Enclosure is a frame for misalignment

A small square has to sit inside a larger pad with the pad showing on every side. A square that hangs off a corner will not serve.

Enclosure is that margin. It is how far the outer shape extends past the inner shape, measured on each side. The number that matters is the smallest margin, not the average, and not the side that happens to look generous. A contact with 0.01 micrometers of metal on one side and 0.19 on the other fails, because the process does not borrow margin from the roomy side. A hang-off of 0.03 micrometers is a margin of minus 0.03, not nearly zero. The picture at the top of this page puts the good case and the bad case side by side: one contact fully inside its pad, one contact hanging off a corner.

Margin on every side

Measure from each edge of the inner shape to the nearest edge of the outer shape. If the rule wants 0.05 micrometers of metal around a contact, then left, right, top, and bottom each need at least 0.05. A contact centered in a pad that is 0.20 micrometers wider and 0.20 taller has about 0.10 on each side, and it passes. Shift it until one side is 0.01 and the opposite side is 0.19, and it fails, even though the pad's extra metal is unchanged. Misalignment between two masks eats the tight side first.

The second picture keeps only the good case, the contact centered in the pad, so the margin is easy to see.

The contact sits in the middle, the pad shows on all four sides, and the contact does not touch the pad edge. The rule does not demand a perfect center. It demands a minimum on every side. Centering is the reliable way to keep all four readings above that minimum together. If a strap forces the pad to be asymmetric, still compute the four margins. The centered look is optional. The smallest side is not.

A via is the usual inner shape. The cut has to be enclosed by metal below and metal above. The inner square in these pictures stands for that cut, and the pad stands for the surrounding metal. Different pairs of layers carry different enclosure numbers. Metal around a via is not the same distance as a pad around a drill, or as one drawn layer around another. Read the pair the rule names. A decorative rectangle on the wrong layer can look like a surround while the real pair still fails.

The corner that hangs off

The right-hand pair at the top is the failure. The inner square crosses the corner of a smaller pad. On two sides the enclosure is zero or worse. The opening sits partly off the metal. In the process the contact can etch into the field beside the pad, the metal cap can be missing on that corner, or the connection can be a sliver that opens when the masks shift by less than the hang-off. Both shapes are present, and they do touch, which is why the drawing looks almost acceptable. Touching is not enclosure. Enclosure is surround.

If the inner square extends 0.03 micrometers past the pad, the margin on that side is minus 0.03. It is not nearly zero. A check will flag it when the rule is on. A glance will often call a slight hang-off acceptable, and the lot pays in via resistance or in opens at that corner. The left pair, and the second picture, are the correction. Shrink the inner square, grow the pad, or move the square back onto the pad until every side shows margin. Growing the pad by sizing, every edge outward, keeps the contact centered. Stretching only the near edge of the pad fixes one side. That is enough only when the other three sides already pass.

Do not hide a hang-off by drawing a second rectangle over the corner and forgetting to make one pad. You may now have two metal polygons. Some checks measure enclosure against each polygon separately, see the contact hanging off each, and flag both. Merge the pad into one outline, then measure. One pad, one contact, four margins.

Measure the four sides in the editor, left, right, top, and bottom, and write down the smallest. If any of the inner square crosses the pad edge, that side is zero or negative, and extra pad on the opposite corner does not restore it. If you covered the corner with a second rectangle, merge the pad to one outline and measure the four sides again. Move or resize until the smallest side meets the rule.

Four sides, and the smallest one

The rule in this drawing is a margin, and a hang-off of 0.03 micrometers is a negative margin, not a near miss. Measure left, right, top, and bottom of the inner square against the pad. Write down the smallest. If that side is 0.03 micrometers of metal past the contact, the other sides can show 0.20 micrometers and the check still fails. Extra pad on the far corner does not pay for the side that went negative.

The left pair, and the second picture, are the repair: the inner square sits inside the pad on every side. Growing the pad by sizing, every edge outward, keeps the contact centered. Stretching only the near edge fixes one side and leaves the other three for you to measure again. If you covered the corner with a second rectangle, merge to one outline first. Some checks measure enclosure against each polygon and will flag both. One pad, one contact, four numbers, and the smallest of them has to meet the rule.