4 Layer Board Design?

Discussion in 'The Projects Forum' started by sailmike, Sep 18, 2014.

  1. sailmike

    Thread Starter Member

    Nov 11, 2013
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    I'm about to design my first 4 layer board. I'm looking for tips/advice/ideas on how to design/route my board. My current plan is to put all 25 LED's on the bottom layer and all the other components on the top layer. The LED's have their own current regulator providing 350mA and all the other components get power from a 5V regulator. There will be a 25MHz clock on the top layer. The top layer will be a flooded ground. The second layer will be a standard routing layer. The third will be the 350mA supply for the LED's. Each LED will have it's own switching transistor that will be on the top layer.

    Does this seem like a good plan?

    Thanks,
    Mike
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Why do you need four layers when one or two will do?
     
  3. sailmike

    Thread Starter Member

    Nov 11, 2013
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    This will be a small board, only 40mm in diameter and it should make routing easier.
     
  4. Lestraveled

    Well-Known Member

    May 19, 2014
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    OK, a 4 layer board is always easier to lay out than a 2 layer board. A 4 layer board is also more expensive than a 2 layer board. PCB layout is a skill that improves through challenges. Try laying it out on 2 layers first. If it just does not work on 2 layers, then go to 4 layers.
     
  5. ronv

    AAC Fanatic!

    Nov 12, 2008
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    I usually try to put the ground and power planes on the internal layers. They are less prone to mistakes.
     
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  6. sailmike

    Thread Starter Member

    Nov 11, 2013
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    Some one on here mentioned in another thread that putting flooded ground on the active component side helps reduce decoupling inductance and resistance. Also, there will be two different power sources, the 5V regulator and 350mA current source. It seems logical to me to put the 350mA current source on the 3rd layer, which is next to the bottom layer the LED's will be on.
     
  7. MrChips

    Moderator

    Oct 2, 2009
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    A 4-layer PCB is used when

    1) the layout on a 2-layer board is overly complex

    2) the application is sensitive to high frequency switching noise.

    Is your situation either or both of these?
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Usually the two middle layers are used for ground plane and power plane.
     
  9. NetDoc

    New Member

    Jan 6, 2014
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    I am trying to get my head around this. Does anyone have a picture of what we're talking about?
     
  10. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Picture of what? I dont have any off hand, but i could make some if you say what exactly is that you want to see.
    mostly for sailmike:
    Since all your components are either on top or bottom, then it is logical to keep the connections in top and bottom layers, as you need much fewer vias (which take a lot of space) than if you have all the signals in the middle layers. You say that you want to lower the inductances, but keep in mind that the return path to ground is just as important as the supply.
    Another thing is that you want your ground plane to be with as continuous as possible. If you had it on the top, then all the component pads and vias will be breaking it, while when it is in the middle it usually can stay pretty much intact.
     
  11. MrChips

    Moderator

    Oct 2, 2009
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    If you don't know then maybe you don't need to know. But if you are really curious for curiosity's sake we will be happy to explain.

    Have you tried to goggle 4 layer board?
     
  12. #12

    Expert

    Nov 30, 2010
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    Might be a stupid question but...why do LEDs need to have a 25 MHz clock? Humans can't see much above 100 Hz, so why go into RF design for something that will work in the audio range?
     
  13. NetDoc

    New Member

    Jan 6, 2014
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    I googled it, but still no clear explanation. I even checked Wikipedia with no luck. One of the reasons I joined this forum was to learn. I may not "need" to know, but I really "WANT" to know.
     
    Last edited: Sep 20, 2014
  14. b1u3sf4n09

    Member

    May 23, 2014
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    Wikipedia usually has the answer, the kicker is knowing where to look: http://en.wikipedia.org/wiki/Printed_circuit_board
     
  15. b1u3sf4n09

    Member

    May 23, 2014
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    Can you provide a schematic for this board?
     
  16. NetDoc

    New Member

    Jan 6, 2014
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    Yeah, that's what I looked at. It does a piss poor job of describing it. It's OK, I get the idea I'm supposed to know or go elsewhere.
     
  17. sailmike

    Thread Starter Member

    Nov 11, 2013
    143
    3
    Sorry I haven't answered in a while. I've been out of town plus I haven't been getting any notices of replies. I've attached a picture of the circuit.

    Thanks,
    Mike
     
  18. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I dont see any reason to put such a simple circuit on a four layer board. You should be able to fit everything on a two layer with ease.
     
  19. sailmike

    Thread Starter Member

    Nov 11, 2013
    143
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    This PCB is going into a flashlight case and I want to utilize as much of the reflector as possible and that means making the PCB diameter as small as possible. Also, the pads for the LED's are large to provide heat sinking. As a result, there's very little spare real estate on the bottom where the LED's are.
     
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