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I've been spending some time with some old PCBs from the 70s. I have seen this pattern on some, which are these short traces that appear as little whiskers in between each of the IC lead/leg through-holes. (This pic is obviously unpopulated.) They aren't connected to anything and are not soldered to anything when the boards are seen populated. What is their purpose? Is this a soldering aid of some kind?

enter image description here Thanks!

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  • \$\begingroup\$ Could they be there to block interference between the pins? I vaguely remember putting unused traces next to ones that carried high frequency signals. \$\endgroup\$
    – Solx
    Nov 10, 2021 at 20:25
  • \$\begingroup\$ While not what's going on there: I've seen something similar in modern use that was to reduce the variation in what fraction of the copper was removed/left across the board. It has something to do with getting more uniform processes by not depleting the enchants faster in the middle of large empty areas. IIRC that's even more important when building ICs. \$\endgroup\$
    – BCS
    Dec 3, 2021 at 20:20

3 Answers 3

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That is a good question.

Olden days (for you :)), we called the PCB design process Artwork. In order to make a "film" for the photo etching process of copper clad PCB, people had to "draw" the film. Actually a film was "developed" from something drawn on a mylar film usually, using drafting pens or using black tapes and ready made various foot prints.

Long story short, the taping required precision :-) when the traces have to pass between IC pins. So, it was convenient to have footprints with the traces already in the middle of pins, with good clearance.

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    \$\begingroup\$ Ah, I think I get it-- so these are essentially artifacts left over from standardized IC footprint masks, and as shown here the PCB designer didn't need to use them to connect traces through the pin area, so they end up in the final design just sitting there unused. Does that sound right? Thanks! \$\endgroup\$
    – Ben Zotto
    Nov 8, 2021 at 1:54
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    \$\begingroup\$ @BenZotto. . yes. They could have been removed. \$\endgroup\$
    – Marla
    Nov 8, 2021 at 1:56
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    \$\begingroup\$ @BenZotto The main "art" was not only the beauty, but the electrical and functional performance, which required good piece of EE knowledge. Meantime, end-users were nothing but just amazed what "technologies" can do. Steve Jobs (spelling?) did that too at his garage, I assume. So, I would say, people were more practical. h h ... \$\endgroup\$
    – jay
    Nov 8, 2021 at 2:09
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    \$\begingroup\$ @BenZotto It would have taken time for someone to remove them from the artwork, and there'd be a risk of them damaging the tracks around it. Safer just to leave it in place. \$\endgroup\$
    – Graham
    Nov 8, 2021 at 15:00
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    \$\begingroup\$ @jay I believe the correct spelling of Steve's last name (at least the one that was likely to be laying out PCBs in his garage) was Wozniak. \$\endgroup\$
    – Glen Yates
    Nov 8, 2021 at 18:08
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They're part of the layout template,

70's was before computerised layout, track layouts were done on paper and then photographically transferred to the circuit board.

If there's a need to run a trace between two pads they offer connection points for that.

pre-patterend sticky tape was used for the part outlines, and fine crepe or vinyl tape was used to lay out the traces. (source: Altium)

enter image description here

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    \$\begingroup\$ Makes ya appreciate "Ctrl-Z" \$\endgroup\$
    – Kyle B
    Nov 8, 2021 at 5:29
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    \$\begingroup\$ for undo you pull on the endof the tape and it comes off clean, but yeah cut and paste is literal. \$\endgroup\$
    – Jasen
    Nov 8, 2021 at 6:13
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    \$\begingroup\$ This, and print layout for books/newspapers/magazines is where the term "cut & paste" came from. It was very literal. When I was in college, I worked for the campus paper laying out ads. I would print the text, cut out a piece of text, run it through the waxer an literally paste it (with hot wax) to the layout sheet. Same thing here. \$\endgroup\$
    – FreeMan
    Nov 8, 2021 at 19:04
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    \$\begingroup\$ @Sixtyfive Didn't Jesus pioneer Ctrl-S? \$\endgroup\$
    – dotancohen
    Nov 9, 2021 at 16:38
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    \$\begingroup\$ The terms to search for are dry transfer or Letraset (a common brand name) \$\endgroup\$ Nov 10, 2021 at 1:08
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those dogbone traces were called solder traps. Before full automation, soldering was done by hand. It was very easy to flow too much solder onto a joint. They prevented solder runoff onto adjacent pins which would in all likelyhood short out the chips.

I ruined alot of 7476 Flip Flop IC chips back in the day ;-)

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