3

I'm assuming (but do not really know) that the original Macintosh used a two layer PCB. So my question is, when did the Macs start using more complicated boards with four (or more) layers?

For those that do not know, a PCB (Printed Circuit Board) is typically stacked in layers with copper traces separating them. Two layer boards (top and bottom) are the simplest (not counting single layer boards) and were widely used back in the dawn of computers (even today, really). So stacking more layers in a PCB helps reduce the overall size of the board. Which saves money but complicates the design.

  • Boards with more than two (or one) have been around since the 1960s, and I'd be surprised if there is any mac with just a two layer board. – Raffzahn Jul 23 '18 at 18:58
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    Yep, I acknowledged my false assumption. :-) Thanks. – cbmeeks Jul 23 '18 at 19:02
  • Multi-layer boards may have been around, but they weren't common until the early 1980s. The Apple /// infamously had issues because of its "fine-line" 2-layer board, which tried to keep board size small by using finer traces instead of more layers. The boards were extremely failure-prone, and eventually Apple reworked them completely. – jeffB Jul 24 '18 at 14:41
  • @jeffB that's a good point. Do you know if the re-worked A/// boards were more layers or just better routing? (or both) – cbmeeks Jul 24 '18 at 14:51
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They always did -- the "original" Macintosh 128k used a four-layer PCB. You can actually see the layer numbers marked to the left of the Apple logo at the bottom of the board. (Only the "1" is clearly visible in this photo, but if you held the board up to a light, you'd see layers 2, 3, and 4 marked in the other three boxes.)

Macintosh 128k motherboard

  • Ah! I did not know that. I falsely assumed that it was a two layer board originally. So the answer to my question is, the very first one! – cbmeeks Jul 23 '18 at 18:15
  • The question this leaves for me is ... why did Apple stick a label with their copyright message on the CPU? – Jules Jul 23 '18 at 22:21
  • @Jules that's a good question – Wilson Jul 24 '18 at 8:38
  • @Jules - possibly because it's the biggest flat space available on the board that doesn't already have text. – Michael Kohne Jul 24 '18 at 13:25
  • @Jules Because that's likely not a Motorola 68k, it's a second-source from Hitachi. The markings are probably to hide that fact. – user71659 Jul 26 '18 at 0:49
1

The original Mac had to use a 4 layer main logic PCB to meet FCC emission requirements. The metal fence is on the PCB for similar reasons.

  • Why would a PCB with fewer layers not meet FCC regulations? – Wilson Jul 25 '18 at 10:26
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    To keep electromagnetic emmisions down you need to run signals over a continuous ground or power plane. On a board of nontrivial complexity you need at least 2 layers for signal routing. That adds up to at least 3 layers but 3 layers boards aren't really a thing due to the way PCBs are made. – Peter Green Jul 25 '18 at 17:39

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