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PCB stack-up is an important design and manufacturing aspect as it directly impacts the signal integrity of your design. Building an efficient build-up mainly depends on selecting the right materials and optimum layer arrangement.
In this article, you will learn:
- 4 steps to build a successful PCB stack-up
- Best practices to arrange your layer stack for good signal integrity
- Different types of layer stacks
Before diving into the steps to design a well-organized build-up, check out our infographic on 8 design rules for an efficient PCB stack-up
4 steps to build an efficient PCB stack-up
Planning, designing, and manufacturing are crucial stages in any product development process. This is the same when it comes to building your circuit board build-up. Your design specifications should always be within the capability of your CM. This reduces the time taken to build your prototype and smoothens the transition from prototype to PCB mass production.
1. Plan your design requirements to ensure manufacturability
Design requirements: Clearly define the electrical and mechanical demands of your board. This includes:
- Type of board: HDI, hybrid stackup, high-speed, rigid, and rigid-flex
- Material specifications
- Number of signal layers and power layers
- Power delivery network (PDN) and its impedance characteristics
- Impedance values for high-speed traces
- Layer assignments and trace widths to maintain controlled impedance
- RF, analog, and digital circuit requirements (operating frequencies)
- Blind or buried via requirements
- Board dimensions and tolerances
- Special requirements for mounting holes or slots
- Class 2 and 3 requirements
Manufacturing capability: Consult your manufacturer at an early stage to understand their capabilities and limitations. Get their insight on layer stack specifications like materials, thicknesses, and copper weights.
Discuss the cost and feasibility of producing your stack-up. Further, verify if it meets industry standards and regulations.
2. Use PCB layout design tools to build your stack-up
After finalizing all the specifications, build the layer stack using circuit board design software. The steps include:
- Create a new project and define the physical dimensions and shape of your circuit board (length and width, with specific cutout regions).
- Arrange layers to attain symmetry using a layer stack manager.
- Set the impedance profile requirement with the type (microstrip or stripline), target impedance, and tolerances.
- Define the required via type (through or microvia), aspect ratio, dimension, and connecting layers. You can also add back-drilling requirements.
- Configure the sub-lamination requirements, if needed.
- Run design rule checks and fix any errors or violations flagged by the DRC.
- Advanced PCB design tools offer simulation and analysis of your circuit design, including stack-up. Perform signal and power integrity analyses to check the board’s functionality.
- Download Gerber or ODB++ files. Here, you can represent the layer stack details in cross-section drawings and tables. You can also include other special requirements and specifications in fabrication notes.
Use our Stackup Designer to expedite your design process. You can build and export your stack-up in IPC-2581 format in a go using this tool.
3. Know the manufacturing tolerances before finalizing the layer stack
Layer-to-layer registration: While manufacturing, the material undergoes an enormous amount of thermal and mechanical stress. This can misalign the layers and offset the vias. If the board is not symmetrical, it can even cause the board to deform or warp.
To avoid this, ensure that all the materials used have similar CTEs, especially if you are opting for a hybrid build-up with different laminates. A maximum of 50µm or 1.9685 mil tolerance is acceptable in layer-to-layer registration.
Finished board thickness: Fabricating a board to an exact thickness may not always be possible. There will be some variation in the specified and finished board thicknesses. Hence, consult your manufacturer to ensure that the variation is within the limits to avoid issues during the final mounting process.
Board thickness tolerances are as follows:
| Board thickness | Tolerance |
|---|---|
| Less than 15 mil | ± 2 mil |
| 15 mils to 31 mil | ± 3 mil |
| 31 mil and above | ± 10% |
Controlled impedance: There can be a slight variation in the specified and manufactured impedance values. The standard tolerance for controlled impedance is ±10%. Sierra Circuits is capable of fabricating circuit boards with a tolerance of ± 5% for controlled impedance.
If you are planning to build an IPC class 3 board, ensure that the dielectric thickness is a minimum of 3.5 mil with 2 plies of prepregs.
4. Repeating the same manufacturing process in high-volume production
Once your prototype is finalized, your design can be sent to mass production. During this, your manufacturer will scale up the process to build multiple boards at once. Repeating the same process from prototype to bulk production is one of the essential traits of a capable manufacturer. Hence, discuss and understand the high-volume production process of the fab house you pick.
Additionally, your stack-up should also be compatible for mass production with readily available material and simplified design.
PCB Material Design Guide
9 Chapters - 30 Pages - 40 Minute ReadWhat's Inside:
- Basic properties of the dielectric material to be considered
- Signal loss in PCB substrates
- Copper foil selection
- Key considerations for choosing PCB materials
Download Now
5 design tips to build your PCB stack-up with good signal integrity
Even though the design rules vary depending on the application and its requirements, here are a few guidelines to help you during the design phase:
1. Select the right dielectric material
Allan Knox, Senior PCB Design Engineer at Sierra Circuits, explained, “Choosing the right laminate is crucial, as everything starts with that. It determines your signal losses, clock settings, heat management, and power handling capabilities.”
While selecting the material, ensure that it meets all the electrical, mechanical, and thermal properties required for your design. For instance, opt for low-Dk material if you are building a board that operates at a higher frequency (in GHz). However, they can be expensive, so understand the trade-off.
