Post-Installation Quality Checks: Touch, Brightness, Dead Pixels, Color, and Proximity Sensors

Quality Control Timing: Two Checkpoints

Run QC twice: (1) immediately after connecting the new display (before final sealing) to catch connector/part issues while rework is easy, and (2) after adhesive has set (or after the device has sat under light pressure per your shop routine) to confirm nothing shifted, pinched, or became intermittent.

• Checkpoint A (pre-seal): device assembled enough to power on safely, display connected, sensors/camera unobstructed, but adhesive not fully committed.
• Checkpoint B (post-set): after adhesive cure/set and final assembly, including cases where pressure or clamping was used.

Tools and Setup

• Solid-color test images (white, black, red, green, blue, gray) and a gradient image.
• Touch test app or built-in diagnostics (if available), plus a drawing app for continuous lines.
• Dark room option (for backlight bleed, OLED uniformity, and dead pixels).
• Known-good charger and a call-capable SIM/eSIM setup (or a test call method) for proximity checks.
• Microfiber cloth and isopropyl alcohol (for cleaning camera/sensor windows).

Checkpoint A: Immediate Post-Install QC (Before Adhesive Commitment)

1) Touch QC: Edge-to-Edge Grid + Multi-Touch

Concept: Touch issues often come from connector seating, digitizer faults, or pressure/pinch points that create “dead zones” near edges. Testing before sealing prevents repeated opening.

Step-by-step

1. Open a touch grid test (or drawing app). Set screen brightness to mid-level to see the grid clearly.
2. Edge-to-edge sweep: drag a finger slowly along all four edges and corners, staying within 2–3 mm of the bezel line. Repeat twice.
3. Grid fill: trace horizontal and vertical lines across the entire display to ensure no gaps. In a drawing app, draw continuous lines without lifting your finger; look for breaks.
4. Multi-touch: place 2, 3, then 5+ fingers (as supported) and move them around. Pinch-zoom in a photo or map to confirm stable tracking.
5. Pressure sensitivity check (if applicable): ensure normal taps register without excessive force; avoid pressing hard (can mask marginal issues).

Pass criteria: no dead zones, no “ghost touches,” no intermittent tracking, and consistent response at edges/corners.

Fail criteria: missed input areas, random touches, touch that drops out when flexing the phone lightly, or multi-touch that collapses into single-touch.

2) Brightness and Uniformity: Multiple Levels

Concept: Uniformity issues can be caused by panel defects, pressure points, or backlight problems (LCD). Checking at different brightness levels reveals issues that hide at max brightness.

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Step-by-step

1. Disable auto-brightness temporarily (so the level stays fixed).
2. Display a white screen at ~25%, ~50%, and ~100% brightness.
3. Look for dark corners, bright spots, blotches, or banding. Tilt slightly to confirm it’s not just viewing angle behavior.
4. Display a gray screen (mid-gray) at low brightness to reveal unevenness and tint shifts.

Pass criteria: no obvious bright/dark patches, no severe banding, and consistent brightness across the panel for the device class.

Fail criteria: strong hotspots, large dark areas, obvious bands/stripes, or brightness that is significantly lower than expected at the same slider position.

3) Dead/Stuck Pixel Check

Concept: Dead pixels show as black dots on bright screens; stuck pixels show as colored dots on dark screens. Early detection avoids customer returns.

Step-by-step

1. Show solid red, green, blue screens: look for black dots (dead subpixels) or off-color points.
2. Show solid black in a dim room: look for colored points (stuck pixels) or glow patterns.
3. Zoom in with your eyes: scan systematically in rows to avoid missing small defects.

Pass criteria: no visible dead/stuck pixels at normal viewing distance; follow your shop’s tolerance policy if applicable.

Fail criteria: any dead/stuck pixel in the central viewing area, clusters, or multiple defects noticeable without searching.

4) Color Tinting / White Balance

Concept: Replacement displays can show warm/cool tint, green/pink shift, or uneven white balance. Some devices allow software calibration; QC determines if the part is acceptable.

