Introduction to Circuit Boards

At virtually all times of the day, we are almost surrounded by technology, and most of it helps in making our daily routines easier. In fact, most of us would probably feel completely lost without our electronic devices.

Almost everyone knows the pieces of technology are essential to them, but what many do not know is these devices need flexible printed circuit boards to keep them running. According to industrial reports, the revenue for printed circuit boards in the US in 2014 alone amounted to nearly $44 billion.

Flexible PCB’s and their Application

Flexible printed circuit boards are a patterned arrangement of components and printed circuitry utilizing flexible base materials. The main advantage over regular printed circuit boards is their flexibility, which allows the flexible printed boards to conform to the three-dimensional space available within devices.

Printed circuit boards are almost always unique, each being designed to exact specifications to meet the requirements of individual devices. The thickness of flexible circuit boards can vary widely, ranging from 1.6 mm to 0.08 mm. Among common household items, you will most likely find flexible printed circuit boards being used in microwaves, clocks, cell phones—their touch-type interface and LCD requires flex circuits.

When are they used?

As long as televisions and computer monitors were using Cathode-Ray Tubes (CRT), there was ample space inside for a rigid printed circuit board. Moving over to LCDs has limited the space inside these devices and forced the use of flexible circuit boards. Desktop printers, computer keyboards, and memory devices all use flexible circuit boards now.

Apart from the everyday technology and electronics, flexible circuit boards are also used extensively in almost all industrial applications involving transportation, military, aerospace & avionics, medical, automotive, defense, consumer electronics, communications, satellites, semiconductors, and many more.

Benefits of Flex PCB’s

With consumers and applications demanding a reduction in the size of electronic devices, the importance of flexible printed boards is projected to rise. Their popularity among manufacturers is gradually increasing as many original equipment manufacturers are gradually becoming aware of the significant benefits of flexible circuits.

Although designed and manufactured in much the same way as their rigid counterparts, there are specific differences between the two processes, which make flex circuits typically more cost-effective. Flex printed circuit boards:

• Reduce production costs by simplifying system design
• Solve internal packaging problems, especially for smaller devices with compact designs
• Offer considerable weight reduction when replacing wire harnesses
• Offer enhanced flexibility, which helps absorb and reduce the effect of vibrations
• Increase the reliability of the device, as reduction in wiring lowers the risk of connection failures
• Can be used in harsh environments, as they are resistant to corrosion, high temperatures, shock, moisture, and water

Properties of Flex PCB’s

Engineers designing equipment look for greater flexibility and stretch, which can only be met by flexible circuits. In contrast to the regular rigid printed circuit boards, flexible circuits can bend and wrap around small or unconventional spaces. This exclusive feature makes flex circuits so attractive to design engineers. All this is making product design engineers and manufacturers recognize and take advantages of flexible printed circuit boards.

Conclusion

At PCB Global, we are aim to assist all our customers unique and varying requirements. We work with a variety of factories that specialise in flex PCB’s which ensures quality and efficiency with regards to supplying you flex PCB requirements. For more information or for a quote please feel free to contact us at sales@pcbglobal.com

Modern electronic modules demand increasingly high levels of density. The miniature sizes of most surface mount devices (SMDs) that have now reached unbelievably small dimensions are primarily fueling this demand. Other advancements such as improvements in packaging technology has led to fine-pitch components such as the Ball Grid Array (BGA) sporting more than 1500 pins and a 0.8 mm pitch on average, covering entire bottom surface of the package.

Microvias, Buried Vias and Sequential Build-up

 All the above has led to an unprecedented reduction in the widths of structures on the PCB, that is, the track widths and the spacing between them. To improve the density further, fabricators are using microvias, buried vias, and a sequential buildup of multi-layer boards to improve the integration. Use of these technologies offers designers more space on the outer layers for placing components. For instance, use of buried vias precludes the use of through-holes, which would have to go through all the layers.

Use of microvias, buried vias and fine structures results in boards with very high-density interconnection (HDI), and Sequential Build Up (SBU) of multiple layers supplements this. For instance, an HDI-SBU multi-layer board will have at least two layers or a multi-layer core and one or more external layers with microvias.

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Advantages of HDI-SBU Technology

PCBs with standard lamination and through vias are less expensive to manufacture, as they have simple via models and use ordinary dielectrics such as FR-4. The manufacturing process is mature, and fabricators can produce high-reliability boards.

