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How do you know the number of layers needed?

amerdsp

Hello all,

   In a previous post, I was advised that for a multilayer board (>4), I

should set up the layers upfront. How does one know the number of layers

needed ahead of time?  Is there a way to plan the layout before diving in?

 

Thank you,

 

A

 

  • On Tue, 14 Apr 2009 14:27:23 -0400, Amer Abufadel wrote:

     

    Hello all,

       In a previous post, I was advised that for a multilayer board (>4), I

    should set up the layers upfront. How does one know the number of layers

    needed ahead of time?  Is there a way to plan the layout before diving

    in?

     

    Thank you,

     

    A

     

    Yes.  Requirements can be determined up front to a significant degree

    instead of diving in to the layout to discover the requirements.  Get the

    as much of the mechanical requirements as can be determined first.  I

    generally attempt a test placement with just the two layers defined.  The

    number of layers needed and routability will generally become apparent. 

    But before defining the internal layers, some of the basic questions are:

     

    Q1) How many rails (supplies) and what are the number of connections?

      Q1a) Do any of the rails require a high degree noise immunity or carry

    large currents?

    Impact: noise and current considerations can lead to a dedicated layer.

     

    Q2) Count the number of controlled impedances and timing critical nets

    and map them into the test placement.  A sketch is good enough.

     

    Q3) How dense is the design, (ie. BGA ball rows and columns - see mfg

    suggested layout in the data sheet, number of parts verses routable

    areas, mounting features that narrow routing)? 

    Impact: there is a wide range of of answers here, too numerous for a

    simple chat.  The number of answers and depth of the "what if" will

    define the experience level of the designer (or visa versa).  Sorry for

    the cop-out but numerous books and industry standardized specifications

    are written on this stuff.  Google on "PCB Layout Requirements".

     

    Hope that helps to get you started.

     

    Greg