I have a Macintosh SE in my collection and I noticed these odd looking traces on the mainboard. Not quite sure what to think of it

odd looking traces

  • 4
    That's perfectly normal and part of the production process.
    – Raffzahn
    Commented Sep 19, 2019 at 22:20
  • 1
    My Commodore PC looks the same.
    – Ziagl
    Commented Sep 27, 2019 at 9:35

2 Answers 2


They were most likely wrinkled like that from the start, due to the way they were manufactured, and thus you should not be worried.

Until the mid-90s, boards often went through an HASL, or "hot air solder leveling" process that put solder on the traces before the solder mask (a protective layer over the board) was applied. This initial solder over the PCB's copper traces helped avoid corrosion and, when near a point where a component would later be soldered, would help wetting so that the component could be more reliably connected to the PCB trace.

Excess solder on the traces would be removed before the mask was applied, but usually not completely on large traces. So, after being masked, when the board was wave soldered to attach the electronic components, the original HASL solder, now underneath the solder mask, would melt again and reform unevenly under the mask during cooling.

There's a more detailed discussion about this on Reddit.


This is actually a comment, but images cannot be inserted in a comment, so I wrote an "answer", just to point out that this kind of trace looking is already known in other computers, like the ZX Spectrum. AFAIK, this is due to the wave soldering process used.

This is how it looks like on the solder side: enter image description here

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