Communicating PCB Layer Stackup Needs to Manufacturers
False or inadequate communication can wreck a project. Just recently, I overheard a husband and wife talking about buying blinds. Seems like a simple topic and a simple task. It probably is, if everyone listens, pays attention, and understands. While she was thinking about decorating her daughter’s bedroom, he was thinking about… hunting. Imagine his excitement as he purchased the limited edition Beak n’ Feather hay bale duck blind and how he couldn’t wait to tell his wife. I’ll leave her reaction to your imagination.
In our business of PCB design, communicating our needs to manufacturers and vendors is a top priority. The context of our requests is sometimes lost either by not providing the correct information, not listing enough information, or not giving any information. In the latter instance, the manufacturer handles decisions about the layer stackup. Problems with performance—or even late phase change orders—can occur because of a misunderstood request that impacts high-speed, high-density circuits. Mistakes and changes can lead to problems with EMI and crosstalk.
What’s the Plan, Stan?
Good communication begins with planning the layout and stackup while using software-based stackup planning tools to optimize the stackup. Those tools assist with determining the best materials for circuits that operate at fast signal speeds and require excellent signal integrity. The layout matches circuit performance with design specifications and shows the physical interconnections between the components. Are you working through a flex or rigid-flex PCB? Or is this a regular board design?
Stackups describe more than the basic construction of the PCB. Properly planned stackups consider methods for minimizing radiation, noise, and crosstalk. In addition, you can address impedance issues and signal integrity issues with the stackup. Your decision about the number of layers needed for your design depends on the number of routed signals, signal frequencies, constraints, and enclosures.
Keeping track of your layer stackup can be a painstaking task.
Do You Have Your Papers?
Good communication also continues with our ability to document the information about stackup. While communication with manufacturers may seem to occur in stages, the best communication occurs across the PCB design, development, and development processes. As you work through the PCB design concept stage, consider the components, packages, pin counts, and the stackup. In turn, your fabrication, assembly, and testing documentation should include the Gerber files for each layer, ReadMe files that show exact specifications for each layer, and NC drill data.
Members of the design team should verify the documented data. In addition, you and the design team members should review basic parameters such as signal speeds or clock frequencies. Your analysis of the design must also cover how the design controls impedance and EMI. Each detail affects the final layout and illustrates the importance of giving the correct information to the manufacturer.
For example, the manufacturer needs to know about the logical structure of the layers, including the distribution of signal layers and the power and ground reference layers. Providing detailed documentation prevents mistakes like placing the layers in the wrong order. You can prevent such errors by placing a layer number on each copper layer and using stacking stripes. When the manufacturer views the board from the primary side profile, the numbers for each layer should be visible. Stacking stripes on the edge of each layer show the physical arrangement of the layer by visually stepping down in order.
The number of layers and available materials for the stackup affects the circuit’s ability to reduce inductance at high frequencies. Communicating with the manufacturer about performance issues requires the design team and manufacturer to know about the different technologies available for the stackup. As you work with the stackup configuration, ensure that your manufacturer understands about the number of layers and any related problems that can occur. Your stackup should have an even number of layers to prevent warping.
Do you trust your design software in multi-layer PCBs? What about through PCB manufacturing?
Details, Details, Details!
When working with manufacturers, you and your design team should understand the manufacturer’s capabilities. As an example, the manufacturer that your team chooses must have the type and quantity of materials needed for the stackup. As you develop your communication about the stackup, focus on the details. Manufacturers need exact specifications about items, such as core thickness and total substrate thickness. Different types of dielectrics and prepregs have different thickness specifications. Total board thickness increases as the layer count grows.
In addition, check to ensure that the manufacturer has the substrate material that your team selected in inventory. If you simply specify FR-4 when you actually need FR408, the manufacturer will miss the fact that you need a high-performance dielectric. You should also communicate about the type of stackup used in your design. While most designs use the foil method, other designs may work with the capped method for the stackup.
Keeping track of various layer stackup needs while keeping manufacturer capabilities in mind can be challenging, to say the least. PCB layout software like Altium Designer® simplifies the process and helps ensure that you have everything you need to take your design from design to prototype to production. Talk to an Altium expert to learn more.