Patterned asymmetric chemical strengthening

1. An electronic device, comprising:

a display; and

a cover glass positioned over the display and having a front surface with a flatness that is no more than 120 microns out of plane and a rear surface opposite to the front surface, the cover glass comprising:

a first compressive stress region extending from the front surface to a first depth in a range from greater than 50 microns to 250 microns into the cover glass;

a second compressive stress region extending from the front surface to a second depth, less than the first depth, into the cover glass;

a third compressive stress region extending from the rear surface toward the first compressive stress region and to a third depth into the cover glass; and

a fourth compressive stress region extending from the rear surface toward the second compressive stress region and to a fourth depth, greater than the third depth, and in a range from greater than 50 microns to 250 microns into the cover glass.

2. The electronic device of claim 1 , wherein the cover glass further comprises:

a first tensile stress region positioned between the first compressive stress region and the third compressive stress region; and

a second tensile stress region positioned between the second compressive stress region and the fourth compressive stress region.

3. The electronic device of claim 2 , wherein a first centerline of the first tensile stress region is offset with respect to a second centerline of the second tensile stress region.

4. The electronic device of claim 1 , wherein:

the second compressive stress region completely surrounds the first compressive stress region at the front surface; and

the fourth compressive stress region at least completely surrounds the third compressive stress region at the rear surface.

5. The electronic device of claim 1 , wherein:

the first depth is approximately equal to the fourth depth; and

the second depth is approximately equal to the third depth.

6. The electronic device of claim 1 , wherein:

the cover glass defines four corner regions; and

the first compressive stress region and the third compressive stress region are located at least partially within one of the four corner regions of the cover glass.

7. The electronic device of claim 1 , wherein:

the cover glass defines a rectangular outer perimeter region;

the first compressive stress region and the third compressive stress region are located at least partially within the rectangular outer perimeter region; and

the first compressive stress region at least partially surrounds the second compressive stress region.

8. An electronic device comprising:

a display; and

an enclosure at least partially surrounding the display and comprising a glass enclosure member having an exterior surface with a flatness that is no more than 120 microns out of plane and comprising:

a first localized compressive stress region extending into the glass enclosure member from the exterior surface to a first depth in a range from greater than 50 microns to 250 microns;

a second localized compressive stress region adjacent to the first localized compressive stress region and extending into the glass enclosure member from the exterior surface to a second depth, less than the first depth;

a third localized compressive stress region extending from an interior surface of the glass enclosure member towards the first localized compressive stress region and to a third depth; and

a fourth localized compressive stress region extending into the glass enclosure member from the interior surface of the glass enclosure member towards the second localized compressive stress region and to a fourth depth, greater than the third depth, and in a range from greater than 50 microns to 250 microns.

9. The electronic device of claim 8 , wherein:

a first tensile stress region is positioned between the first localized compressive stress region and the third localized compressive stress region; and

a second tensile stress region is positioned between the second localized compressive stress region and the fourth localized compressive stress region.

10. The electronic device of claim 9 , wherein the fourth depth is greater than the second depth.

11. The electronic device of claim 9 , wherein the first localized compressive stress region is at least partially surrounded by the second localized compressive stress region.

12. The electronic device of claim 8 , wherein:

the glass enclosure member defines a camera window;

the electronic device further comprises a camera positioned below the camera window;

the first localized compressive stress region is positioned at least partially within the camera window; and

the second localized compressive stress region surrounds the first localized compressive stress region.

13. The electronic device of claim 8 , wherein:

the glass enclosure member defines at least a front surface and a rear surface of the enclosure.

14. The electronic device of claim 8 , wherein:

the glass enclosure member defines a cover sheet having a front surface and a rear surface;

each of the first localized compressive stress region and the second localized compressive stress region extends into the cover sheet from the front surface of the cover sheet; and

each of the third localized compressive stress region and the fourth localized compressive stress region extends into the cover sheet from the rear surface of the cover sheet.

15. The electronic device of claim 14 , wherein:

the cover sheet has a length of at least 100 mm and a width of at least 40 mm.

16. A method of forming a cover sheet for an electronic device, the method comprising:

positioning a first mask along a first surface defining at least a portion of a front surface of the electronic device;

forming a first front compressive stress region having a first thickness along the first surface by exchanging ions into the cover sheet;

removing the first mask;

forming a second front compressive stress region having a second thickness, less than the first thickness, adjacent to the first front compressive stress region by exchanging ions into the cover sheet;

positioning a second mask along a second surface opposite the first surface;

forming a first rear compressive stress region having a third thickness and extending from the second surface toward the second front compressive stress region by exchanging ions into the cover sheet;

removing the second mask; and

forming a second rear compressive stress region having a fourth thickness, less than the third thickness, and extending from the second surface toward the first front compressive stress region by exchanging ions into the cover sheet, wherein after the forming of the second rear compressive stress region each of the first front compressive stress region and the first rear compressive stress region has a thickness in a range from greater than 50 microns to 250 microns and the first surface of the cover sheet has a flatness that is no more than 120 microns out of plane.

17. The method of claim 16 , wherein:

the cover sheet comprises alumina silicate glass; and

forming the first front compressive stress region comprises:

immersing the cover sheet into a first bath comprising sodium ions; and

subsequent to the immersing the cover sheet into the first bath, immersing the cover sheet into a second bath comprising potassium ions.

18. The method of claim 17 , wherein:

the first bath includes a sodium concentration of greater than 30% mol; and

the second bath includes a potassium concentration of greater than 30% mol.

19. The method of claim 16 , wherein:

the cover sheet defines four corners;

the first mask leaves each of the four corners exposed along the first surface; and

the second mask covers each of the four corners along the second surface.

20. The method of claim 16 , wherein:

a first tensile stress region is formed between the first front compressive stress region and the second rear compressive stress region;

a second tensile stress region is formed between the second front compressive stress region and the first rear compressive stress region;

the first tensile stress region is offset with respect to a centerline of the cover sheet in a first direction; and

the second tensile stress region is offset with respect to the centerline in a second direction opposite to the first direction.