515

As promised earlier, i started the temperture controll project.
The last Atmel AVR in DIP package i had went to some ‘black box’. Have no sources for firmware, so i don’t want to touch it now.

It happens i’ve got Atmega8 in TQFP package. The only problem is the connection of TQFP to veroboard is either complicated or expensive. I’ve decided to create my own modular and testing board for Atmega. It has: Atmega8, place for crystal, power supply, ISP connector, place for EEPROM – currently 24c16; RS-232 tranceiver, reset button; every pin of the MCU is routed to the connector. The board is 80x50mm – a quater of standart Euro PCB.

Here it comes. Before sanding, some tracks corrected, some still not. Hope the routing is correct 😉

The process is simple:

  1. create the board using Eagle editor;
  2. print it mirrored on an OHP tranparency. I use quite old Epson Stylus Color 440 printer, set media to “Photo Quality Ink Jet Paper” and print at 720dpi. Setting of ‘OHP transparency’ gives horizontal lines, which may be quite ok for OHP, but mkes no sense for contact exposure using light box;
  3. expose the UV-sensitive PCB. My result if ~12 minutes, while there are no signs of exposue on the board;
  4. remove exposed areas using 10% solution of liquid glass at temperature of ~50 deg Celsium. Make sure every exposed area is clean from residues of UV resin;
  5. etch the board in FeCl3. Heat it to ~45 deg Celsium for better results. I use indirect heating: a tin can with water placed on gas stove and PVC can with etcher inside

What i learned at last for today:

  • keep an eye on drill diameters: usually they are too big for some parts;
  • drill of 0.6mm is fine for vias. Just make sure diameter is at least 1.2mm
  • keep spacing around vias bigger (about 0.025″) for it will merge with other tracks otherwise;
  • get a better printer. Ink-jet is not suitable for fine PCBs
  • keep min spacing of 0.025″ or tracks will merge
  • also: tracks of 0.01″ tend to dissolve, if etching takes longer. Use wider tracks – 0.012″ in the case a lot of copper is to be removed
  • i need a better light box: the lamps are too close to the PCB and the light field is not uniform in current setup

So, inputs and cotroll board is on the way.