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Layout: Screen FPC Extender

A project log for FunKey project - All your games on your keychain !

Fun-on-a-Keychain is back, packing all your games and multiple emulators in a cute and extremely tiny handheld console.

Squonk42Squonk42 08/12/2019 at 21:110 Comments

LCD Screen

The chosen LCD screen for the FunKey console is awesome:

Hinge

The FunKey console uses a foldable design in order to reduce the device size when not in use, and maximize both the screen and keypad size when playing. The screen flat cable must then go into the hinge, and in order to avoid too much stress that would eventually lead to broken cables, it must be "rolled" into it like a flypaper in order to divide the stress over the longest possible length.

Unfortunately, the stock LCD screen ribbon cable is not long enough. This is of course something that the manufacturer can customize, but this costs a fixed tooling fee of $800... We plan to go this way for mass production, but this sounds like a lot of money for the prototyping phase only.

FPC Extender

So, we decided to use cheaper alternatives for our prototypes:

The problem is: I never designed FPCs before ;-)

I got some basic hints from one of my colleague:

However, a few questions remained unanswered:

  1. what material and thickness to use for the stiffeners?
  2. what material and thickness for the flex itself?
  3. which stack-up?

I decided to crawl the Web for more information, and here is what I found, I hope this may help some other PCB designers:

Stiffeners

Stiffeners are used over some parts of the FPC cable for various reasons, and I found this page very informative. Basically:

The stiffener thickness is variable, but is generally between 0.2 mm and 2 mm. I chose FR4 stiffeners with a 0.2 mm thickness as it is the thinnest, but also because this is the thickness for the original SAEF screen stiffener, too.

Flex

For the flex cable itself, I found this slideshow containing a lot of details.

I learned that:

With all these elements now clear, I decided to use PI with rolled-annealed copper foil ("RA") to reduce costs.

Stack-Up

For the layer stack-up and dimensions, I found these 2 links useful:

https://www.epectec.com/flex/material-stackup.html

http://www.asia1tec.com/goodies/FPC_material_datasheet.pdf

The second link shows a stack-up for a common double-sided FPC thickness (without the stiffeners) of 0.196 mm and provides the constitutive materials, for which I found the corresponding datasheets:

https://www.sas-electronics.de/downloads/FL_SF302.pdf

https://www.pcbglobal.com/files/SF302C%20Coverlay.pdf

Please note that with a SF302 0511210DR laminated PI core instead of the SF302 101820DR one, the total thickness lowers to 0.151 mm when keeping the same COverlay. This will probably be my final choice to achieve a greater flexibility.

Layout

Here is my custom layout. Please mind the gap in the hatched ground plane on the bottom side, which is designed to "guide" where the flexion (roll) should take place:

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