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Why is the set of white labels on a PCB called the "silkscreen"?

enter image description here

Jesus Castane
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Randomblue
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1 Answers1

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It's called silkscreen because this is the name of a type of printing: screen printing.

Silkscreen printing

The traditional screen printing process is like this: Inside the frame is a sheet of fine woven material (the silk) which is porous enough that you can push paint through it using a squeegee. A mask is placed between the silk screen and the paper. It's all pressed together, then the squeegee is wiped across the screen. Paint is pushed through the mask onto the paper, forming the image. Really, the only job of the screen is to protect the mask from the squeegee.

A similar process is used when printing the white paint on PCBs. However, instead of a separate mask, the screen is covered with a photosensitive layer, which is developed to produce the required mask as part of the screen.

Silk screen with photo mask

Of course, for PCB manufacture, this process is done by machine:

PCB silk screen printing machine

If you look closely at a PCB, you can see that the silk screen image is actually quite rough, and it's apparent that the image was pushed through a net.

PCB silk screen closeup

Rocketmagnet
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    Do these machines still use the silk screen method? It wouldn't surprise me if technology brought us more efficient ways that don't require custom masks. – jippie Feb 20 '13 at 13:17
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    @jippie - Yes. See added image. Actually a custom mask is more efficient for produce large numbers of PCBs. Screen printing it still what's used in industry, rather than, say, ink jet printing. It's just so much faster. – Rocketmagnet Feb 20 '13 at 13:27
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    I understand your point, but still many companies do one-of runs these days and that is making me wonder too. – jippie Feb 20 '13 at 13:31
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    Some of the material here is wrong. In screen printing a mask is not used below the screen. This would raise the screen up off of the media and the ink may not adhere correctly. The screen is actually the mask. The screen is coated with material that when exposed to light hardens and fills in areas of the screen. The screen IS the mask.The screen does not "only protect the mask". The point of using a screen is that it blocks ink so you don't put down a huge amount of ink which would be wasteful and take longer to cure.Most prototype board houses use silkscreen as that would take far more work. – reportingsjr Feb 20 '13 at 18:05
  • @reportingsjr - If you read my answer fully, you'd see that that's exactly what I said. There's even a picture to illustrate it. Please re-read my answer and remove the down vote. – Rocketmagnet Feb 20 '13 at 18:09
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    `A mask is placed between the silk screen and the paper. It's all pressed together, then the squeegie is wiped across the screen. Paint is pushed through the mask onto the paper, forming the image. Really, the only job of the screen is to protect the mask from the squeegie.` I don't think they've ever used a separate mask. The mask is basically part of the screen. Originally, I think the masking was hand-painted onto the screen, but now it's generally done using photographic techniques. – Connor Wolf Feb 24 '13 at 04:36
  • The opening paragraph of the wikipedia article is very poorly written. `Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate.` seems to indicate a separate stencil. However, there is no other mention to such a separate stencil. – Connor Wolf Feb 24 '13 at 04:40
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    I think @reportingsjr's point is that there is no process that works as you initially describe screen printing. While you correctly describe the process used for PCB silkscreening, the general description of the silkscreen process is inaccurate. – Connor Wolf Feb 24 '13 at 04:42
  • I'm also curious, how are silkscreens used for custom one off boards? Isn't it expensive to produce one? – capcom Feb 24 '13 at 13:09
  • @ConnorWolf - I understand his point, but it seems kind of pedantic, because my answer does explain the way silkscreen printing is done on a PCB. – Rocketmagnet Feb 24 '13 at 16:31
  • @Rocketmagnet - The fact that you correctly explain *PCB* silkscreen printing is irrelevant. The question is are you correctly explaining silkscreen printing in general (the opinion in the comments seems to be "no"). – Connor Wolf Feb 25 '13 at 01:04
  • [comment removed since I missed one of the responses] Question: does anyone know if laser-cut film is now being used in screen printing? In the past, hand-cut film was popular in fine-art printing. – lyndon Feb 25 '13 at 12:24
  • @ConnorWolf - Re: "...there is no process that works as you initially describe...". I beg to differ. This is precisely the process we used when I did screen printing at school. I'll let my glamorous assistant [demonstrate](http://www.youtube.com/watch?feature=player_detailpage&v=ND1AUJhdUZc#t=438s). – Rocketmagnet Feb 26 '13 at 23:09
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    @Rocketmagnet - Well shit! I stand corrected. So much for my google-fu. In that case, I have no objection to your description. – Connor Wolf Feb 27 '13 at 05:47
  • @ConnorWolf - You're a good man. – Rocketmagnet Feb 27 '13 at 10:42
  • @Rocketmagnet It may be worth noting that some PCBs are "screenprinted" with direct legend printing (DLP), essentially a giant inkjet printer with UV-curable ink. OSHPark, the likely fab for the PCB shown (identifiable by their signature purple soldermask), has used this process for years, though I'm not sure if they did back when that photo was taken. It can still produce a pixelated look like in the photo, presumably due to the print resolution. – Dominick Pastore Dec 05 '21 at 13:38