Lower dielectric constant in pcb

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Hi, the text book says that "the use of lower dielectric constant material will decrease the cross talk for the same characteristic impedance". The most commonly dielectric material used in the pcb is FR4 with dielectric constant around 4. Which dielectric materials have lower dielectric constant and how much they are expansive compared to FR4. Any example of high speed pcd design in which it is really important to use lower dielectric material constant instead of FR4 ?
 

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Using a lower dielectric constant material will decrease the crosstalk for the same characteristic impedance. The dielectric constant, also known as the relative permittivity, is a measure of a material's ability to store electrical energy in an electric field. A lower dielectric constant means that the material can store less electrical energy and thus reduce crosstalk.

In high-speed PCB designs, it is important to use materials with lower dielectric constants to minimize crosstalk, reduce signal distortion, and maintain signal integrity. For example, in designs with high-speed serial interfaces such as USB, HDMI, or PCIe, using a lower dielectric constant material can help improve the signal quality and reduce bit errors. It can also be important in designs with multiple high-speed signals running close to each other, such as in a DDR memory interface or a high-speed backplane.
 
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Thanks for reply. Which dielectric with lower dielectric constant is commonly used after FR4 for high speed signal designs including HDMI, PCI, 10GigB, USB etc. in the design, and what is the cost comparison compared to FR4 ?
 

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joniengr said:
Thanks for reply. Which dielectric with lower dielectric constant is commonly used after FR4 for high speed signal designs including HDMI, PCI, 10GigB, USB etc. in the design, and what is the cost comparison compared to FR4 ?
Click to expand...

Hi,

You can use Rogers laminate like RO4350 , the dielectric constant is about 3.5+/-0.05 under 10GHz.
The PCB price will increase a lot if you use this type of low Er laminate to replace FR4 .

I hope my answer can help you .


Louis.
 

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The application include high speed ADC design up to 2.5 GSPS connected to ZYNQ Ultrascale+ on a custom develop board having 2 GByte LPDDR4 RAM. The custom board also contains peripherals like 10GigE, USB 3.0, PCIe, and SD-Card. This is a complex deign. That's why I am wondering if FR4 is still an option or a stackup with lower dielectric constant material should be considered. I have heard about Rogers laminate like RO4350. Price wise what is the comparison with FR4 ?
 

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Ask supplier for best tradeoffs in price and frequency but Teflon is very expensive so consider best epoxy fibre materials used in MOBO's for 5GHz to 10GHz signals . There are dozens of boards and pre-preg laminate materials to choose from.

https://www.pcbway.com/advanced-pcb-capabilities.html. The problem is loss tangent and dielectric constant varies with frequency and also the Glass Transition Temperature Tg may be compromised.

The price of total copper area * layers is the major cost in FR4 but high end dielectric adds up.
 

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FR4 is a generic term, ISOLA etc. have many variations...
The type of glass weave in the FR4 will vary it's Er.
Again, from a real world view, the percentage of boards that don't use a FR4 based laminate is small, and even then it is often only on the outer layers, with an FR4 core.
When boards are this critical I tend to work closely with a couple on manufacturer's (single source should be avoided if possible), and get their recommendations based on the design's requirements. They often have Polar software, that will give you better results for PCB structure/gaps etc. So discuss with the manufacturer, your requirements, you will then end up with a manufacturable design, at the best cost.
 

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Lower dielectric constant in the PCB will decrease the cross talk. What are the other advantages of using lower dielectric constant in PCB layout ?

Using lower dielectric constant is better for PCBs involving high speed signals ? What are the limitations of FR4 ?
 

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Thanks for reply. This is very useful information. I have described my application in post #6. Can you please suggest which dielectric should I use in the stack up and what is the cost comparison with FR4 ?
 

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Talk to a PCB manufacturer or two, they will help you, they will have recommendation's of what materials they can work with. They will also give you a stack up with figures from Polar, that you can enter into you signal integrity software and use to build an accurate stack up in the PCB design.
 

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