Exploring Advanced Techniques in PCBs

A PCB or Printed Circuit Board is commonly at the core of most electronic devices. Typically the heart of the system, these PCBs can have diverse functionalities like providing computing power, regulating the power input, or controlling the activities of the device. Coming in a wide variety of shapes, sizes, and kinds, the complexity of the boards vary depending on the device requirements. Irrespective of their looks and functions, all boards share some common attributes. All boards have some electronic components soldered on to the board surface, with metal traces traversing different layers in the board and electrically interconnecting these components. In this article Rush PCB Inc explore advanced techniques that designers and manufacturers of PCBs use to fabricate printed circuit boards.

The entire story of a PCB begins with its schematic circuit design and ends with an assembled and fully functional printed circuit board assembly. In between, there are various important milestones like layout, routing, BOM creation, material selection, PCB fabrication, assembly, and testing.

The evolution of new emerging technologies in the electronics industry presents a continuous challenge to the design and fabrication of circuit boards. Advancements in various fields like medical, aerospace, telecommunications, and computing are pushing the frontiers of design and manufacturing to achieve new complexities in PCBs. For instance, designers now use advanced routing techniques for complex and highly dense circuits. Examples of such modern designs include wearables, IoT, and smartphones. While these new device types require newer levels of electronic technology, they also need newer and more complex PCB designs.

Routing for Advanced Components

With electronic devices improving their functionality while reducing their form factors, component manufacturers support them with highly sophisticated components that pack a lot of functionality and power in newer packages like the BGA or ball grid array. Unlike the regular integrated circuits with pins on their edges, BGAs have an array of closely spaced solder balls under their body.

Depending on their complexity, BGAs can be large components with increasing pin counts, but reducing pitch. These components demand precise routing skills for successfully routing traces from all the pins under the BGA. Mostly used for memory components, BGA routing demands newer methodologies of trace routing for accommodating their double data rate connectivity.

Designers developing boards for such advanced components will require using newer routing strategies and techniques, and must become familiar with them. Not only must they learn about trace routing for BGA packages, but also have a better understanding of DDR routing technology.

DDR Routing Techniques

Connecting high-speed memory circuits require one of the most exacting PCB trace routing. Specific routing patterns are necessary for DDR or double-data rate memory to work properly. Primarily, it involves matching trace lengths for achieving proper signal integrity and capturing each edge of the clock signal.

Apart from requiring placement precision, the memory chips must have adequate room for routing all connections. It is typical for designers to add or subtract to the trace length to get them all to match. This is known as trace tuning. It may also require the designer to put in specific routing patterns, depending on the type of memory circuit they are using.

Using Proper Tools

Precision routing requires maintaining the constraints of routing architectures to achieve a superior PCB design. Whether routing complex and dense DDR memory, breaking out fine-pitch high-pin-count BGA ICs, or routing for medical wearable devices, designers must use considerable skills. Fortunately, most modern PCB design tools come equipped with enormous amounts of functionality to enable designers achieve their objectives.

For instance, most PCB design tools allow assigning design rules to classes, components, nets, and include high-speed design rules as necessary for the design. They also have helpful features such as assigning specific areas of the board where traces will automatically reduce their width to allow routing high-density BGAs. Most PCB design tools can automate the trace tuning process to effectively match trace lengths.

Advanced PCB Design Considerations

While schematic designs define the functionality of a board, its layout determines its performance in terms of signal and power integrity. The designer achieves the latter with proper stackup design and effective routing techniques so that the finished layout conforms to the functionality defined by the schematics. With a proper PCB design tool, a designer has access to a full set of tools for routing, stackup design, and post-layout analysis tools. These not only allow the designer to complete the design, but also verify the performance of the board. There are primarily six key areas that the designer must ensure the PCB design meets:

Stackup and Routing

Proper stackup and routing is critical for board performance, more specifically for those operating at high speeds and high frequencies. Proper routing is important for impedance control, signal integrity, and crosstalk suppression throughout the board. Proper stackup design is necessary to achieve good power integrity in boards.

Signal Integrity

Any signal, whether high- or low-frequency, must appear without change at the end of its travel. This may require using proper channel compensation and clock recovery techniques, especially when using schemes involving multilevel signalling.

Power Integrity

Power availability all over the board must be stable and without any additional noise. Supply voltages must be within the specified tolerances, and should not dip when components draw current.