Step-by-step

1. Show a pure white screen at ~50% brightness.
2. Check for overall tint (too warm/yellow or too cool/blue) and uneven tint (one side pink/green).
3. Show skin-tone photos and a neutral gray gradient to confirm real-world appearance.

Pass criteria: neutral whites and consistent tint across the panel; minor variation acceptable if consistent with OEM behavior for that model.

Fail criteria: strong green/pink cast, obvious half-screen tint split, or color shifts that make content look incorrect.

5) Flicker at Low Brightness

Concept: Some displays use brightness modulation that can cause visible flicker, especially at low brightness. Excessive flicker can be a part-quality issue or a compatibility problem.

Step-by-step

1. Set brightness to the lowest 0–20% range.
2. View a mid-gray screen and watch for pulsing or banding.
3. Use a second phone camera in slow-motion mode to detect flicker/banding more clearly (if permitted by your workflow).

Pass criteria: no visible flicker during normal use; camera-detected flicker that is not perceptible to the eye may be acceptable depending on device behavior.

Fail criteria: obvious pulsing, rolling bands visible to the eye, or flicker that causes discomfort.

6) OLED-Specific: Burn-in / Image Retention Quick Check

Concept: OLED can show image retention or uneven aging. A replacement panel should not show persistent shadows after brief static images.

Step-by-step

1. Display a high-contrast static image (e.g., white screen with dark UI elements) for 1–2 minutes.
2. Switch to a uniform gray screen.
3. Look for faint outlines of the previous image (status bar icons, navigation bar).

Pass criteria: no persistent outlines after a short exposure; brief, very faint retention that disappears quickly may be acceptable depending on panel type and device norms.

Fail criteria: clear persistent shadows, uneven patches that remain, or visible burn patterns out of the box.

Functional Checks Tied to the Display Assembly

7) Front Camera Clarity (Through the Display/Cutout Area)

Concept: Smudges, dust, misaligned seals, or incorrect lens window alignment can soften images or create haze.

Step-by-step

1. Open the front camera and take a photo in good lighting.
2. Check for haze, blur, vignetting, or flare.
3. Switch between photo/video; verify focus and exposure behave normally.

Pass criteria: sharp image comparable to pre-repair expectations; no foggy look.

Fail criteria: persistent haze/blur, dark corner shading, or visible obstruction patterns.

8) Proximity Sensor Behavior During Calls

Concept: The proximity sensor should turn the screen off near the face and back on when moved away. Misalignment, contamination, or incorrect screen/sensor window properties can cause the screen to stay on (cheek touches) or stay off.

Step-by-step

1. Place a test call (voicemail or another phone).
2. Bring the device to your ear: the screen should turn off promptly.
3. Move it away: the screen should wake reliably.
4. Repeat 3–5 times; test with slight angle changes.

Pass criteria: consistent screen off/on behavior with normal movement.

Fail criteria: screen stays on near face, screen stays off after moving away, or behavior is intermittent.

9) Ambient Light Sensor Response

Concept: Auto-brightness should respond smoothly to lighting changes. A blocked sensor window or incorrect alignment can cause slow or incorrect brightness changes.

Step-by-step

1. Enable auto-brightness.
2. Shine a light near the sensor area, then cover it with your hand.
3. Observe brightness changes within a few seconds; repeat twice.

Pass criteria: noticeable, stable adjustment without oscillation.

Fail criteria: no response, extreme jumps, or constant hunting up/down.

10) Earpiece Audio Path (Top Speaker)

Concept: The earpiece mesh, gasket alignment, or debris can muffle sound. Display assembly work often disturbs this area.

Step-by-step

1. During the same test call, set volume to mid then high.
2. Listen for muffling, rattling, distortion, or very low volume.
3. Confirm the sound is coming from the earpiece (not loudspeaker) by covering the bottom speaker area briefly.

Pass criteria: clear voice audio at normal volume levels.

