The bridge crossing the Tacoma Narrows in Washington state is a magnificent structure in an idyllic setting. However, it is not the first bridge that was built there. In November of 1940, the original bridge collapsed after only four months of service due to a design flaw causing the bridge to twist spectacularly in high winds. Such disasters are uncommon, so it’s sobering when something as dependable as a bridge can fail due to improper design.
In PCB design, it isn’t uncommon for junior designers to quickly throw components into a layout to move to the “fun” part of routing the traces. With PCB design, as with bridges, all the work can be wasted effort if the substrate isn’t designed correctly. Therefore, it is essential first to configure the layer stackup of the board to support the needs of the design. Here are some more details about the importance of PCB stackup design and what to be aware of before going into the layout.
PCB Layer Counts; Cutting Expenses or Making Expensive Cuts?
Considerations for PCB stackup design often revolve first around the cost of manufacturing multilayer circuit boards, and most people want to find ways to reduce the bottom line. The quick answer is yes. You can save money by eliminating layers in your board layer stackup. What is less understood is that reducing the layer count may increase other costs while saving manufacturing costs. Consider these possibilities with fewer PCB layers:
- Vias: With fewer layers for routing, more vias may need to be added to the design to connect all the nets. Not only will this increase the fabrication costs of the board with extra drilling, but the additional vias may also end up affecting the circuit performance.
- Copper: Reducing the layers may force you to get creative in the layout of your power and ground planes. This creativity will probably affect the power integrity of the board, and it may cause problems for manufacturing with unbalanced copper.
- Routing: Without the extra layers to route orthogonally, you will probably need to be creative in winding traces around. As stated above, this will mean increasing the number of vias and degrading the signal integrity of the board.
The problem gets worse when considering high-density designs. These layouts are packed with fine-pitch, high pin-count parts, out of which it’s difficult to route. Multiple via escape patterns are usually required to bring out all the larger devices’ nets, which means additional layers. There will also be specific routing topologies to contend with, such as fly-by or T-topologies associated with DDR routing. Add to that multiple power supplies, areas of circuit isolation, and thermal considerations, and it’s evident that settling on a layer stackup configuration involves much more than just the cost. The circuit performance aspect of the design will also need consideration, which is covered in the next section.
The Importance of PCB Stackup Design for Circuit Performance
Most circuit boards today have some elements of high-speed circuitry involved with them, meaning that the signal integrity of the board will be a primary consideration. High-speed transmission lines usually require an adjacent ground plane if routed on the board’s surface in what is known as a microstrip layer configuration. If the high-speed transmission lines are routed on internal layers sandwiched between two ground planes, this is known instead as a stripline configuration. These layer configurations will provide an excellent reference plane for clear signal return paths, eliminating the most prolific cause of electromagnetic interference (EMI) in circuit boards. The ground planes also help shield the high-speed transmission lines from incoming interference and radiate their own interference.
Sensitive high-speed signals require added space to protect them from other forms of noise and interference:
- Trace separation: When high-speed traces are too close to each other, one signal may overwhelm the other and impose its signal characteristics on it instead. This imposition may cause the victim signal to mimic the aggressor signal instead of producing its expected behavior.
- Impedance matching: Going through different areas of the board can change the characteristic impedance on high-speed signals. This change can cause signal reflections which could distort the original signal and interrupt the intended function of the circuit.
- Circuitry isolation: Power circuitry and analog circuitry can all create their own brand of noise, impacting the regular operation of digital circuitry. It is imperative to keep the trace routing of these different areas isolated from each other.
The board’s power delivery network (PDN) is another area that can suffer if it isn’t designed correctly into the board layer stackup. There must be enough layers to support the multiple power nets in a high-speed design so that power is delivered cleanly throughout the board. High-speed digital devices have fast switching speeds, which may not always return to their low state. This “ground bounce” effect can cause the ground reference level to rise in the board, creating false states and corrupting the signals sent and received in the devices. One way to counter this problem is to configure the board with large ground planes and make sure each ground pin of the high-speed devices is directly connected.
All of these issues need careful consideration when designing PCB stackup. In controlled impedance routing, the board layer materials, configuration, and thickness all require calculating together to arrive at the correct trace widths. This amount of configuration often leaves designers with a boat-load of questions, but fortunately, there’s a resource that can help.
PCB Materials, Stackup Configurations, and Other Questions that Need Answers
Some designers don’t realize how many different PCB materials are available when building a circuit board. Traditionally, FR-4 or one of its epoxy resin variants is used to fabricate a PCB. However, depending on the circuitry you are designing, FR-4 may not give you the performance you need. FR-4 does not provide as tight control over dielectric constants as other materials and suffers greater signal loss with less mechanical stability. You may find yourself needing to look into more advanced materials such as:
- Enhanced Epoxy: A step up from standard epoxy resins, these materials have better electrical properties and do well with complex multi-layer high-speed circuit boards.
- Polyimide: These are highly robust circuit board materials and have excellent thermal and mechanical properties, perfect for harsh operating environments.
- PTFE (Polytetrafluoroethylene): Known by the more common name of Teflon, these board materials are well-suited for high-frequency RF circuitry with their low dielectric loss.
Seek out a trusted information resource to learn which material best suits the needs of a design and which layer stackup configuration will be the most optimum. At VSE, we have been helping high-speed designers understand the importance of their PCB stackup design for many years. Our electrical and manufacturing engineers will help determine which options are best to give a board the best performance and quality.
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