Mini LED bonding on transparent FPC
This is “R” from the Sales Technology Group.
This time, we will introduce an example of bonding a mini LED on a transparent FPC.
Due to the size of the mini LED (from 0.1 mm square), the LED alone is almost invisible. Therefore, we thought that if it was bonded on a transparent board, a transparent display could be created when it was not lit.
First of all, please see the operation of the created demo product.
The created transparent FPC uses wiring with a pattern width of 0.04 mm (40 μm).
40 μm wire is thinner than a human hair thickness (50-100 μm).
The ideal is "invisible wiring", but this level of wiring width is usually the limit when creating an FPC.
In the capacitive touch panel, there is an example where wiring of about 4 μm, which is an order of magnitude thinner than this FPC, is used. In addition, blackening may be applied to make the copper color less noticeable, and if the panel is made with such technology, the wiring will be almost invisible. However, if the wiring is too thin, it will not be possible to pass current, so it may be necessary to balance visibility and current capacity.
In the panel created this time as well, the matrix-driven common electrode concentrates the current of 18 LEDs, so three 40 μm wide wires are connected in parallel.
The LED bonded on the FPC can be easily removed by external force if it is left exposed.
Since the chip size is too small, the connection area is small and the strength is inevitably low.
Therefore, to protect the chip, we covered the LED with a slightly thicker OCA and transparent film. The LED chip itself is about 0.1 mm thick, and we used a soft OCA with a thickness of 1.0 mm to ensure that the area around the chip is filled with OCA and that no external force is applied.
Since both FPC and protective film are bendable materials, we were able to create a transparent display that can be bent as shown in the photograph.
Normally, even if a hard LED chip is bonded on the FPC, bending the bonding part will destroy the bonding part, so it is better to avoid it. In the case of a mini LED, the chip size is small, so even if the FPC is bent, the displacement of the chip is not large, and the protective film and thick OCA do not seem to peel off easily.
We would like to make a transparent electric bulletin board by arranging a lot of LEDs on a larger board, but there is a budget for making a demo product, so it is on this scale.
Since the mini LED itself can be bonded on a 30 cm square board, it can also be bonded on a large board. You can also make a larger display by tiling. If you are interested, please let us know.
