Tuesday, 30 September 2014

Dual Sided PCB – A first foray – Part 1

Now that I am fairly confident in my ability to produce a single sided printed circuit board, it’s time to move on to the more complex issue of producing a 2 sided printed circuit board.

The plan is to make a circular PCB with a shift register and 8 LED. The circuit will be driven by an ATTiny85 that attaches via 3 jumpers to the board (so that I can free up some space on the board) and to power via another 2 jumpers.

Shift Register - Round PCB - Copper Both Layers

Above is the circuit design showing both the top and bottom layer.

I started out by using a hole saw to cut the double sided copper clad board to the right shape. The main downside to this method is that you end up with a pilot hole in the middle of the board. Fortunately, the pilot hole ends up underneath the 16 pin DIP and not much routing needs to be considered.

Normally, when transfer printing a single layer PCB, the bottom layer is printed out as a mirror image so that when it is transferred to the board, it’s around the right way. For dual layer, it seems that the top layer is printed out in positive and transferred to the PCB so that it will match up with the bottom layer. I printed out both top (positive) and bottom (mirror) onto an A4 sheet of paper, held the sheets with the toner side facing each other and held it up to the light and it looked like the above image … so I think that this is the right way to go.

I also included three “register” points on the design so that I could align the two faces. I plan to cut the A4 sheet down to the design and then hold both top and bottom layer to the copper clad board using the register points for aligning them.

The other three holes in the design are for mounting screws.

Shift Register - Round PCB - Copper Top

The top layer includes the ATTiny85 connections (4 o’clock on the design), most of the shift register connections and the anode connections for the LED.

Shift Register - Round PCB - Copper Bottom Mirror

The bottom layer includes the resistors (I’m using 0805 SMD), the cathode connections for the LED, and the power connections (note that the bottom layer as shown above is NOT mirrored … if you are going to use this design, you will need to mirror the image first).

One of the things that I haven’t quite worked out yet is the connection of the DIP on two layers. I may have to do some rearranging of traces, bending of DIP legs so that they can be surface mounted on the top layer.

This design requires:

  • 8 x 0805 SMD resistors (appropriate for your LED) I’m using 100Ω;
  • 8 x LED;
  • 1 x 16 pin DIP;
  • 1 x 3 pin header (connects to the ATTiny85);
  • 1 x 2 pin header (connects Vcc and GND);
  • 1 x 74HC595 Shift Register IC;

To run the circuit, I’m going to have the ATTiny85 on a solderless bread board running my modified Shift Register sketch.

Well … that’s the plan. I’ll do the toner transfer and etch tonight and see how far I get along with the build.

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