Copper plane selection mainly depends on the current that flows through your board. If you’re working on a high-current board, you need to choose a thicker copper plane. To learn more, see role of PCB trace current capacity in design.
2. Estimate the optimum number of signal layers
PCB stack-up is dependent on the number of signal layers present in the design. For instance, high-speed or high-power applications may require a greater number of layers compared to low-speed boards.
Low-pitch, high-pin-count complex devices like BGAs usually require a greater number of signal layers. Furthermore, signal integrity requirements like extremely low crosstalk may also require a higher number of signal layers.
Analog and digital signals will require separation between the two types and are likely to increase the number of signal layers.
3. Calculate the required number of ground and power layers
Using ground and power planes allows the designer to allocate signal layers purely for signal routing. They also reduce the DC resistance in the power and ground rails, thereby ensuring fewer DC voltage drops at the devices.
The ground layer is the plane of copper that connects the ground connection of the power supply. A power layer is a flat plane of copper that connects to the power supply rail.
The planes also provide signal return paths for time-varying and high-frequency signals. This helps to considerably reduce noise and crosstalk, thereby improving signal integrity. Power planes also improve the capacitive decoupling ability of the circuits. In addition, they also improve EMC performance by reducing EM radiation.
Read our article on 7 considerations for pcb power supply design.
4. Ensure uniform impedance throughout the high-speed traces
A uniformly controlled impedance is important for achieving good signal integrity. To attain the targeted impedance, you need to carefully design the trace geometry, including the trace width, spacing, and layer stack.
If the reference plane for the trace is not on the next layer over, the nearest copper feature can act as the reference and cause distortion. To avoid this, always have a solid ground plane right below the controlled impedance trace.
Always consider the trapezoidal effect of the trace while calculating the impedance.
5. Avoid placing adjacent signal layers to reduce EMI
Another key aspect of the layer stack design is the sequential layer arrangement. Here, you need to place alternating layers of signal, ground and power planes to reduce EMI effects. Consider these guidelines to arrange the layers for good signal integrity:
- Place signal layers below the ground plane for tight coupling
- Keep a minimum distance between the power and ground planes
- Avoiding placing two adjacent signal layers to avoid EMI issue
- Maintain symmetry in your stack-up to avoid manufacturing failures
- Don’t split the power plane as it causes noise issues
- Place two adjacent power planes as far as possible to avoid unwanted coupling
Different types of PCB stack-ups
With advancements like sequential lamination, laser drilling, and flexible substrates, we are able to build PCBs that are compact and flexible. Here are a few different types of layer stacks:
Standard multilayer PCB build-up
These stack-ups are fabricated in a single lamination cycle. It can accommodate more complex circuits when compared to single-layer boards. Due to single lamination cycle, the board undergoes less thermal and mechanical pressure. Additionally, it does not require laser drilling. Hence, board production becomes more cost-effective with less turnaround time.
On the contrary, you can only implement through-hole vias so there will be less utilization of space.
Flex and rigid-flex stack-up
They are usually built with a polyimide material, which offers flexibility. It also includes adhesives, stiffeners, and overlays. You can design the complete board with flexible material, or a part of it can be rigid. These boards are thin and flexible and are used in medical, aviation, and space applications.
HDI build-up for a compact board
HDI stack-ups are fabricated using a technique called sequential lamination. These boards feature microvias that have an aspect ratio of 0.75:1. This will help to increase the component and trace density. As the board is sequentially built, you can have a larger variety of design choices. It simplifies the design architecture.
In HDI, the number of ball grid arrays (BGA) or the highest pin count device determines the number of layers. Other factors affecting stack-up include the number of signal, power and ground layers.
Microvia structures can greatly impact the manufacturing process since they directly affect the number of lamination cycles. Any layer on which a microvia starts or stops requires a sub-construction, and each sub-construction will require an extra lamination cycle.
During each lamination, the board will be exposed to thermal and mechanical stress. This will reduce the reliability of the board. It also increases the cost and manufacturing time. Hence, arrange the vias such that it undergoes the least number of lamination cycles.
HDI PCB Design Guide
5 Chapters - 52 Pages - 60 Minute ReadWhat's Inside:
- Planning your stack-up and microvia structure
- Choosing the right materials
- Signal integrity and controlled impedance in HDI
- Manufacturing considerations for higher yields
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Stack-up for high-frequency applications
Here, the signal loss in the material becomes crucial. Hence, the layer stack is built with a low dk material that offers the least signal distortion.
If there are limited traces that operate at high frequencies, then you can even opt for a hybrid stack-up where only selected layers are built with low-Dk material. RF circuits and high-speed circuits are also built with the same technology.
Sample PCB stack-ups
Here are a few samples of different types of circuit board build-up
Standard 4-layer stack-up
6-layer rigid-flex stack-up: 6 layers with 3rd and 4th layers as flex
Hybrid PCB stack-up: Stack-up made up of Rogers 4350 and Isola 370HR material for high-frequency circuits
HDI stack-up: 16-layer board with through-hole, buried and staggered blind vias
Any mishap in the stack-up preparation can cause EMI and signal distortion in your design. If the errors are not identified during the design phase, you may have to discard the entire board. Hence, careful planning from the initial stage will help you build the right stack-up in the first attempt. If you require any assistance in building your layer stack, please comment below. We will be glad to help you out!
