Rigid to flex PCB transition

A project log for Flex PCB Solar Motorized Earring

A tiny, solar powered, motorized spinner, made from a flexible PCB by folding into a rounded cube. An electronic wearable, an earring.

Lloyd KonnekerLloyd Konneker 05/28/2019 at 20:470 Comments

The prototype (seen in photos) uses a rigid PC and requires much handwork and soldering.

The main goal of this project is to study whether a flex PCB will bend into a structural, rounded cube.  That would save manual fabrication of a cube from rigid PCB.

The bendable parts are absent copper, except for traces that need to cross.  The bendable parts only have traces on one side so that they can bend more.  Ideally the traces would be on the inside of the bend so they are in compression.  In tension, they might break open more easily.  But I decided not to use vias to get to the inside of the bend.

The bend radius is 2mm, which is not aggressive.

The other parts of the design (sides of the cube) have copper zones on both sides so that they might be stiffer.

Typically, I fill complete ground zones on both sides anyway, to minimize noise and to reduce copper waste in etching.  Here, some of the zones are “no net” because I didn’t want to run traces across the bendable part to ground all the zones.

The sides have SMD solar cells.  After bending, the sides will still have bending moments. Solder joints and components are not supposed to carry structural loads.  The bending moment would tend to tear open the solder joints to the solar cells.  Maybe solar cells oriented the other direction would tear open less easily.

The design has three tabs that fit into slots.  The minimum slot width that the OSHPark fab supports is 20 mil.  This is greater than the about 7 mil thickness of the flex board itself.  Thus the slots are not ideal for structurally gripping the inserted tabs.  I considered just drilling the ends of the slots at the minimum drill of 10 mil, and using an Xacto knife to cut a slot between them.