Today, I went to make the PCB of my AVR USB Programmer (USBASP). BUET Students usually go to ‘Toru Bhai’ for making PCBs. I did my design in Proteus Isis and Ares. I talked to Toru Bhai, and he asked me to bring print out of the masks I designed. Basically 3 masks are required for single layer PCB:
Bottom Copper
Grinder used to smoothen the sides
Top Components (Silk Screen)
Solder Resist
Although, Toru Bhai is reluctant to do the Solder Resist, as he claims it is too expensive. As I went today in his ‘factory’, in 6/1/F, Ram Krishna Mission Road, he didn’t have much work load and insisted me to wait and take my PCB with me. So I watched the fabrication process. Surprisingly, the screen printing is very much similar to VLSI fabrication.
Basically, a thin fabric with tiny holes in it is first covered in paint, and the tracing paper with the print out of the pattern is held in front of it. The screen is enclosed in glass, and exposed to solar light. The region covered by ink in the tracing paper remain unchanged, but the region exposed to light hardened. The screen is then washed, and the region covered by ink on tracing paper during solar exposure has its paint removed.
Then the screen is used to imprint the pattern of the tracing paper on copper board. The paint has to be insoluble to water. After the print, the board is submerged to FeCl3 (Ferric Chloride). Regions not covered by paint has its copper removed, as Copper reacts with FeCl3 to produce Cu-Chloride.
Afterwards, the paint is removed. Then holes are drilled to it, and the top layer patterns are drawn accordingly.
I got some pictures of the process:
The lab:
A screen looks like this:
Fitting the screen
Screen ready for action
Patternizing
FeCl3 Etching
Drilling Holes
Grinder used to smoothen the sides
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