may have regarding the application, use and outcome of the purpose of your ceramic PCB. For any enquiries or if you would like to arrange a quote for your ceramic PCB, please don’t hesitate to contact us as sales@pcbglobal.com

Stackup in the design of a multilayer Printed Circuit Board (PCB) is an important factor from the point of view of the Electro-Magnetic Compatibility (EMC) of the product in which the PCB is used. Much of the radiation from the PCB can be reduced with a good stackup, whereas a poor stackup can worsen the radiation considerably.

Basic Factors to Consider

While designing the stack-up of a PCB, a designer must consider the following factors;

·      The number of layers to be used

·      The number of power and ground planes

·      Sequence of the planes and spacing between the planes.

While the spacing between the planes is of importance to the PCB manufacturer, the other factors are important for an optimum design of the PCB and its EMC performance.

Design of Multilayer PCB’s

Signals

In a regular PCB, the designer decides on the number of layers based primarily on the number of signals he/she has to route and their frequency of operation. For a rigid flex PCB, the designer must consider an additional factor—the nature of the bend of the PCB. For instance, a flexible part of the PCB may be joining two of its rigid parts, and the placement of the flexible part in the stackup in relation to the rest of the rigid part helps to minimize stresses and determines the final form and fit of the PCB.

Layers

The total number of layers to be used also depends on whether the PCB will finally reside within an unshielded enclosure or a shielded one, and the type of emission class the product is required to meet. Requirements that are more stringent need higher number of layers, with larger number of ground and power layers, making the stackup design more critical. Usually, the stackup is more of an optimum achievement within the restrictions of time and cost.

Planes

Introduction of multiple power and ground planes in the stackup of multilayer rigid flex circuits leads to a significant reduction of radiated emission. This is because the presence of the planes allows improved signal routing configurations. The designer has proper control over the impedance and is able to reduce the ground noise significantly by using a large ground plane.

Balanced/unbalanced Construction

While designing the stack-up of a rigid flex PCB, it is important that the designer maintain a symmetrical cross-section of the board to enhance the mechanical strength and to prevent warping of the board. Sometimes, designers also use an unbalanced construction as this allows stackup configurations that are more suitable to the design.

Signal Layers

While deciding on the stackup of a multilayer rigid flex PCB, the designer usually keeps the signal layer adjacent to a plane, which couples the signal layers tightly to their respective planes. While this process allows the ground and power planes to couple closely together, it also allows the designer to route high-speed signals within buried layers located between the planes. This helps to reduce cross-coupling and interference.

Stack-up

The stack-up design requires a close coordination between the designer and the PCB fabricator. The spacing between the planes within a stackup is important for maintaining the impedance, and for achieving the required overall PCB thickness.

4,6,8 & 10 Layer Standard stack up examples as per below

Conclusion

The design, construction and purpose for Rigid Flex PCB’s leads to endless possibilities for their use. For more information on Rigid flex PCB’s please visit our blog at http://www.pcbglobal.com/17/blog.htmlor if you would like some advice on how to get the most out of your PCB stack up for your Rigid Flex PCB’s, please feel free to contact the PCB Global team at sales@pcbglobal.com

Although the design of aluminum based printed circuit boards (PCBs) is no different from that for a traditional FR-4 board, the similarities are limited to the imaging and wet-processing operations. To make the design cost-effective and manufacturable, an additional secondary mechanical operation is necessary. Additional considerations are necessary for the solder mask, legend printing, and mechanical fabrications.

Structural Considerations

PCBs constructed with a metallic support base separated by a thin dielectric from the copper conductors of the circuit are also called Insulated Metal Substrates (IMS). Usually, aluminum is the choice for the carrier material because of its lower costs compared to other metals. The dielectric separating the substrate from the conductors has substantial influence on the total performance and it determines the thermal resistance. Depending on the requirement, the dielectric can comprise layers of filled or unfilled epoxy resins.

Although filled resins offer a 3-10-fold higher thermal conductivity as compared to that from FR-4 material, this depends on the filling material and the quantity used. Additionally, the filling material influences the cycle resistance of the board. One of the major considerations the designers face with IMS PCBs is the coefficient of thermal expansion or CTE. As the board transfers heat to the metal part, it expands at a rate different from that of the dielectric and the copper traces. The differences in CTE between the various constituents of the PCB create stress on the solder joint.