Fail criteria: very low output, distortion, or intermittent cutout when pressing lightly near the top edge.

11) Fingerprint / Face Authentication (If Integrated)

Concept: Some devices integrate fingerprint sensing with the display or require precise optical clarity. Face authentication depends on front camera/sensor performance.

Step-by-step

1. Fingerprint: attempt unlock 10 times. If it fails repeatedly, delete and re-enroll fingerprints and test again.
2. In-display fingerprint: test at different finger angles and with typical screen protectors (if the customer will use one).
3. Face authentication: test in good light and moderate indoor light; confirm it unlocks promptly and doesn’t loop.

Pass criteria: consistent unlock success comparable to expected device behavior.

Fail criteria: repeated failures after re-enrollment, sensor not detected, or authentication only works under narrow conditions.

Checkpoint B: Post-Adhesive-Set QC (After Final Assembly)

This second pass focuses on issues caused by pressure, slight shifts, or cables being pinched during final closure.

Repeat the High-Risk Tests

• Touch edge test again: especially corners and near the connector side.
• Uniformity at low brightness: pressure points can appear as bright/dark patches after clamping.
• Proximity sensor call test: adhesives/foams settling can change sensor behavior.
• Front camera quick photo: confirm no new dust or haze appeared.

Physical Interaction Checks

• Lightly press around the perimeter while on a touch grid: touch should not cut out or create ghost touches.
• Check that the display does not lift at edges and that tapping does not create rainbowing/newton rings (pressure contact artifacts).

Troubleshooting Map: Symptom → Likely Cause → Actions

Pass/Fail Criteria for Returning the Device

Minimum Pass Criteria (Returnable)

• Touch: full edge-to-edge response, stable multi-touch, no ghost touches during normal handling.
• Display: acceptable uniformity at low/mid/high brightness; no obvious lines, severe blotches, or distracting tint splits.
• Pixels: no dead/stuck pixels that are visible in typical use (apply your shop’s written tolerance if you have one).
• Flicker: no user-visible flicker that would be noticed in normal use.
• OLED retention: no persistent image retention after brief static test.
• Front camera: clear image without haze/obstruction.
• Proximity/ambient sensors: reliable behavior in call and lighting tests.
• Earpiece: clear call audio at normal volume.
• Biometrics (if used): fingerprint/face unlock works consistently after re-enrollment if needed.

Automatic Fail Criteria (Do Not Return)

• Intermittent touch, dead zones, or ghost touches.
• Any lines across the display, repeated flicker visible to the eye, or major brightness non-uniformity.
• Proximity sensor failures that cause screen-on during calls (risk of accidental inputs) or screen-off lockouts.
• Front camera haze caused by internal contamination that cannot be cleaned externally.
• Biometric function missing or unreliable when it is a normal expected feature for the device configuration.

QC Checklist Template (Printable)

Checkpoint A (pre-seal)  Date/Tech: __________  Device: __________  Part: __________
[ ] Touch: edge sweep all sides + corners
[ ] Touch: grid fill (no gaps) + drawing continuous lines
[ ] Touch: multi-touch pinch/zoom
[ ] Brightness: 25% / 50% / 100% uniformity on white
[ ] Gray uniformity at low brightness
[ ] Dead/stuck pixels: R/G/B/Black scan
[ ] Color tint: white + skin tones
[ ] Low-brightness flicker check
[ ] OLED only: quick retention check
[ ] Front camera clarity test photo
[ ] Proximity sensor call test (3–5 cycles)
[ ] Ambient light sensor response
[ ] Earpiece audio during call
[ ] Fingerprint/Face auth test (and re-enroll if needed)

Checkpoint B (post-set)
[ ] Repeat touch edge + multi-touch
[ ] Repeat low-brightness uniformity/gray
[ ] Repeat proximity + camera quick check
[ ] Press-perimeter test (no touch dropouts)

Result: PASS / FAIL   Notes/Actions: ______________________________