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Thermal reliefs in plane connections

Discussion in 'CAD' started by Joerg, Jan 23, 2008.

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  1. Joerg

    Joerg Guest

    While checking a layout for a client I saw this for the umpteenth time:
    Trace connections to a plane via thermal relief, not in a solder area
    where it would thermally matter.

    Question: Was it taught once upon a time that thermals must be used
    everywhere? Or is there some other reason why it's done so often?
  2. Joerg,
    I would have to believe that it is the old problem of a little knowledge
    being misapplied by the uninitiated. A general comment on a rule or practice
    then being taken to further extremes and actually misapplied. Sometimes it
    may be software initiated, some packages actually have default rules that
    start out your design with thermals on 'all' plane connections. i.e. Protel
    P99SE (quite possibly the current AD software also), the default plane
    connection style rule on a new PCB is for "Board" (everything, vias and
    pads) to be thermally relieved. So the less knowledgable and experienced get
    introduced to the silly practice by their acceptance of software package

    Certainly seems silly doesn't it? Then there is the problem of all the
    discontinuities caused in the planes by these unnecessary thermal reliefs.
    It is fairly common to see required thermal reliefs that are actually
    starved connections because of all the unecessary reliefs impeding a good
    connections to those required points.
  3. David Brown

    David Brown Guest

    It is easier to have a consistent style of trace to plane (or to
    polygon) connection throughout the board. For many types of board,
    thermal reliefs are just as good as direct connections - they are only a
    problem if the relief wires are very thin, or if you are working at very
    high frequency, high currents, or with high accuracy analogue signals.
    Personally, I use thermals everywhere if I am doing a simple card, and
    direct connections everywhere if it is higher speed or has smaller


  4. DJ Delorie

    DJ Delorie Guest

    Makes it easier to wire in a fix later.
  5. qrk

    qrk Guest

    I would guess that the layout person is inexperienced. In Orcad
    Layout, you need to specifically check off a box to get rid of thermal
    connections for a particular pad stack.
  6. Joerg

    Joerg Guest

    Yes, I've had one case where a supply plane split open into four parts
    because the fab house took the liberty to "improve" the clearances. They
    ended up doing another free blitz run for us.
  7. Joerg

    Joerg Guest

    Not really anymore. Most boards have this all covered and vias are so
    small anyway. Plus, don't we all design perfectly so no rework is ever
    needed ... ?
  8. Marra

    Marra Guest

    My CAD software automatically has an underlap from the power plane to
    the pads.
    SO the only connection to the pwoerplane is via a normal track.
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