To minimize the effect, copper is preferred to aluminum as the base material, as it has a lower CTE. However, this increases the cost of the PCB. Other design variants include IMS with exposed copper, and aluminum with thin insulation layer.

IMS with exposed copper is an optimized variant. Here, the metal of the copper substrate protrudes partially through the insulation layer, and there is no insulation to impede the transfer of heat. Therefore, electronic components can operate at higher currents or higher power levels. However, this requires the connected thermal contact points to be electrically neutral.

Aluminum with a thin insulation layer is a modified form of IMS technology, where a thin layer of ceramic or aluminum oxide is used, rather than the dielectric made of epoxy resin. Depending on the application requirement of dielectric strength, the layer thickness may vary from less than 40 µm to 125 µm. This helps to achieve thermal conductivities of approximately 2 W/mk.

Design Considerations

Considering reliability, experts recommend housing the control part on a separate standard PCB rather than placing it together with the power part on the aluminum based PCB. One of the most important criteria for aluminum PCBs is the minimum distance for drill holes, as the base substrate is a conducting metal.

For double-sided aluminum core circuits, this requires insulating the aluminum core against through-plating. Usually, the aluminum core must be pre-drilled, and excess resin used when press-molding the aluminum core with prepregs.

This opens up the possibilities of manufacturing a multilayer PCB with an aluminum core. In addition, it is also possible to produce multilayer rigid-flex PCBs, by using an aluminum core of 0.5 mm thickness.

Conclusion

Special design rules may apply for your aluminum PCB dependent on the purpose of the PCB and the outcome you are wanting to achieve with your specifications. Standard aluminum PCB’s can be ordered online on our website http://www.pcbglobal.com/quote/aluminium-pcbs/or for any requirements outside the online capabilities, please email your design file to sales@pcbglobal.com

If you are looking for small volume electronic products that must be highly reliable while operating at high frequencies and high insulation in environments with high pressure, high temperature, and high pressure, Metal Core PCBs (MCPCBs) may be a good choice. However, there is other alternative — a ceramic PCB.

Characteristics

A brief overview of the basic structure of ceramic PCBs offers an insight into why they offer such excellent performance. Usually, ceramic PCBs are made from 96-98% Alumina (Al2O3), Aluminum Nitride (AIN), or Beryllium Oxide (BeO). Although for thin or thick film technology, silver palladium (AgPd) is preferred as the conductor material. For the requirement of direct copper bonding, copper is used. Ceramic PCBs can operate in the temperature range of -55°C to +850°C, and they have excellent thermal conductivity ranging from 24-250 W/m-K, depending on whether the ceramic material is Alumina, Aluminum Nitride, or Beryllium Oxide. Ceramic materials exhibit great compression strengths of above 7000 N/cm2, with breakdown voltages of up to 28 KV/mm for 1.0mm thickness. The thermal expansion coefficient under operating temperatures of 50-200°C is about 7.4 ppm/K.

Types of Ceramic PCBs

Depending on the manufacturing method, three basic types of ceramic boards are available in the market:

• Thick Film Ceramic Boards
• Thin Film Ceramic Boards
• DCB Ceramic Boards

Thick Film Ceramic Boards

These are so called because of the thickness of their conductor layer, which may exceed 10 microns, but not more than 13 microns. The conductor layer is usually silver or gold palladium, and printed on the ceramic substrate.

The advantage of thick films on ceramic boards is manufacturers can put interchangeable conductors, semi-conductors, conductors, electric capacitors, or resistors on the ceramic board. After completing the steps of printing and high-temperature sintering, all the components on the board can be laser-trimmed to their desired values.

Thin Film Ceramic Boards

The thickness of the conductor layer in thin film ceramic boards is less than 10 microns and deposited on the ceramic substrate using thin film manufacturing technologies such as electroplating, sputtering, or evaporation. The thin films are useful in producing on-board passive networks, assemblies for micro-components, and hybrid integration of circuits formed by packaging.

Depending on the concentration of component parameters and the distribution of the passive networks, thin film ceramic PCBs may be further categorized into lumped or distributed parameters. While lumped parameters cater to frequencies lower than that used for microwaves, the distributed parameters are meant for operating within the microwave band alone. Usually, the equipment used for manufacturing thin film ceramic boards is more expensive than those used for making thick film types. In addition, the cost of production is higher for thin film technology.

Thin film ceramic PCBs are very useful for analog circuits such as for microwave circuits, as they need to exhibit high accuracy, greater stability, and excellent performance.