Return Currents

Designers must prevent interference from return currents by providing proper reference planes and return path management. This is an important consideration, especially with circuit boards operating at high frequencies and high edge rates, such as in mixed signal boards, where there is a possibility of interference between analog and digital sections.

EMI/EMC

If the board has to pass EMI/EMC tests, the designer must take adequate steps to effectively reduce conducted and radiated EMI within the board, while improving its EMC to external signals,

Thermal Management

With high-density component placement, and reduced form factors, boards can run dissipating considerable heat, especially with high-frequency operations and high data rates. The designer must consider effective thermal management strategies to remove the heat generated and keep the board temperature within the specified safe operating limits. While this may involve using both active and passive thermal management techniques, it also depends on the application environment.

While passive thermal management techniques may make use of heat sinks and fans to cool hot components, active thermal management may be more extreme. Designers may use alternative substrate materials like ceramics, as its higher thermal conductivity allows a more effective removal of heat. Designers may also use MCPCBs or metal core PCBs if the application allows. These PCBs typically have a thin substrate backed by an insulated metal plate that allows faster dissipation of heat.

Multi-Board Stackup Design

Most high performance PCB designs are typically multi-board rather than stand alone systems. For instance, the boards may be part of a complex arrangement of several rigid-flex boards. Working on multi-board systems increases the complexity of the design, where all the factors listed above can create a complex behavior of radiative emission patterns.

Designing high-performance PCB systems may require the designer to pay careful attention to the layout, in view of the complex interplay between stackup, routing, signal integrity, power integrity, and EMI/EMC. A proper design layout can help to solve problems related to signal noise sources and power integrity. On the other hand, an improper design layout may result in aggravated noise and EMI. Starting the design with a well-thought-out stackup is helpful, as this eases subsequent strategies for a high-performance PCB design.

A proper stackup also helps determine routing and return path strategy in a PCB. Most high-speed boards are likely to use impedance controlled differential pair routing for multiple signal lines. With strategically placed ground and power planes, the board will run with adequate power stability.

In high-frequency boards, the designer may have to consider using alternative substrate materials to keep losses in check. This may include using materials with high- or low-k dielectrics. Most designers prefer using materials with high dielectric constants and very low loss tangents, as this combination offers higher interplane capacitance, thereby improving power integrity and reducing signal losses. However, it may not be possible to satisfy both requirements at the same time, in every board, and for all frequencies.

Impedance Controlled Routing

Traces typically behave like transmission lines, especially those carrying high frequencies. Unless terminated properly, reflections can occur in these traces, disturbing the signal integrity. To prevent this, designers must use careful impedance controlled routing throughout the board.

High-performance PCBs operating at high frequencies and modulation, typically use differential pair traces to route signals. Designers need to eliminate skew and common mode noise in these signal lines by using precise coupling and length matching techniques. Here, the stackup is important as it determines the trace dimensions necessary to ensure consistent impedance throughout the board. It is necessary to route traces over the shortest possible path, thereby improving signal integrity.

HDI Routing

Data center environments typically use more advanced boards that must accommodate multiple channels with high data rates. Additionally, these boards also feature small form factors. For such boards designers must employ HDI design along with controlled impedance routing to allow the design to fit within the required space. Several designs are resorting to the use of rigid-flex or flex PCBs to accommodate the board in the limited space.

Reducing form factors and increasing complexity in mobile devices is forcing designers to design with HDI technology. Here, the designer uses laser-drilled blind and buried vias to route traces between layers. The designer also uses higher layer counts where each layer is very thin. The substrate is typically fully flexible.

EMI/EMC Shielding

With increasing component density and small form factors, high performance PCBs need more attention to EMI/EMC. In a board, EMI can arise due to multiple reasons, including switching ICs, resonance on mismatched lines, radiation from strong transmissions, noise from power lines, and many more.

While smaller form factors rule out the use of old-school shielding cans, designers must use more sophisticated techniques for reducing EMI/EMC in high performance boards. These include fence design, stripline routing, and a careful stackup design, while eliminating potential noise sources. For such boards, designers prefer using differential pairs, as they not only resist common mode noise, but also have significantly reduced radiation when compared to single-ended transmission lines.

Conclusion

Creating advanced high performance printed circuit boards requires the use of right design tools, and a thorough design evaluation before the first prototype run. Rush PCB Inc recommends using simulation techniques to create a baseline performance metric against which the designer can compare the results of testing the first prototype.