I wouldn’t call myself a rockhound, but I’ve always been fascinated by crystals. From an early age, I’ve been captivated by the way that these rocks appear to change color when rotated. Of course, now I know a bit about refraction, reflection and the electromagnetic spectrum and how differing wavelengths define the color we see. Nevertheless, it’s still intriguing how a simple change in orientation can give you a brand new perspective.
When designing single or two-layer PCBs, it is typical to view your design from a two-dimensional perspective. However, the increasing demand for more compact electronics products requires PCBs with multiple layers and a three-dimensional design perspective. This design perspective adds issues, such as SMD packaging and layer stackup to your design considerations. Before looking at how best to create your PCB stackup, let’s see how the design perspective changes for multilayer boards.
The PCB Stackup Design Perspective
Regardless of the number of layers that comprise your board design, specifications for pad spacing, clearances, trace widths, copper weights and drill hole sizes must be tailored to your contract manufacturer (CM). If your design requires multiple layers, you will need to consider additional design for manufacturing (DFM) specifications for signal, power and ground routing through vias, as well as the PCB stackup. These additions entail incorporating verticality into your design perspective.
The PCB stackup design perspective that includes both vertical and horizontal considerations is a three-dimensional approach that significantly impacts board fabrication and PCB assembly. For fabrication, the number of layers, their arrangement or stackup and type of material must be determined. These choices require coordination between thicknesses restrictions; material parameters; such as dielectric constant, the coefficient of thermal expansion and electrical strength, signal type isolation and drill hole options. For PCB assembly, via routing options and their effect on the soldering process are of great importance. The capabilities of your CM limit the choices for the vertical aspect of your design.
Now, we can look at the best ways to implement the PCB stackup design perspective.
Top 4 Tips for PCB Stackup Design
Following the tips below in conjunction with the DFM guidelines and support of your CM is the best way to implement the PCB stackup design perspective into your overall board design.
Tip #1: Determining the number of layers
The first consideration for your PCB stackup is determining how many layers are needed. This includes considering signal (high speed, low speed), power (for high power boards or when power supplies are a part of your circuit) and ground layers or planes. It is highly recommended not to mix signal types on inner layers. For most boards, you can use the pin density to get an accurate layer count:
After calculating the pin density, you may use a readily available lookup table to find the number of signal layers and total layers or consult your CM for stackup support. You may be required to furnish additional information, such as control impedance values.
Tip #2: Determining the layer arrangement
After determining the number the layers, you need to determine how they should be arranged or stacked. Good rules to follow here are:
- Route high-speed on minimum thickness microstrips.
- Place signal layers next to internal power layers for tight coupling.
- Power and ground layers should have minimal spacing between them.
- Avoid having two signal layers adjacent to each other.
- Make the stackup symmetric from the top and bottom layers inward.
Tip #3: Determining layer material types
An important consideration for your PCB stackup is the thickness of each signal layer. This should be established in conjunction with determining thicknesses for prepreg and core(s). There are standard thicknesses, as well as other properties, for different circuit board material types. Your process for selecting materials should include these electrical, mechanical and thermal properties.
Tip #4: Determining routing and vias
Completing the PCB stackup design is the determination and routing of the traces. This includes determining copper weights, where to put vias and what type of vias to implement. As for all specifications for your design, you should work with your CM in making these determinations as some CMs avoid certain via types, such as via-in-pads.
DFM for HDI Printed Circuit Boards
PCB Stackup Design Example
Many possible PCB stackups may be realized, depending on your implementation of the above tips; however, the following illustration is a good reference example for an 8-layer PCB stackup.
Example 10-layer PCB stackup
When multiple layers are involved, your board design requires you to incorporate a PCB stackup perspective. This perspective is a three-dimensional approach to PCB design where the internal structure is as important as the surface layout. By applying the tips above, you can easily overcome the nuances associated with this design approach and including your CM from day 1 of design should ensure that your PCB stackup is the best manufacturable implementation for your requirements that meet your CM’s capabilities.
|Tempo‘s Custom PCB Manufacturing Service
At Tempo Automation, we can construct your desired PCB stackup design to 64 layers. To get you started on the best path, we furnish information for your DFM, enabling you to easily view and download DRC files. If you’re an Altium user, you can simply add these files to your PCB design software.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on the PCB stackup or how to create the best layer stackup for your design, contact us.