DCB Ceramic Boards

Direct copper bonded (DCB) technology represents a special process where a copper foil is bonded on to the ceramic core (AIN or AL2O3) on one or both sides. The bonding takes place under high temperature and pressure.

This type of bonding not only gives the super-thin DCB substrate high bonding strength, but it also has excellent isolation, high thermal conductivity, and fine solderability. Showing high current loading capacity, the DCB ceramic board can be etched similar to normal FR4 PCBs are.

Conclusion

At PCB Global, we have the capability not to only provide ceramic PCB’s, but to also assist you with any design specifications or inquiries you may have regarding the general use and outcome of the purpose of your ceramic board and also determining if a ceramic PCB is the right choice for your requirements. For any questions or if you would like to arrange a quote for your ceramic PCB, please don’t hesitate to contact us as sales@pcbglobal.com

PTFE or polytetraflouroethylene, commercial name Teflon, has an inert molecular structure that makes it an excellent material for use as non-stick coating. PCB fabricators are increasingly using PTFE laminates compared to conventional FR4 materials, because of the unique properties of PTFE that allow it to be used for high frequency applications. Although fabricating PCBs made of PTFE is very similar to those followed for conventional PCBs, fabricators need to be careful in handling the rather soft material, and fine-tune their processes with special emphasis on those areas where PTFE differs from traditional materials because of its unique properties and chemistry.

For instance, PTFE laminates are soft and the surface is able to bend, wrinkle, or dent more readily than their FR4 counterparts. While such surface imperfections are acceptable in consumer electronic circuits, they can significantly affect functional performance in high frequencies. Therefore, PTFE laminates need a flat support while storing, to prevent them from sagging or drooping, as this can set over time.

Surface Preparation for Metallization, Marking, and Multilayering

It is not advisable to prepare the copper surface of a PTFE laminatemechanically. Equipment such as bristles, pumice scrubbers and composite brushes suitable for conventional rigid material should not be used as the soft PTFE substrate could stretch to absorb the stresses leading to unpredictable dimensional results.

To prepare the PTFE surface, the standard process that the PCB industry uses is Sodium Etchants or Plasma Gas Cycling. These processes strips or removes the fluorine from the PTFE surface making it suitable for metallization, marking, and multilayering.

To avoid problems associated with registration resulting from dimensional stretching, fabricators use soap or a degreasing bath to remove the potential organics. They also use chemical cleaning to remove anti-tarnish coating on the copper foil. This typically removes about 40 millionths of an inch from the surface of the foil to promote photoresist adhesion.

Lamination

PTFE and copper films can bond without the use of bonding films and/or prepregs. Usually, fabricators use temperatures of 700F and pressure of 450-500 psi as a starting point for the lamination process. The temperature and pressure changes with ceramic filling and other compositions of the PTFE laminate.

Fabricators also use bonding films with lower melting point for reducing the processing temperatures to about 250-425F. Others may use ceramic filled bonding plies as woven glass reinforced prepreg, requiring process temperatures of 550F.

Drilling

Although there are no hard and fast rules while drilling copper laminated PTFE substrates, it is essential to employ new tools at all times. Typically, slow infeed and higher chiploads are preferred for eliminating spurious laminate fibers or PTFE tailing.

Fabricators achieve additional benefits such as easier drilling and cleaner holes with ceramic-filled laminates, as this material has a modified dielectric constant, and lower CTE. However, ceramics filling increases drill wear by 25-50%.

Metallization and Copper Plating

As pure PTFE laminates have a very high Z-axis CTE, it is necessary to use high tensile plated copper on the walls of through holes. Copper of high ductility reduces the chances of pad lift, barrel cracks, and blistering, as PTFE has an inherently low modulus.

Soldermask

Fabricators use a standard PTFE plasma cycle process prior to application of soldermask to enhance the SMOBC adhesion to the copper. For best results, application of soldermask should preferably be completed within 12 hours of circuit etching.

Conclusion

At PCB Global, we currently fabricate Teflon PCB’s for microwave applications and the defence industry and have the knowledge, experience and capability to advise our customers on the use of Teflon base material PCB’s and how this can be implemented for their custom PCB design for their intended application. For more information or any inquiries of Teflon PCB’s, please proceed to contact us or simply email your Teflon base design file for a fast quotation to sales@pcbglobal.com