In this log, we want to share our experience of bringing our FunKey Project from its early-prototype stage to a “Ready For Manufacturing” / “Community-Ready” state necessary to launch a successful crowdfunding campaign that in turn should provide us with the lever required to launch a product in mass production.

Where Do We Come From

People following the Funkey Project certainly remember that the original motivation came from @Sprite_tm‘s 2016 Hackaday superconference project of a pocket retro gaming console, that lead @c.Invent to develop the #Keymu – open source keychain-sized gaming console as a Proof-of-Concept that entered the 2017 Hackaday Prize. The corresponding Youtube video is now peaking at 4.8M views and #6 in Youtube’s Popular Videos – Game Consoles & Mobile Phones category!

Unfortunately, the tiny (and expensive) Intel Edison CPU module at the heart of the console was abruptly discontinued, which forced him to find a replacement module. Given the tiny dimensions, this prove to be very difficult, but @Squonk42 joined him to develop a new board around the LicheePi Zero module. Eventually, a new #Funkey Zero console which was used to validate the new CPU choice took part in the 2018 Hackaday Prize:

At this point, it became clear that the main feature of the #Keymu – open source keychain-sized gaming console was its foldable design, providing a maximum size for both the screen and keypads in the smallest form factor. It also became clear that such a hinged design required the help of a professional mechanical designer, and this challenge motivated @David.Larbi who joined the team at the beginning of this year. Read More

LCD Screen

The chosen LCD screen for the FunKey console is awesome:

• 1.54″ diagonal, 33.72 mm x 31.52 mm
• 240 x 240 pixels
• 262K colors
• wide view angle of 80°
• contrast 400:1
• IPS technology
• simple SPI interface, up to ~60 MHz
•  ST7789V controller chip

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:

• for the Revision A, we used individually soldered thin enameled copper wires and an ultra small PCB placed into the hinge: definitely not something to use for more than 2 boards!
• for the Revision B, we decided to invest some money into a customized FPC (Flat Printed Cable) as we can get 5x FPC prototypes for ~$100 at PCBWay

The problem is: I never designed FPCs before 😉

I got some basic hints from one of my colleague:

• the copper density must be as constant as possible
• the traces must use smooth curves instead of sharp angles to avoid tearing during flexion

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: Read More

The previous log detailed the screen FPC extender, used to adapt the stock LCD screen to the FunKey main PCB.

Before launching the FPC fabrication which is quite expensive (~ $100 / 5 pieces), we decided to have a dry run using a paper mock-up. Here is the result:

We covered the old Rev. A PCB with a paper print of the new Rev. B one, and created a paper version of the flex, gluing the connectors on them at the right positions. As you can see, the result is not too bad.

This was until we tried to open the lid flat:

Well, the FPC is a little bit too long, causing a “wave” that totally defeats the original purpose of the “flypaper design”: the small bend radius would certainly cause a failure sooner or later.

Moreover, the FPC is a little bit too wide, too, so it interferes with the internal ribs somehow.

So, back to the drawing board, we decreased the length by 2 mm, and the width to 4 mm, here is the result:

Much better! And the behavior when opening / closing the lid is also improved:

Here are the corresponding layouts: