High-speed designs and RF designs need to have consistent impedance for routed signals, and the impedance challenge is normally approached in two possible ways. These approaches include controlled impedance design, where the manufacturer mixes material options to hit an impedance target; the other is controlled stackup, where the designer accesses dielectric data for materials.
Controlled stackup design requires the designer do some homework and have a knowledge of how dielectric materials are stacked and combined in a stackup. Controlled impedance puts more on the shoulders of the manufacturer and requires them to do some verification on a test coupon. Make sure you know how to specify your PCB stackup for fabrication if you take either approach.
Two Ways to Build a Stackup
There are in general two approaches to create a PCB stackup, known as controlled stackup and controlled impedance. In the first method, designers have to select materials on their own and calculate impedance, while in the second approach, the manufacturer selects materials and thicknesses to hit an impedance target.
It is not necessarily the case that one approach is “better” than the other. Most designers take the controlled impedance approach and devise a target for their trace width/spacing for single-ended and differential channels. However, if you select “impedance control” options when getting a quote from a manufacturer, it’s important to know that this will bring additional engineering costs. This is because they have to do some calculation, measurement, and analysis of your stackup as described below.
Controlled Stackup Approach
The controlled stackup approach gives the designer more freedom to build their board, but it requires that the designer locate specific materials that comply with their calculated trace width and spacing. Controlled stackup also gives a designer more control over the design and is recommended for advanced products that have specific SI targets.
In a controlled stackup approach, the designer only needs to specify the following data:
- Layer thicknesses on all layers that need set impedance
- Dielectric constant on all layers that need set impedance
- Specific product names that provide the above specifications
This would be provided in a stackup table, either exported as an image from your CAD software or created in a program like Excel. If you need specific impedance values, you can also use the manufacturer’s standard layer stack options to build the board.
Example PCB stackup table.
In order to get the board produced, the fabrication company needs to have the required materials in stock. Oftentimes, if they do not have your specific brand/product in stock, they can suggest a suitable replacement material that matches most or all of the target specs.
Controlled Impedance Approach
In the controlled impedance approach, the design is created such that you specify an impedance goal in the design on particular layers. Here, you aren’t out shopping for materials or calculating any impedance, you are simply selecting the width/spacing and target impedance you want in the design.
When you specify the impedance needed on each layer, the controlled impedance approach demands that you do not mix different impedance targets on the same layer. The manufacturer will take these targets, and they will select material Dk and thicknesses that attempt to hit this target.
This approach requires that you do some work to estimate impedance because you need to know whether the width/spacing target is practical. If you don’t do this, the manufacturer may find that the impedance target requires a set of materials that are unavailable. For example, if your width/spacing target and the target impedance demand a material with 1 mil thickness and Dk = 2, the design will not be buildable because there are no PCB materials that meet these specifications. Make sure you know what ranges of linewidth/spacing are practical on available PCB materials.
Once the stackup is designed by the manufacturer, they will perform some measurements on a test coupon to verify the trace width target is within tolerance. This is done through a measurement of return loss or with a time-domain reflectometry measurement (the latter is more common). If the width target is incorrect, the stackup can be adjusted, or the design may be sent for modification of the width/spacing in the PCB layout.
Whatever your assembly requirements, or if you need venting below zero clearance components, make sure you use the industry’s best CAD tools in OrCAD from Cadence to prepare your PCB design for volume assembly. OrCAD is the industry’s best PCB design and analysis software with utilities covering schematic capture, PCB layout and routing, and manufacturing. OrCAD users can access a complete set of schematic capture features, mixed-signal simulations in PSpice, and powerful CAD features, and much more.