3 ms·

Can anyone explain to a hardware idiot how he fixed the short (where Apple cut the board)? I don't understand why scraping off layers like that made it go away.

Can anyone explain to a hardware idiot how he fixed the short (where Apple cut the board)? I don't understand why scraping off layers like that made it go away. Why can the board still work after you've taken out a chunk of it (as long as you take out enough chunks)?

RodgerTheGreat 2y ago
The damage to the board was caused by a tool like a pair of tinsnips, crushing the layers of the board together while cutting. The tiny layers of copper are folded together at the edge of the cut, causing shorts. By sanding away the board at a much shallower angle than the original cut it is possible to expose an edge along which no conductive layers are bridged. As for how the board still works when that area is cut out, probably just a degree of luck that none of the traces that were severed were essential?
- Wowfunhappy 2y ago
  Ah, thank you! So it wasn't actually the cut that "broke" it, it was the fact it was crushed.
- simondotau 2y ago
  Based on appearances, the board has a tight cluster of components laid out similarly to a contemporaneous iPad. The rest of it seems like a vast empty wasteland, lightly populated with debug terminals, ports, and compatibility shims which make it work inside a Mac mini chassis. Chances are the breakage caused a power rail to short and the “repair” fixed the short while leaving one or two debug points inoperable.
runjake 2y ago
I believe Wowfunhappy is referring to this section of the video: https://www.youtube.com/watch?v=reQq8fx4D0Q&t=422s https://www.youtube.com/watch?v=reQq8fx4D0Q&t=422s
jwagenet 2y ago
When you clip the board, the conductive layers are also being smooshed together and touch despite the non-conductive layers, which seems to persist during the enlargement of the cut. I imagine the goal of the flat filing is to remove more material from the top layers an “terrace” the layer lines until they don’t touch. As far as the repair is concerned, it seems like the cut only goes through continuous ground or voltage planes. So in a sense, it is lucky no important traces or logic lines are harmed in the process.
nxobject 2y ago
I'm surprised that it wasn't any closer to any other non-power/ground traces.
gorbypark 2y ago
It's like an imaginary (horribly made) club sandwich! From the bottom up, there's a layer of bread, then a layer of ham, then another layer of bread, then a layer of cheese, then the top piece of bread. The ham and cheese are your conductive layers, one carrying current the other the ground. When you first make the sandwich, everything is good because the ham and cheese are not touching each other. However, when you cut it in half, the bread, ham and cheese all smoosh together at the cut point and the ham and cheese electrically short out. Now, if you are super careful, you can probably go in with a super sharp knife, cut it again very carefully, and make it so the ham and cheese don't actually ever touch. Apple (or whoever) purposely put a cut in the sandwich knowing the ham and cheese would touch, making the sandwich inedible. This YouTuber comes along with a really sharp knife and cuts the sandwich again making sure the ham and cheese never touch. It just so happens when Apple made the cut, they didn't cut anything important, other than "smooshing" the layers (ie: they didn't cut through any traces or capacitors, etc).

RodgerTheGreat 2y ago

The damage to the board was caused by a tool like a pair of tinsnips, crushing the layers of the board together while cutting. The tiny layers of copper are folded together at the edge of the cut, causing shorts. By sanding away the board at a much shallower angle than the original cut it is possible to expose an edge along which no conductive layers are bridged. As for how the board still works when that area is cut out, probably just a degree of luck that none of the traces that were severed were essential?

Wowfunhappy 2y ago
Ah, thank you! So it wasn't actually the cut that "broke" it, it was the fact it was crushed.
simondotau 2y ago
Based on appearances, the board has a tight cluster of components laid out similarly to a contemporaneous iPad. The rest of it seems like a vast empty wasteland, lightly populated with debug terminals, ports, and compatibility shims which make it work inside a Mac mini chassis. Chances are the breakage caused a power rail to short and the “repair” fixed the short while leaving one or two debug points inoperable.

runjake 2y ago

I believe Wowfunhappy is referring to this section of the video: https://www.youtube.com/watch?v=reQq8fx4D0Q&t=422s https://www.youtube.com/watch?v=reQq8fx4D0Q&t=422s

jwagenet 2y ago

When you clip the board, the conductive layers are also being smooshed together and touch despite the non-conductive layers, which seems to persist during the enlargement of the cut. I imagine the goal of the flat filing is to remove more material from the top layers an “terrace” the layer lines until they don’t touch. As far as the repair is concerned, it seems like the cut only goes through continuous ground or voltage planes. So in a sense, it is lucky no important traces or logic lines are harmed in the process.

nxobject 2y ago

I'm surprised that it wasn't any closer to any other non-power/ground traces.

gorbypark 2y ago

It's like an imaginary (horribly made) club sandwich! From the bottom up, there's a layer of bread, then a layer of ham, then another layer of bread, then a layer of cheese, then the top piece of bread. The ham and cheese are your conductive layers, one carrying current the other the ground. When you first make the sandwich, everything is good because the ham and cheese are not touching each other. However, when you cut it in half, the bread, ham and cheese all smoosh together at the cut point and the ham and cheese electrically short out. Now, if you are super careful, you can probably go in with a super sharp knife, cut it again very carefully, and make it so the ham and cheese don't actually ever touch. Apple (or whoever) purposely put a cut in the sandwich knowing the ham and cheese would touch, making the sandwich inedible. This YouTuber comes along with a really sharp knife and cuts the sandwich again making sure the ham and cheese never touch. It just so happens when Apple made the cut, they didn't cut anything important, other than "smooshing" the layers (ie: they didn't cut through any traces or capacitors, etc).