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

enter image description here

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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

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    \$\begingroup\$ 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. \$\endgroup\$ – jippie Feb 20 '13 at 13:17
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    \$\begingroup\$ @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. \$\endgroup\$ – Rocketmagnet Feb 20 '13 at 13:27
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    \$\begingroup\$ I understand your point, but still many companies do one-of runs these days and that is making me wonder too. \$\endgroup\$ – jippie Feb 20 '13 at 13:31
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    \$\begingroup\$ 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. \$\endgroup\$ – Connor Wolf Feb 24 '13 at 4:36
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    \$\begingroup\$ @Rocketmagnet - Well shit! I stand corrected. So much for my google-fu. In that case, I have no objection to your description. \$\endgroup\$ – Connor Wolf Feb 27 '13 at 5:47

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