Mirka Halas wrote:
>
> Hello Designers,
>  I am in process of converting from Gerber 274 to 274X,
> and I opened
> can of warms, which is called thermal reliefs.
> So far we used thermal relief and antipads developed by the rule of
> thumb, that it should be oversized by 20 mils from the pad.
>  I would like to understand the calculation, and where I can shrink
> the dimensions to get
> more plane surface for our high speed boards.
> I read IPC-D-275,IPC-2221 and IPC-2222, and do not see calculation for
> the outside diameter
> of thermal relief. 60% of the land is calculated for the web width,
> but what is the hight of the web?

        Karl's response is probably the best analysis I've
        seen on the characteristics of the thermal relief, "but"

> Is it the same as the diameter of Clearance area in planes?
> Is IPC's calculation for the clearance area in planes too conservative.
> One manufacturer can do
> minimum antipads with diameter of the drill plus .016".
>
        Minimum per the drill size, per your vendor's capability
        (.016 would be reduced producibility in many shops), but
        maximum should be smaller than:

        (Pos pad) + 2*(pos trace spacing) - 2*(registration allowance)

        You really don't want to allow the signal wires to sneak
        plane clearance underneath them on a high speed design.

        This applies to the outer diameter of your thermal as
        well - suddenly even the simplest pad geometry gets pretty
        darn complicated, huh?
--

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

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