A buyer-side guide to holes in glass back covers — why a cut-out is the most expensive feature on the part, how CNC grinding and laser drilling differ, why the order of operations decides strength, and what to write on the drawing before tooling is cut.
A hole in a glass back cover looks like a subtraction: take a sheet, remove a circle, ship it. In production it behaves like an addition. Each hole adds machining time, tool wear, breakage risk, an inspection step and a new place for the part to crack. When a programme moves from one large aperture to a porous pattern of six or eight small ones, the drawing changes by a few lines and the commercial risk changes by a category. This guide is for the buyer who has to price that change and then defend it.
1. Why a Hole Is the Most Expensive Millimetre on the Cover
Start with the reference part. The large-hole and porous covers in this range are 168.14 mm by 75.14 mm by 0.55 mm, made from electronic-grade float glass in fine-sand finishes, with customisable colour and logo. At 0.55 mm the sheet is thin enough that it is forgiving in bending and unforgiving in handling. Every operation after the glass is cut is an opportunity to lose the part.
Drilling is the least forgiving of those operations, for three reasons. It removes material from a brittle body without giving it anywhere to relieve stress. It creates a free edge, and a free edge in glass is a crack waiting for a reason. And it is usually done on a part that has already had value added to it — printed, coated, or at least inspected — so a break at this stage costs more than a break at cutting.
That is why a quotation with "one hole" and a quotation with "six holes" are not separated by a small increment. They are separated by cycle time, tool consumption, breakage allowance and inspection labour. A buyer who asks only for the unit price will be told a number that assumes everything goes right, which is not how a drilling line runs.
2. What a Hole Actually Does to a Sheet of Glass
Glass does not yield. Metal redistributes stress by deforming; glass accumulates it until something gives. A hole concentrates that stress, and the classical factor for a circular hole in a plate under tension is about three — the material immediately beside the hole sees roughly three times the nominal stress. In a thin cover that is handled, dropped and thermally cycled, that multiplier is the reason failures start at apertures rather than in the middle of a face.
Two consequences follow directly. The wall quality of the hole matters more than the hole's dimensional accuracy, because a microcrack in the wall is a stress raiser sitting inside an already concentrated field. And the distance to the nearest edge matters more than the hole diameter, because a hole close to an edge merges two stress fields and the ligament between them carries the load.
This is also why a drilled cover and a moulded cover are not interchangeable in a reliability discussion. A moulded polymer part has a different failure mechanism and a much larger strain to failure. Glass wins on scratch resistance and feel and loses on tolerance to a bad edge.
3. CNC Grinding or Laser Drilling — Which Route for Which Hole?
Both routes are used in production, and the choice is driven by hole size, edge requirement and volume rather than by preference. The table below is the comparison to have in front of you when a supplier proposes one and you want to know why.
| Criterion | CNC diamond grinding | Laser drilling |
| How it cuts | Core drill or milling tool with abrasive and coolant | Localised ablation, no mechanical contact |
| Wall quality | Excellent; can be polished to a fire-polished look | Rougher; may show a recast layer and microcracks |
| Typical hole size | Medium to large apertures, camera and battery openings | Small holes and dense arrays, where mechanical tools would be too slow or too fragile |
| Cycle time | Longer; scales with hole count and depth | Short; one of the reasons dense patterns are economic at all |
| Tool wear | Real cost; tool life drives unit price | No mechanical wear, but optics and consumables degrade |
| Main risk | Chipping at exit, breakage on thin sections | Heat-affected edge, taper, and cracks that only appear in reliability testing |
A hybrid route is common for demanding parts: laser to open the hole quickly, then CNC to finish the wall. It sounds like double work and is often cheaper than either route alone, because it separates the fast operation from the precise one. If a supplier proposes it, that is a sign they have made this trade before.
4. The Order of Operations Decides Whether the Hole Is Strong
This is the single most revealing question you can ask a glass supplier: is the hole formed before or after strengthening? The answer tells you whether the strength of the finished part is designed or accidental.