As the layer count increases, fabricators start facing problems with boards using standard lamination and through vias. The costs skyrocket, and few fabricators can reach good yields. They face difficulties such as delamination at high temperatures, increase in layer count as large vias reduce the ability to route, implementing BGAs with pin pitch below 1 mm, capacitive coupling from through-hole vias, long via stubs creating impedance mismatches, and many more.

HDI-SBU boards solve most of the above problems. Microvia technology makes vias much simpler to fabricate, with potentially shorter via stubs. As microvias are dimensionally much smaller than through-hole vias are, designers have much more area for routing traces, thereby offering them the only practical way of implementing multiple fine pitch components such as the BGA.

Advantages in relation to the Manufacturing Process

With smaller feature sizes for traces and vias, HDI-SBU boards enable designers achieve much higher route densities than before, resulting in fewer number of layers. Therefore, although the HDI-SBU manufacturing processes are more expensive compared to the cost of fabricating standard boards, the high-density boards can be made in much smaller sizes and lower number of layers, thus helping to offset the cost. In fact, multi-layer HDI-SBU boards with larger number of layers are cheaper to manufacture compared to standard boards with the same number of layers. Moreover, newer materials are now available, making HDI-SBU boards compatible with RoHS processes, offering better performance at high temperatures, and improved signal and power integrity at high frequencies.

Applicable Standards for HDI-SBU

Japan Printed Circuit Association publishes IPC/JPCA-2315, jointly with IPC, and these standards provide easy-to-follow tutorials on selecting HDI and microvia design rules and structures.

Conclusion

The team at PCB Global are able to assist with any queries you may have in relation to if HDI-SBU PCB’s are necessary or beneficial to your PCB specifications, requirements and outcomes. Please feel free to contact the team at sales@pcbglobal.comfor any more information on this area of PCB manufacture.

If you are using plated through holes (PTH) in thick backplane/midplanes and printed circuit boards (PCBs) with a high layer count, they might be distorting the high speed, and high frequency digital signals passing through them. Depending on the stub length of the PTH, the distortion may be severe enough to prevent digital receivers from distinguishing between a logical one and a logical zero. The situation becomes worse with increasing data rates, as the distortion introduced by the PTH stub also increases, usually at an exponential rate.

Potential Issues

The via stub introducing the undesired distortion is the portion of the PTH via that is not in series with the circuit. As the via stub does not serve any useful electrical function in the circuit, PCB manufacturers remove them by the back-drilling technique or controlled depth drilling, using a conventional NC drilling equipment. The technique uses a drill bit, with diameter slightly larger than the onethat created the original via hole, to remove the copper plating from the via stub.

Back Drilling and Bit Error Rate

Bit error rate in high-speed PCBs depends on deterministic jitter, which is a type of signal distortion and is particularly problematic as data rates increase. As the stub length of PTH contributes significantly to the signal distortion, they affect the bit error rate as well. Removing the via stubs in the path of high speed signals by back drilling helps to reduce signal distortion, thereby improving the bit error rate considerably, often by several orders of magnitude.

Back drilling of PTH vias introduces other operational advantages as well. As the technique improves impedance matching, the signal attenuation reduces. This allows the channel bandwidth to increase. In addition, the smaller stub end reduces the EMI/EMC radiation, via-to-via crosstalk, and excitation of resonance modes.

Stub Length and Distortion

As high-speed signals traveling along a trace come across a PTH with a stub, reflections from the stub end mix with the signal and create distortions. The amount of reflection depends on the equivalent impedance of the stub, which in turn, depends on the physical length of the stub. Longer the stub, greater are its shunt capacitances that reduce the equivalent stub impedance, thereby increasing the reflections.

The simplest solution to the above problem is to reduce the length of the stub—by back drilling. The residual stub length, left over after the back-drilling operation, is much smaller, resulting is smaller reflections, and hence, improving the signal integrity.

Alternative Methods

Manufacturers use several alternative methods, as back drilling can be an expensive operation. These techniques involve alternative stackup arrangements and laser-drilled vias. The designer can move traces to layers close to the end of the via stub to reduce the stub length. Although several alternative methods do exist, these techniques may not be viable from cost and manufacturing standpoints, especially for high-density and backplane/midplane PCBs. The only option in these cases is to remove the via stub by back drilling.

Conclusion

The actual length of the via stub remaining after the back-drilling operation is dependent on a number of variables. One of them is being aware of the physical location of the signal layer accurately to which the drill must travel, and this introduces a level of uncertainty. If there are any issues that are related to this design, PCB Global’s Computer Aided Manufacture (CAM) team will find this in the initial processing phase and be able to assist with reconfiguring your design to match your desired outcome and your PCB requirements.

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.