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Thread: PCB design - thermal relief spoke width & anti-pad clearance??

  1. #1
    Join Date
    Dec 2010
    SF Bay Area
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    Question PCB design - thermal relief spoke width & anti-pad clearance??

    I'm finalizing my board and about to send for fabrication. During final review & inspection, I noticed there are areas ("gaps") in the copper pour that are created by thermal relief ("anti-pad spacing") where pads are clustered, such that thermal relief spacing around the individual pads is merged to create larger open spaces. (See attachments for a simple relief pattern around two pads, and other where clustered pads create a larger gap). I contacted Oshpark for advice, and their response was not very specific. I'm currently using 20 mil spoke width and 20 mil anti-pad clearance. These were the default values when I loaded a default technology file in the design tool. Oshpark said those values would be "fine", and pointed out that too wide spoke or to short pad clearance would make soldering difficult. Yes, but there should be some guidelines to optimize for electrical and thermal properties.

    I've come to form the opinion that PCB layout is a mixture of engineering and art. True masters in the field have developed what seems to be an aesthetic and practical knowledge that becomes a very-difficult-to-describe set of best practices. I've also concluded that there are a few folks visiting this forum that have developed such mastery by years of vocational (or avocational) experience.

    I've found some simple formulas with unclear/incomplete commentary that look like a rule-of-thumb for spoke width is 60% of the shortest pad dimension divided by the number of spokes; and anti-pad clearance is 25% of the shortest pad dimension. But I had to piece that together from several differing and incomplete bits I found online, so it may be totally wrong.

    If anyone has suggestions here I'd appreciate the input, and will take it into consideration before releasing my design to fab. Thanks in advance.
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  2. #2
    Join Date
    Oct 2012
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    Default Re: PCB design - thermal relief spoke width & anti-pad clearance??

    The reason behind the thermal relief is when soldering, especially thru hole parts, is that the heat isn't soaked into the plane (power or ground), preventing a good solder joint, and also as important, is to remove the part. Also relate this to how much current you are trying to source or sink. If this is a main connection point for power into the board, just make sure that the amount of copper you have for thermal relief can actually carry the current. For a pullup, pulldown, you just need a connection to the plane.

    From what I see, what you have is just fine.


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