The sound sequence is cut and drill, grind and polish the edges, then strengthen, then print or coat. When the hole exists before strengthening, the ion-exchange process puts the hole wall into compression along with the rest of the surface, and the wall is protected in the same way as the face.
Drill after strengthening and you cut through the compressive layer, leaving a wall with no protection and, usually, a fresh set of microcracks. The part may pass a visual inspection and fail a drop test. There is a legitimate exception — some processes re-polish and re-strengthen a post-formed hole — but that is a deliberate extra step, not something that happens by default. Ask which one you are buying.
One absolute rule sits beside this. Thermally tempered glass cannot be cut, drilled or ground after tempering; it releases its stored stress and breaks into fragments. That is the physical basis of the "safe particle" behaviour that tempered glass is bought for. If a drawing requires a hole, the hole must exist before the tempering step, full stop. Our article on how back cover manufacturing differs from screen protectors covers the wider process sequence.
5. Large Hole and Porous Arrays Are Two Different Businesses
A single large aperture and a pattern of small holes both count as "holes" in a quotation, and they behave nothing alike on the line. The large-hole cover has one big cut-out, usually for the camera island or a battery window, and its problems are fixturing, warpage and exit chipping. The porous cover has many small holes, and its problems are cumulative position error, tool life and inspection time.
| Aspect | Large hole | Porous array |
| Dominant risk | Warpage and exit chipping on a long cut | Cumulative position error across the pattern |
| Fixturing | Support around a large opening is the challenge | Many small features need many references; datum strategy matters |
| Inspection | A few measurements, each critical | Many measurements; sampling strategy decides the inspection cost |
| Breakage pattern | Cracks running from the corner radii | Ligament fracture between adjacent holes |
| Best route | CNC grinding, with generous corner radii | Laser, or laser plus CNC finishing on visible holes |
The porous variant is where buyers most often underestimate cost, because the drawing looks like a pattern and the quotation looks like a single operation. Ask for the number of holes, the smallest ligament between them, and the inspection sample size. Those three numbers, not the hole diameter, set the price.
6. How Tight Can Hole Position Be — and What Does the Extra Tolerance Cost?
Position tolerance is the number buyers fixate on, usually because the camera module is unforgiving. It is worth understanding that tolerance is not free and it is not linear: going from a comfortable band to a tight one can double the cost, and going one step further can change the process entirely.
As a working frame, general-purpose CNC drilling holds position in the low hundredths of a millimetre on a well-fixtured thin part, and a supplier with good metrology can do better. Laser can hold position tightly because there is no tool deflection, but its wall quality is the limiting factor rather than its accuracy. The exact numbers must come from the supplier against your drawing, because fixture, glass thickness and hole size all move them.
Two practical points save a lot of arguing. First, a tolerance without a datum is not a specification; state which surfaces and which edges define the coordinate system, and whether position is measured from the outside profile or from a machined reference. Second, separate position from size and from form. A hole can be in the right place, the right diameter and still be unacceptable because the wall is tapered or chipped.
7. Minimum Edge Distance: The Rule Buyers Break Most Often
A hole placed close to the outside edge leaves a narrow ligament of glass between the hole wall and the profile. That ligament carries the load, cannot be reinforced, and is where the part will break. The common design rule of thumb is to keep at least one glass thickness of material between the hole edge and the part edge, and preferably one and a half to two thicknesses where the area sees handling or drop loads.
On a 0.55 mm cover that is a very small distance, which is exactly why the rule gets broken. A camera aperture pushed 0.3 mm closer to the corner to match a new module can look fine on a render and fail in a drop test six months later. If industrial design requires the hole to move, the honest response is to change the corner radius or the edge treatment at the same time, not to accept the ligament and hope.
The same logic applies between holes in a porous pattern. Ligament width between adjacent apertures is a design variable, and a pattern that looks even can have a thin spot where two holes approach each other. Mark the minimum ligament on the drawing and inspect it.
8. What Actually Breaks at the Hole
Failure at a hole is not one mechanism, and the fix depends on which one you have. Five appear often enough to name.
