One guideline that is sometimes brought up in terms of high speed design and routing is the use of laminates with low Dk values. The main material set the industry has gravitated towards is PTFE with a ceramic filler, which may be reinforced with spread glass. PTFE materials offer many advantages in high speed or RF systems, and in a system that might experience a large temperature change, but at the expense of greater cost compared to an FR4 material.
Because of the tradeoffs, it’s natural to ask the question: when should PTFE be used? It’s best to look at how professional development teams are using these material sets, and specifically in cases where the PTFE material has a low Dk value. While we can’t cover every instance of PTFE PCB material usage in this article, we’ll cover a few of the important cases.
Not All High Speed Designs Need PTFE
Back to the common guideline for a moment: it is sometimes stated that high speed designs should use a lower loss laminate material like PTFE, and that the low Dk value is important for signal integrity. In reality, the fact that some PTFE-based materials have a low Dk value is not so important for signal integrity, and the justification that is sometimes used is that the signals move faster.
The reality is that there are other considerations involved in selecting PTFE that have nothing to do with modifying the electrical length of transmission lines. Instead, designers should just build to the system impedance and make sure their material selection aligns with one of the areas outlined below.
Probably the most common case where designers will cite the need for PTFE PCB materials is in GHz-range RF designs. This is appropriate because the losses in the system will generally scale with frequency, and PTFE materials are known for their low losses. Therefore, it makes sense to reduce this particular source of loss by using a low loss tangent material like a ceramic-filled PTFE dielectric.
Below this solder mask layer is a PTFE layer that provides low dielectric losses.
At these frequencies and with PTFE PCB layers, the dominant loss mechanism is not reflection or dielectric losses. Instead, the copper and surface roughness losses can be dominant. The surface roughness losses come from two sources:
- From the skin effect at high frequencies
- The roughness of the copper, which has to do with its production method
- From the surface finish, which can create additional roughness during deposition and it can have magnetic losses
In these designs, it’s common to see larger layer thicknesses, both because the layer count can be lower, and because the
High Layer Count, Low Layer Thicknesses
Digital systems with high layer count are common, and this forces many designs to adopt HDI techniques. For example, components with many pins or with very fine pitch BGA packages will need to use high layer count and/or thin layers to get controlled impedance traces into these packages and around the board. When the impedance of these traces needs to be controlled, which is common in advanced digital systems that use many high-speed protocols, then the layer thickness and Dk value of the layers becomes very important in determining impedance and routability.
This is where routing with a low Dk PTFE PCB material is useful. On these materials, the trace widths can be made wider to hit a specific characteristic impedance target. This is beneficial because, while the trace density is smaller, the manufacturing requirements are looser. In products that will scale to millions of units, this ensures higher quality and a less specialized process will be required, both of which help reduce costs.
This PCB for a smartphone can have very high layer count on PTFE laminates that require small linewidths.
The wider traces are allowed because the impedance of a trace is inversely proportional to the square root of the dielectric constant (√Dk). So, if the dielectric constant is lower for a given layer thickness, then the trace can be made wider.
The final place where PTFE will be preferred is in designs where the routing length will be very long, especially in designs where the signal must pass over one or more connectors. Some examples include backplane/daughterboard systems and other multiboard systems.
In these designs, the insertion loss will be the major factor determining losses, so using a smaller Dk value will reduce a portion of those losses. Note that the same copper loss factors mentioned for mmWave designs above still apply in long channels. This difference in these systems is that the long channels may often be digital, and they might be routed on thick layers as differential pairs. This reduction of losses with a PTFE material is important and it allows designers to focus on the other loss factors that will arise in long channels (reflection and copper losses).
PTFE-based laminates, both glass-reinforced and non-reinforced, are very useful materials that feature very attractive electrical properties, especially in high speed designs and in RF layouts. To summarize, there are a few important situations where PTFE laminates with low-Dk are preferred over a low-loss FR4 material:
- When layer counts are high and thicknesses are small
- On RF boards that need very low loss using larger layer thicknesses
- When channels are long and insertion loss dominates
Think carefully about relying on these products as the additional cost might put your product at a competitive disadvantage. However, in some products, the lower loss and wider trace widths enabled with low-Dk and low-Df materials is very important. Another option in cases where layer thicknesses get small is to use flexible polyimide as the base material, as is currently done in smartphones and other mobile products.
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