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May 1999

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Subject:
From:
Jeff Seeger <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 25 May 1999 11:08:47 -0400
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,

          Stack up design for PCBs:  Beyond the IPC Recommendations

        The Greater Boston Chapter of the IPC Designer's Council will
        hold its last meeting of the season Thursday, June 3rd at 6pm.

        Eric Bogatin, noted SI and EMC lecturer and consultant, will
        speak on the limits of common techniques for predicting board
        impedance, and give insight into mediating inevitable cases
        where engineer and fabricator disagree.

        Abstract:

        Controlled impedance circuit boards are the norm. Establishing
        the design  rules for the PCB stack up is often  hit or miss.
        Whatever rule of thumb a designer uses, fab vendors have their
        own fudge factors to add.  Often these are based on empirical
        results from building test boards. The IPC tried to assist in
        the design process by publishing the IPC-2141 Controlled Impe-
        dance Circuit Boards and  High-Speed Logic Design guide. They
        offer analytic approximations for common PCB stack-ups such as
        stripline and microstrip. However, they beg the question, under
        what conditions can these equations be accurately used?

        In this paper, these recommended approximations are introduced
        and compared to results from field solver calculations that have
        been verified accurate to better than 1%. For the range of typi-
        cal geometries used in circuit  board fabrication, the IPC equa-
        tions are typically accurate to 5% for the limited range of well
        defined cross sections. The predicted delay is accurate to about
        20% for some situations.  The precise accuracy at various geome-
        tries will be reviewed.  For simple structures  that match the
        ideal cross sections, the IPC equations may be used for estab-
        lishing the first order design conditions. However, when higher
        accuracy is needed or real world structures need to be designed
        that these equations do not describe, such as solder mask over
        surface  traces,  asymmetric stripline  and  coupling  between
        traces, a field solver is needed. Examples of how simple to use
        field solvers can give accurate results for all stack up geome-
        tries is presented.  These techniques apply to conventional as
        well as the new build up multilayer, MCM and hybrid substrates.

        Biography:

        Dr. Bogatin  has been involved with  signal integrity  and EMI
        issues for almost 20 years. After receiving his Ph.D. in Physics,
        he has  worked at AT&T Bell Labs, Raychem Corp, Sun Microsystems
        and  Ansoft Corp.  Bogatin Enterprises  was founded in 1988 to
        provide  expert consulting  assistance in the field  of Signal
        Integrity and EMI.  Activities include training courses, tech-
        nology analysis and integration, technical reports, and marketing
        assistance  for clients  offering products for  the high speed
        industry. Dr. Bogatin's training courses are offered through the
        Technical Proficiency Institute.  He can be reached by email at
        mailto:[log in to unmask] or at www.BogatinEnterprises.com .

        Agenda:

        Note there is a meeting fee of $10/members $20/non-members.

        6:00 pm,    Arrivals, Pizza and soft drinks available
        6:45        Jeff Seeger - Chapter Update
        7:00        Eric Bogatin, Stack up design for PCBs

        RSVP:

        Non members are welcome at meetings, but expect some pressure
        to join...<grin>.  Non-members RSVP please, reply/sender-only
        mailto:[log in to unmask]

        President's Note:

        I am very grateful for having attracted Dr. Bogatin to come
        from Kansas enlighten us on how to succeed with impedance when
        arguments start.  Your attendance and meeting fee will help to
        defray his travel expense.

        Directions:

        Lockheed/Martin in Lexington MA.  This facility is located off
        route 128 at the route 2A/Concord/Hanscom exit.  Proceed West
        200 yds, turn Left onto Forbes Road.  The meeting is in the
        first building on the left (after the guard shack).

        Mission:

        Physical Design in this era is not for the faint-of-heart.
        Old assumptions run out of legitimacy as speeds and margins
        converge on "need for perfect".  Work with us to learn where
        perfect might be found.

        Your help and participation have made the 1998/1999 meeting
        season a great pleasure to organize and host.

        Thanks for a great year; we look forward to seeing you at
        this meeting and again next fall, where we'll try to prepare
        ourselves for the professional side of the next millenium.

--

      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800

        President, Greater Boston Chapter, IPC Designer's Council

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