[SI-LIST] Re: [OFF TOPIC] SMD Land patterns

Martin Euredjian wrote:
> 
> Going beyond raw specifications I'm trying to understand some of the choices
> made when designing land patterns for various types of SMT devices.
> 
> snip <
> 
> So, rather than actual footprints I am looking for any kind of study/paper
> that looks into the consequences of varying the geometry of the patterns.
> In other words: How does pattern geometry affect such areas as soldering,
> rework, reliability, testing, manufacturing, etc.?

        Martin,

        The problem with footprints is the answer is in shades of
        grey rather than black-or-white.  The change from one var-
        iation to another does not necessarily result in problems,
        but does move the yield in terms of DPMO.

        Having worked with IPC-SM-782 from inception as well as a
        number of CEM and OEM specific guidelines we've come to
        think of footprints in levels of qualification:

        generic (the part will attach, data sheet)
        broad concencus (IPC, "reasonable yield" nearly anywhere)
        narrow concencus (OEM negotiated with multiple CEMs)
        factory specific (CEM authored)

        As you move down this list you can expect higher yield if
        you are in the matched manufacturing environment.  Aspects
        such as rework and inspection access are more an issue in
        determining whether your product is disposable post-test,
        while area reserved for testability prevents fixture or
        component damage.

        We try to start with JEDEC package types and confirm them
        to IPC (or other required) specification.  The destination
        is really IPC-610 and/or J-STD-001.  Close the loop (very
        important) with manufacturing dialogue and inspection.

        Hope this perspective is useful,
-- 
 
      Jeff Seeger                         Applied CAD Knowledge Inc
      Chief Technical Officer                  Tyngsboro, MA  01879
      jseeger "at" appliedcad "dot" com                978 649 9800
            Printed Board Implementation Services since 1983
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