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I am trying to construct a 4-layer board for an RF application. I have decided to use Rogers RO4350 material due to the high frequency signals on the top layer. I have also considered combining Rogers and FR4 material since the high frequency signals will be only on the top layer (less expensive.) The most suitable stackup for the moment is:

  • Rogers RO4350B of 4mil (0.101mm) thickness
  • Rogers Prepreg RO4450B of (30mil to 40mil) thickness
  • Rogers RO4350B of 4mil (0.101mm) thickness

enter image description here

As you can see, RO4350 dielectrics are quite thin so we need the prepreg to add mechanical strength on our board. My question is: Since we use a prepreg in the middle instead of a thick core, will the above stackup have an appropriate mechanical strength or should a thick core be used instead of prepreg?

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  • \$\begingroup\$ I'm curious whether 30~40mm was a typo. How do you even make practical vias on a 30mm thick PCB? \$\endgroup\$
    – Wesley Lee
    Oct 7, 2019 at 8:48
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    \$\begingroup\$ Yes it is a typo. The correct value is 30-40 mils! Thanks for that \$\endgroup\$ Oct 7, 2019 at 9:08
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    \$\begingroup\$ The most qualified to answer your question is your PCB fab shop. I think RO4450B bonds to roughly 4 mils, so you will need 8-10 plys to reach 30-40mils. It sound really expensive and impractical. I have never seen more than 4 plys of prepreg in a stack-up, so it might not even be possible. If it is a requirement to have cores on the outside, consider a six layer stack-up to have a thicker core material in the center as a stiffener. You should consult your PCB fab shop early on in the process. It's a bummer finding out that your PCB cannot be manufactured after you are done designing. \$\endgroup\$ Oct 7, 2019 at 10:27
  • \$\begingroup\$ Thank you! Thank was very helpful \$\endgroup\$ Oct 7, 2019 at 10:53
  • \$\begingroup\$ RO4450B has a thickness of 0.0036" about 10x smaller than listed even after the edit: agssales.com/ro4400.pdf \$\endgroup\$
    – Voltage Spike
    Oct 7, 2019 at 15:08

2 Answers 2

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The pre-preg is malleable when your PCB fab shops receive it. But after curing (which happens during the PCB manufacturing process), its properties aren't particularly different from a core's.

I wouldn't worry about the difference in mechanical properties after curing unless you have some very special mechanical requirements.

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Normally this stackup is ok, if your design will work in a special envirment , then you can consider fr4+rogers hybrid press

this pic show Dielectric Strength

enter image description here

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  • \$\begingroup\$ Where was this figure sourced from? A reminder that all sourced images and quotations must have a citation or source indicated. \$\endgroup\$ Dec 15, 2023 at 9:58

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