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PCB Silkscreen

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  • Robert Hedan
    One aspect of PCB-making that has been REALLY bugging me is the silkscreen process. I m starting to make PCBs with lots of SMD components so labelling would
    Message 1 of 1 , Mar 29, 2006
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      PCB Silkscreen

      One aspect of PCB-making that has been REALLY bugging me is the silkscreen process.  I'm starting to make PCBs with lots of SMD components so labelling would make assembly so much easier for me.  Here's the fruit of all my research, you have no idea how many different approaches and materials I've used to mask the solder pads (would you believe hair gel with a syringe for eardrops?  serious, it was the next best thing).

      http://groups.yahoo.com/group/Electronics_101/files/Robert%20Hedan/

      Filename: Silkscreen black on white small.JPG

      I use transfer paper to make my silkscreen so it is important that the paint be able to resist the heat from the laminator.  What better paint than engine paint?  This particular brand can withstand up to 1200 F if cured properly, but I suppose the most ordinary engine paint will suffice 'cause I didn't even bother curing this paint at all.  I just waited until it was no longer tacky and proceeded with the toner transfer.

      White was the only colour in the display at the time for the top quality brand, but there all sorts of colours available in the ordinary brands.  road-sign yellow would probably look REAL good with black lettering, much easier to read than conventional green.  I ended up using ordinary clear adhesive tape for masking.  I would lay out a strip on the back of a scrap PCB and cut out thin strips with a steel ruler and an exacto knife.  Then I'd just place the strips over a series of pads and cut to length.

      The 3 lines of text are font size 12, 10 and 8, but I'm sure I could have used size 6 with equally good results.  You can see how the lettering does not fall apart where there are gaps in the copper.  There are 2 main problems with using toner transfer directly onto copper: the paper does not follow the contour of the surface and the toner does not bond to the bare epoxy.  Both of these issues are solved pretty nicely using this paint.

      It's a compromise, I really wanted to be able to mask pads individually.  But that is extremely difficult and I can easily mask a lot of pads real quick using this technique.  For larger components like ICs, I'm just going to mask the entire IC and its pads.  I had not tinned this scrap piece of PCB for this test, but good pieces will be tinned so having some partly exposed copper will not pose any oxidation problems.

      Yes, I'm an idiot and forgot to print the text in mirror image.  So I did the next best thing and cheated when I reduced it for upload to the site, I flipped it.

      Robert
      :)

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