Exit chipping. The drill breaks through and takes a small piece of the far face with it. It is a cosmetic reject on a visible face and a crack initiator on a structural one. The fix is backing material, feed control and, on thin glass, drilling from both sides.
Wall microcracks. Invisible at low magnification, fatal under thermal cycling or a drop. They are the reason wall quality should be inspected at magnification, not by eye.
Taper. A laser-drilled hole is often wider at the entry than at the exit. If the drawing assumes a cylindrical wall, the effective aperture at the functional side is smaller than specified, and the camera or sensor sees a smaller opening than intended.
Recast and heat-affected layer. Laser ablation can leave a thin altered layer with different stress and different etching behaviour. It may show up later as a delayed crack or as a coating adhesion failure at the hole edge.
Ligament fracture. In dense patterns, the material between holes simply runs out. This one is a design error, not a process error, and no amount of process control fixes it.
9. How to Inspect a Hole Properly
The inspection plan is where a hole specification becomes enforceable. Visual check with a light box is necessary and completely insufficient. Add the measurements below, and agree the sample size before the first lot rather than after the first complaint.
| Characteristic | Method | Acceptance thinking |
| Hole diameter | Pin gauge or optical comparator | Check both entry and exit if taper is possible |
| Position | CMM or vision system against a stated datum | Report the value, not pass/fail, so drift is visible |
| Wall condition | Magnification, 10× to 30× | Chip size limit and crack prohibition stated separately |
| Chamfer / radius | Optical or profilometer | Both sides if the part is visible from both |
| Residual stress indication | Polarised light viewing | Useful as a process-monitoring signal, not a pass/fail alone |
| Coating at edge | Magnification after printing or coating | Bridging, bald spots and ink run-off are separate defects |
| Ligament width | CMM between adjacent holes or hole to edge | The design rule made measurable |
10. Does the Print Reach the Hole Wall?
Most back covers carry a printed mask, a colour layer or a logo, and the hole is the hardest place to print. Three defects are common and all three are visible to the customer: ink that stops short of the wall and leaves a bright ring, ink that runs over the chamfer and forms an uneven halo, and ink that bridges a small hole entirely.
The process answer is a combination of print registration tolerance, a chamfer that gives the ink somewhere to terminate, and a defined keep-out. The purchasing answer is to put the cosmetic criterion on the drawing in words the inspector can apply: how wide a bright ring is acceptable, whether ink on the chamfer is a defect, and at what magnification it is judged. Without those, "clean edge" means whatever the inspector had for breakfast.
This is also where the dust-free silk-screen capability that suppliers advertise stops being a marketing line and becomes relevant. Dust trapped under the print shows up as a speck, and a speck next to a hole reads as a defect even when the hole itself is perfect.
11. Holes and Sealing: Where Water Gets In
Every hole is a potential ingress path unless something seals it. On a back cover, the seal is usually a gasket or an adhesive bead around the aperture, compressed between the cover and the frame or module. The glass does not seal; the joint does.
Two details decide whether the joint works. The bonding land around the hole must be flat and wide enough for the gasket to sit on; a chamfer that runs too close to the edge removes the land. And the surface finish of that land must suit the adhesive; a fine-sand texture that looks good on the outside can be the wrong surface for a structural bond on the inside.
As with any ingress claim, be precise about what is being tested. A cover cannot be waterproof on its own; an assembly can. If a supplier offers an ingress rating for the part, ask what was actually tested and on what.
12. What Holes Do to Yield and Price
A drilling quotation has four components, and buyers usually see only the first: machine time, tool consumption, breakage allowance and inspection. Machine time and tool consumption are visible in the rate. Breakage and inspection are where programmes quietly lose money, because they are absorbed into a margin and then discovered when volume ramps.
Ask for two numbers that suppliers rarely volunteer: the first-pass yield at drilling, and the tool life in holes per tool. Both are process-specific and both improve with volume, which is why a small trial order can look expensive against a forecast price that never materialises at the quoted yield.
