Home made flexible circuit boards

I got a couple of pieces of flexible copper-clad kapton sheet from ebay a few weeks back, and just got round to trying it out….

We have a new project at Build Brighton hackspace to make a POV fan to display SMS text messages sent to it. There is a long way to go, but one of the first steps is to make the LED array to be fitted to a fan blade. This needs to be as light and compact as possible, but I also wanted it to have sixteen 5mm RGB LEDs (thats a total of 48 LEDs to drive). SMD shift registers and resistors, together with a flexible (but more importantly thin and light) board seemed just the ticket.

I expected the sheets to be horrible to work with, and a nightmare for lifted traces, but actually it all went pretty well. I designed the layout for 8LEDs in EAGLE and then edited the image to double up the number of LEDS (my free version of EAGLE doesn’t allow a layout big enough). I used press-n-peel blue toner-transfer film in my old HP LaserJet 1000 printer and ironed the toner onto the copper-clad film (just as I do for rigid PCBs) and etched in ferric chloride. Some gentle cleaning with fine steel wool and the result was pretty good.

I’ve had a bit of practice with hand-soldering SMDs.. I use a 0.4mm needle tip and a high temp (400 celsius) with 0.015″ silver bearing solder. The high temp helps zap the solder without heating the part for too long, as long as you’re quick. I do all the work through a 10x loupe and make a lot of use of tweezers and a flux pen, and to be honest I actually quite enjoy it!

There are 48 x 0805 resistors on the board and six 74HC595D shift registers, each with a 100nF 0805 bypass cap. The 5mm through hole RBG LEDs were surface mounted along the edge of the board and the shift regs were wired up with Kynar wire-wrap flex (I decided not to risk drilling the film). I used a wooden jig to space and hold the LEDs for soldering.

I had a handy roll of kapton tape which I used to insulate the ground track so it didn’t short on the LED anodes passing right above it. Since the tape is heat resistant it behaved itself while the LEDs were soldered. This solved an otherwise tricky routing problem.

Soldering went fine, though it was obvious that things heat up much more quickly on the film than they would on a solid PCB, and solder stays molten longer since the heat has nowhere to go. I also think I should use a lower temp as the backing did seem to be getting slightly warped, but nothing serious.

However everything seemed to go OK and my first experience of making a flexible board went pretty well!

As for the POV fan project, I will keep ya posted

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