Josse @Elmatica

Welcome to Josses Corner!

Impedance Design of Rigid-Flex PCB

Meet Josse, one of our Senior Technical Advisors at Elmatica. This is his corner, where he will share his expertise every month!

After several years in the industry of Printed Circuits, from 1977 to be accurate, I have gathered some extensive  experience within the world of design and manufacturing of PCB. No point to sit on this experience myself. The format to share information in 2017, is through a blog, and voila – this is my corner.

Printed Circuits as we know them today, have developed from a very simple board, to the advanced circuits in 2017, where there are almost no limit when it comes to technology and thickness of boards.

When the industry in 1947 was introduced to a board consisting of two layers, with a plated through hole connecting  them, it was seen upon as a revolution in the industry. Little did we know, that this would be normal in 2017.

The development moved fast after this. Multilayers was introduced in the 60s, Hot Air Soldering and Liquid Photo Imageable came in the 70s. The same decade as Gerber introduced their, Gerber 40 and the Gerber RS-274D, a huge step forward for both humanity and everyone in the industry. 🙂

I will present an update on technologies, my opinions and experience within different field of this “mysterious” industry of Printed Circuits, each month on the blog.
Hope you will enjoy it, and learn something new.
First out, some light information about Impedance on Rigid-Flex printed circuits.

Impedance Design of Rigid-Flex PCB

The above forms are showing some common impedances used on flex, and the approximately  corresponding track/gap
to use to get them.


FLEXIBLE DESIGN CONSIDERATIONS: IMPEDANCE AND CAPACITANCE CONTROL
One can target an impedance by adjusting trace and dielectric spacing dimensions. Actual measurements vs. theoretical calculations can vary, but nominal results can be tweaked. The best result you will normally get from the manufacturer of your PCB/FPC who also have the experience with production and relevant material knowledge.  
Double-layer and multi-layer flexible circuits are ideally suited for providing interconnections that are specifically designed to provide desired levels of signal integrity. Construction techniques commonly referred to as stripline or microstrip,  are particularly well suited for these applications.
In certain cases, it is not possible to achieve desired impedance due to thin dielectrics. Then it is possible on a multilayer pcb to have a reference layer that is not the closest to your impedance traces, but lies deeper into the stackup. The downside is that you must have a clear view between the impedance traces and the reference plan. Meaning you can not use this in between area for other routing.

Microstrip

Typically a 2 layer flex with 1 signal layer and one ground layer.  Mostly used in lower frequency applications in the MHz signal speed area.  Crosstalk can be a concern, are flexible enough for dynamic applications.
What you get: It is more flexible, better for power lines and typically less expensive

 

Stripline

Typically a 3 layer flex with one signal and two ground layers.  Mostly seen in the GHz frequency applications and less crosstalk concerns. Mostly used for less dynamic or static applications.
What you get: Better signal integrity characteristics,  the flex is more expensive and less flexible.

Copper used on flex is normally rolled annealed (RA) and on materials for rigid boards it is electrodeposited (ED).  Compared to ED copper the grains is much bigger and ductility is larger, and makes this copper more suitable for Flex. This will also have impact on design and fabrication.


Contact Elmatica.com with your request on Flex and RigidFlex PCBs.   
Josse@elmatica.com

You can find more general stuff on flex at DuPont.com:
http://www.dupont.com/content/dam/dupont/products-and-services/electronic-and-electrical-materials/flexible-rigid-flex-circuit-materials/documents/DEC-Using-Flex-in-High-Speed-Applications.pdf

Josse, Senior Technical Advisor @Elmatica

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