pcbpcb-designpcb-layerssd
I'm designing a PCB with Hirose microSD card set DM3AT-SF-PEJM5. In its datasheet, page 3, there are several "no conductive traces" area, which are marked as "keep out" zone in the footprint I found.
My questions are:
Why these areas are marked as "no conductive traces"?
If I design a multilayer PCB, say 4 layers, and place the microSD card set on bottom layer, then I cannot route tracks in any of the 4 layers , or just in the bottom layer?
Yes, one of the reasons for using 2-layer PCBs is to avoid the jump wires. On a complex board there would be lots of them in all directions. Your layout doesn't need the second layer, though; it's still simple enough to be routed on a single layer without wire bridges. If you get stuck you may have to move some components to another position, but it can be done.
I would suggest to try it instead of going the easy way of the 2-layer board. It's about practice. If you don't succeed in doing this on a single layer, you'll never be able to do a more complex board on a 2-layer PCB either.
A professionally made multilayer PCB will have copper-plated vias to connect the top trace with the bottom trace. If you plan to make the PCB yourself you won't have the through-metallization, but since you only use PTH parts you can solve that by soldering the wires on both sides of the board. For some components the wire may not be reachable on the component side. Just make sure there's no trace to the hole on that side then. If you would need vias apart from the components' mounting holes you can solder a thin wire on both sides of the hole.
2-layer PCBs will often use both layers for routing, and even component placement. On densely populated boards this may not leave much room for copper planes, like for ground or Vcc. If these are really required designers often will go to a 4-layer board. A ground plane not only helps in the design of HF designs, but ground is often the net with the most connections, so it takes a lot of routing away from the outer layers as well.
Today most PCBs use SMT technology, and SMDs will often be placed on both sides of the board. First the components on one side are fixed with glue dots, then the board is flipped and the parts on the other side are placed. The glue is needed to avoid the components falling of when the board is turned upside down. Some glue dot machines can place up to 50000 glue dots per hour, that's 14 per second!
They actually say "No conducting traces". It means that metal parts of the connector (may) touch the PCB in those areas, and conductive traces may be shorted by them.
In practice it means no routing there, not even with a solder resist over the traces. Solder resist does not guarantee electrical insulation.
Best Answer
The reason for not having conductive traces under the microSD card connector is because it is a push-push connector. This has moving mechanical parts which could touch the surface of the PCB during operation, meaning that these areas could be scratched over time and expose any conductive surfaces and potentially short circuit with the body of the connector.
Only the layer where this connector is mounted requires these keepouts.