Dictionary of Engineering Terms
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Minimum Annular Ring
The minimum annular ring on a plated thur-hole is the minimum amount of copper (at the narrowest point) between the hole and the edge of the pad after plating of the finished hole. MIL-STD-275E and IPC class 3 calls for a minimum annular ring of 0.05mm (0.002") edge of hole to edge of pad. MIL-STD-275E also calls out .005 inches for via to trace distance.
Minimum Annular Ring; PWB Internal Via
Both diagrams indicate minimum annular ring measured from the outside ring of the pad and to the inside diameter of the plated thru-hole for an external pad [below], or to the center of the plated via for an internal pad [above].
Minimum Annular Ring; PWB External Pad
Minimum Annular Ring applies regardless of the shape of the PCB pad. The next page shows two examples of different pad shapes surrounding a plated through hole. As the two graphics indicate, the number of board layers does not effect Minimum Annular Ring. The first picture shows a multi-layer board while the second picture shows a two layer board. Note the drill hole does not have to be centered in the lands.
More Printed Wiring Board Terms [different dictionary section]
In general this is a term or issue an engineering doesn't really need to worry about, in many cases because most engineers don't lay-out their own printed wiring boards. This measurement is normally preset in what ever CAD software being used to produce the board layout. One of a number of layout constraints added to the system when first used, and never needed to be worried about again. Anytime an operator would try to manipulate or move the copper pad in relation to the trace an error will occur, called a DRC [Design rule check].
So the only check that needs to be done is to insure that the system is setup with the proper constraints. It would be impossible to check the hole-to-pad distance on thousands of pads and via on a PCB by hand. The company used to fabricate the PCB may also check the minimum annular ring dimensions, as they apply to their design rules. In which case their fab rule my differ from the designers rules, and non-conforming boards may not be fabricated.
MIL-STD-275E - Printed Wiring for Electronic Equipment
More Printed Wiring Board Information and specifications.
