Introduction

Most reputable, customer oriented printed circuit board (PCB) manufacturers prefer to use microvias when building cost-effective products for their customers. High-density, multi-layer PCBs use microvias, as they represent the most affordable yet highly function-able interconnect solution.

Fabricators use stacked pads in multilayered PCBs that they puncture and connect electrically with a series of copper tube-lined holes. The conductive holes are the vias, and traditionally, there are three types of these—through, blind, and buried, as in Fig, 1.

Fig. 1: Traditional Types of Vias

1. Through vias: these connect two exposed surfaces and any inner layer, going through the entire board.
2. Blind vias: theseconnect one exposed surface with one or more inner layers, but do not go through the board.
3. Buried vias: these connect some inner layers with each other, but do not extend into the exposed top or bottom layer.

Application

Traditionally, PCB manufacturers drilled all vias with mechanical drills. However, with increasing density, and the advent of fine-pitch components such as BGAs, they needed very small diameter vias, and mechanical drills were rather fragile. This prompted fabricators to change over to lasers instead of using regular drill bits, as lasers could drill holes with diameters as small as a few micrometers.

Although lasers can drill holes of extremely small diameter, they have a shortcoming—the depth of the laser-drilled holes is limited to a single pair of layers. That means manufacturers have to fabricate layers in drill pairs—1-2, 3-4. 5-6. etc., so they use the laser to punch through two stacked pads to create the microvia, before they laminate the pairs of layers together, as in Fig. 2.

As evident from Fig. 2, now fabricators have the freedom to make any type of via structure shown in Fig. 1, using the new microvia technique. In fact, now they have the ability to mix and match the old technology with the new to arrive at the cheapest solution.

Accordingly, the board manufacturer can have staggered or stacked microvias, and even mix stacked or staggered microvias with buried vias, if they so wish. To connect several layers or those that are not adjacent, the fabricator merely sets up staggered or stacked microvias. Stacked microvias sit right on top of each other, while staggered although placed atop each other, do not maintain the same z-axis.

Advantages of Microvias

According to IPC standards, manufacturers have to make buried and blind vias less than 150 micrometers in diameter. Therefore, microvias are unbelievably tiny structures, allowing them to connect high-density layers of advanced PCBs.

The miniature size and comparatively improved capabilities of microvias are also a part of the reason for the increasing processing power of our digital age. By using microvias, manufacturers are able to bring down the number of layers used by a PCB, reducing its cost, thereby decreasing the physical size of electronic gadgets, at the same time improving its electrical characteristics and functionality.

Why Edge Plate?

A plated through hole(PTH) is a common feature in multilayered printed circuit boards. Fabricators drill holes through the stack and electroplate the walls with a layer of copper connecting two pads, one on the topmost layer and the other on the bottommost layer of the board. The two pads may further connect to other copper traces or planes on the two layers and if necessary, to traces or planes on some inner layers as well. PCB manufacturers can extend this technique to edge plating; connecting the top and bottom planes of a PCB, by electroplating around its external edges.

The Process

Edge plating requires precision handling of the boards, while fabricators face several challenges, chiefly around preparing the edges for plating, and creating a lifetime adhesion for the plated material. This requires a controlled process during circuit board fabrication to limit any potential hazard for PTH and edge plating. The most significant concern is the creation of burrs, which leads to discontinuities in PTH walls and limits the life of adhesion of the edge plating.

Applications

Several industries require edge plated boards, especially in applications that require better support for connections such as for boards that slide into metal casings. Edge plating has other uses as well as it improves the current-carrying capabilities of the board, provides edge connection and protection, and offers the possibility of edge soldering to improve fabrication.

Although edge plating on printed circuit boards is a simple addition in most cases, fabricators need specialized equipment and trained personnel for the process. Designers must take care that internal power planes do not come up to the edge, and fabricators must make sure there is a gap before they take up edge plating. Designers must make sure there is a band of copper on both edges of the top and bottom side, as the plating will connect to these copper bands.

Limitations

As fabricators need to hold the board within the production panel during processing, they will not be able to plate round the complete length of the edge, therefore, some gaps are necessary for placing rout tabs. Manufacturing a board with edge plating requires routing the board profile at the place the edge plating is required before starting the process of through-hole plating. That precludes V-cut scoring on boards that need to undergo edge plating.

Working Towards Success

Designers must confirm with their fabricators the possibility of manufacturing PCBs with edge plating and the extent to which the fabricator can edge plate the PCB. Designers should indicate clearly in a mechanical layer where they need the edge plating and the type of surface finish they need on it. Most fabricators prefer a selective chemical nickel-gold as the only surface finish suitable for round edge plating.

Additional Function

Edge plating is the copper plating connecting the top to the bottom surface of a PCB, running along at least one of its perimeter edges. This has an additional function, that of preventing electromagnetic emissions from radiating or leaking out the edges of a backplane—it is a practical solution and a cost-effective one.

PCB Global

PCB Global has experienced many PCB designs that has required Edge Plating. When processing an order online, please note in the ‘special instructions’ that your PCB requires edge plating, also to ensure there are no grey areas. An “Edge Plating” mechanical layer should be provided clearly highlighting the areas that require the edge plating. 

Example below details the Edge Plaiting requirement on Mechanical Layer 5, highlighted in Pink:

For technical details on the edge plating process and specifications, please feel free to contact the sale team at sales@pcbglobal.comand we will be able to assist accordingly. 

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