.. _pcb-exposure-box:

PCB Exposure Box
================

:date: June 10, 2010

.. raw:: html

   <p>
       <a href="http://www.flickr.com/photos/armandas/4689191648/" title="PCB Exposure box by Armandas, on Flickr">
           <img src="http://farm5.static.flickr.com/4035/4689191648_505338dc8b.jpg" width="500" height="333" alt="PCB Exposure box">
       </a>
   </p>

This summer I finaly decided to start making my own PCBs. The preferred
method for making PCBs at home is photolithography. The PCBs used have a
layer of photo resist, which is exposed to a source of light through a mask.
The exposed areas are then removed using a special developer chemical.
Lastly, the PCB is dipped into the etchant and the copper is removed from
the areas unprotected by the photoresist.

The first thing I needed was a light source. I decided to build a small box
for exposing my PCBs.

3D modeling
-----------

I began by creating a 3D model of the box usign Google Sketchup. This
allowed me to spot any mistakes and tailor the design to suit my needs.

.. image:: static/exposure-box-model.png

Bill of materials
-----------------

I used the following parts to make this box:

   * Chipboard
   * Plexiglass for the window
   * Fluorescent lamp
   * Reflector from some old lamp
   * IEC socket and cable
   * Power switch
   * Latch
   * Pair of hinges
   * Sponge cloth
   * Screws, glue

Results
-------

Below is the picture of a test PCB. The safe limit of track width seems
to be around 0.2-0.25mm. This could be influenced by the source of light.
While experimenting with the timing, I got an impression that UV light
could do a better job as it would allow reduced exposure times.

.. raw:: html

   <p>
       <a href="http://picasaweb.google.com/lh/photo/F72CrDlzytrvC3T4mxqYwg?feat=embedwebsite">
           <img src="http://lh6.ggpht.com/_is7PcgDfyqQ/TByx6jpL3RI/AAAAAAAABZ4/MjMzVZcFPQw/s400/pcb_test_1.jpg" alt="PCB test">
       </a>
   </p>


Track widths from top to bottom are:

   #. 1.27mm
   #. 0.64mm
   #. 0.38mm
   #. 0.25mm
   #. 0.20mm
   #. 0.15mm
   #. 0.10mm

Links
-----

   * `Mike's article on PCB making at electricstuff.co.uk <http://www.electricstuff.co.uk/pcbs.html>`_
   * `A slightly more detailed picture of the box <http://picasaweb.google.com/jarusauskas/Stuff#5481253467338333426>`_