There is also a design lever that costs nothing. Generous corner radii on a large aperture, a slightly larger hole where the optics allow it, and a pattern that keeps ligaments even will all raise yield without changing the machine. Buyers who treat the drawing as fixed give away that lever before the first quote.
13. Packing Drilled Covers So the Holes Survive
A hole is a weak point, and packing that is adequate for a plain cover can be inadequate for a drilled one. Vibration allows parts to move against each other, and contact lands at the edge or at the hole, which is precisely where the material is least able to take it.
The packing options on these covers are stated as standard — pearl cotton bag plus cardboard box — and reinforced, adding a PE bag, desiccant, foam box and carton. For drilled parts the reinforced route is usually the right starting point for sea freight, not because of humidity alone but because the foam box limits relative movement. Whichever you choose, specify parts per bundle, orientation, and whether any interleaf is used. Our guide to glass component packaging and shipping protection covers the wider logistics picture.
14. Where Buyers Lose Money on Drilled Covers
Specifying a hole after tempering. It cannot be done on thermally tempered glass, and doing it on chemically strengthened glass silently removes the protection at the wall.
Tolerance without a datum. Two suppliers can both claim compliance and produce parts that do not interchange.
Thin ligaments to chase a design. The cost appears months later as a drop-test failure or a field crack, never on the quotation.
Buying porous patterns on hole count alone. Ligament width and inspection sample size drive the real cost.
Ignoring the print at the hole edge. A bright ring around an aperture is the defect customers photograph.
Accepting a visual-only inspection plan. Microcracks are invisible at arm's length and decisive in reliability.
Using plain-cover packing for drilled parts. Breakage in transit is charged to the buyer, not the carrier.
15. The RFQ Sheet for a Drilled Cover
Drawing with a datum. Outline, hole positions, diameters, chamfers and the coordinate system they are measured from.
Hole schedule. Count, size range, minimum ligament, and which holes are optical (camera, sensor) versus mechanical (button, port).
Process intent. Whether the hole is formed before strengthening, and whether any post-drilling re-polish or re-strengthening is included.
Edge criteria. Chip size limit, crack prohibition, chamfer width, and the magnification at which they are judged.
Print interaction. Keep-out around apertures, acceptable bright ring width, and whether ink on the chamfer is allowed.
Yield and packing. First-pass yield expectation, tool life information if offered, parts per bundle, tray or foam format, and the drop test applied to the carton.
For the design context around apertures and structural continuity, one-piece glass back cover design is a useful companion read; for material selection, see material selection for custom back covers.
16. Frequently Asked Questions
Can you drill a hole in tempered glass? Not after thermal tempering — the part releases its internal stress and breaks. Holes must be formed before tempering. On chemically strengthened glass a hole can be machined afterwards, but the wall loses its compressive layer unless it is re-polished and re-strengthened.
Is laser better than CNC? For small holes and dense patterns, usually yes on cost and speed. For visible apertures where wall quality is judged by eye, CNC or laser-plus-CNC finishing usually wins. The right answer depends on which hole you are talking about.
How close can a hole be to the edge? A common rule of thumb is at least one glass thickness of ligament, and preferably one and a half to two where the part is handled or dropped. On a 0.55 mm cover that margin is small, which is why it must be checked rather than assumed.
Why do holes sometimes look fine and still crack later? Microcracks in the wall, a heat-affected layer from laser processing, or a wall that lost its compressive layer after strengthening. None of these are visible in a normal incoming check.
Do you make porous patterns to a customer drawing? Yes — the porous and large-hole covers in this range are made to model-specific drawings, with colour and logo customisation. The drawing and the hole schedule are what the quotation is built on.
Sourcing Glass Covers With Holes or Cut-Outs?
Send the drawing, the hole schedule and the edge criteria. We machine large apertures and porous patterns in electronic-grade float glass, with CNC engraving, dust-free silk screen and colour customisation, and we will tell you which holes should be laser-drilled, which should be ground, and where the ligament is too thin.