2.3 Mounting PCBAs, Displays & Modules

Mounting PCBAs, displays, and modules is the moment when design intent becomes tangible—and where small oversights can ripple into major failures. Precision in alignment, care in ESD handling, and consistency in torque application decide whether boards sit flat, connectors last, and seals hold against dust, water, and interference. Optical elements like displays demand cleanliness and centered reveals to preserve the product’s appearance, while thermal and EMI interfaces require just the right compression to function as designed. Treating this stage as controlled, repeatable craftsmanship ensures that delicate electronics integrate seamlessly into durable assemblies.

2.3.1 What “good” looks like (in one line)

Boards sit flat, connectors mate with a click (not a bend), displays center in the window without dust or light leaks, seals compress evenly—and you stayed ESD-safe the whole time.

2.3.2 ESD-safe handling (non-negotiables)

ESD mat + wrist strap checked at start of shift; ionizer on for plastics/films.
Handle boards by edges, not components; use ESD totes/racks (no cardboard).
Humidity 40–60% RH helps; avoid sweater weather near open displays/boards.
Only ESD-safe gloves/finger cots; no talc or silicone hand cream (label fish-eyes, lens smears).
Keep protective films on lenses until the last safe moment; peel low and slow away from gasket lands.

2.3.3 Alignment & fixturing (make the hardware do the aiming)

Use dowel/locator pins or zero-point nests to set board and display position—don’t “eye-ball.”
For blind-mate connectors (mezzanines, backplanes), add guide pins/funnels; never rely on screws to pull parts into alignment.
Displays: use a window template/bezel gauge so the active area centers before adhesive touches.
Keep first clamp before first bend on any attached harness (19.4/19.5): clamp within 50–80 mm of the connector.

2.3.4 PCBA mounting sequence (repeatable, gentle)

Prep: verify standoff height and flatness on the chassis (23.1). Confirm screws/torque from the map (23.2).
Dry fit: lay the board on standoffs; check hole alignment with a plastic pick—no forcing.
Start by hand: insert all screws 2–3 turns by hand to avoid cross-thread.
Pattern: seat screws lightly corner-to-corner, then final torque in a cross pattern.
Connect: mate board-to-board and harness connectors squarely; listen/feel for latch click; engage TPA/CPA where present.
Verify: board not bowed; gap over standoffs even; no harness strain on headers.

Starter torque (validate on your joint):

M2: 0.2–0.3 N·m, M2.5: 0.35–0.6 N·m, M3: 0.6–1.0 N·m (23.2.5).

2.3.5 Connectors you’ll meet (and how not to break them)

Mezzanines / board-to-board: press straight down using thumb pads near the connector; never at the board edge. Use guide pins if available.
FFC/FPC (ZIF/LIF): open latch fully, insert to the line, close latch evenly; tug test (light) to confirm capture; route the tail with no fold at the latch.
Wire headers: support the board behind the header during mate; no harness pulling as a lever.
Coax (U.FL, etc.): align carefully, press until click; pop up with a nylon tool, not your nails.

2.3.6 Thermal interfaces while mounting (TIMs without mess)

Pads: peel liners last; place with tweezers; avoid stretching; no fingerprints.
Paste: bead diameter gauge from the TIM kit (22.3); apply pattern per SWI (lines/X), two-pass torque on heatsink (50% → 100%).
Checks: light, even squeeze-out at edges; no paste on connectors; sensors read sane after power-on.

2.3.7 Displays & lightpipes (centered, clean, sealed)

Prep

Clean window and lens with the approved wipe (many anti-glare coatings hate IPA—use OEM wipe).
Stage gasket/adhesive and alignment template; keep dust down (ionizer + sticky mat).

Mount

Align with template; tack top edge first, then roll down—avoid traps/bubbles.
If using foam/EMI gasket around the display, confirm gap-free corners (no stretched miters).
Seat lightpipes fully; verify they don’t preload the PCB.

Finish

Peel the outer film only after a dust check under diffuse light.
Run a light leak test (dim room, max backlight): no bright seams; uniform bezel glow.

2.3.8 Sealing (IP and EMC) done right

O-rings: light oil if spec’d; compress 15–25%; use criss-cross torque.
Foam gaskets: aim 25–35% compression; do not crush—check with feeler/tape stack on first article.
EMI fingers/foam: require clean bond pads (23.1.5); verify continuity < 0.1 Ω across the seam.
Adhesive beads: steady bead diameter per SWI; no gaps/strings; keep off label zones. Cure per chemistry before test.

2.3.9 Quick verifications (catch issues now, not at test)

Flatness/bow: lay a straightedge; bow ≤ 0.5 mm across the card unless drawing says otherwise.
Earth bond: PSU tray or board lug to chassis < 0.1 Ω (22.1/23.1).
Connector seating: all latches engaged, TPA/CPA clicked; ZIF bars level.
Display fit: reveal uniform ±0.3 mm; flushness ±0.25 mm within bezel.
Seal path: continuous contact print (talc or witness tape) around the frame—no holidays.

2.3.10 Acceptance cues (starter tolerances—tune to your spec)

Feature	Accept	Reject
PCBA seating	All screws torqued; no visible bow; standoffs fully bearing	Bowed board, lifted corner, cross-thread, cracked boss
Mezzanine mate	Parallel join line; full latch engagement	One side high, intermittent function on tap
FFC/FPC	Tail at depth mark; latch level; no kink at exit	Half-latched; tail skew; crease at latch
Lightpipe/display	Centered in window; no dust; no pressure marks	Light leak seam; specks under lens; skewed reveal
Seals (IP/EMI)	Compression even; corners tight; bond <0.1 Ω	Gasket gaps; crushed foam; high resistance
Harness strain	First clamp ≤ 80 mm from backshell; no bend at cup	Wire bending at solder cup; floating, un-clamped

2.3.11 Common traps → smallest reliable fix

Trap	Symptom	First move
Using screws to “pull” misaligned parts	Stripped threads, warped boards	Add guide pins/nests; loosen, re-align, hand-start
Bowed PCB after torque	Intermittents, cracked vias	Verify standoff heights; reduce torque; add mid standoff
ZIF half-latched	Flicker/no image	Open, re-insert to mark, close evenly, tug test
Dust under lens	Customer returns	Late film peel; ionizer on; lint-free wipe; clean room corner if needed
Over-compressed gasket	IP fails later	Torque in two passes; verify 25–35% compression with tape gauge
EMI gasket on paint	High bond Ω	Expose bond pad; clean; re-seat; measure <0.1 Ω
Harness preload on headers	Cracked solder joints	Add clamp; reroute; service loop per drawing

2.3.12 Pocket checklists

Before mounting

ESD checks green; ionizer on; gloves clean
Chassis threads/standoffs verified; bond pads clean
Fasteners & torque map at station; bits OK

PCBA install

Dry fit OK; screws hand-started; cross-pattern torque
Connectors mated square; TPA/CPA engaged
TIM pads/paste per kit; two-pass torque on heatsinks

Displays/modules

Window & lens clean; template in place
Gasket/adhesive applied without gaps; corners tight
Reveal/flush check; light-leak check; film peeled last

Sealing & finish

O-ring/foam compression in range; criss-cross pattern
EMI seam <0.1 Ω; earth lug <0.1 Ω
Harness clamps set; no bend at joints

With disciplined handling, guided alignment, and verified sealing, assemblies avoid stress cracks, light leaks, and noisy intermittents. The result is hardware that looks clean, functions reliably, and keeps its integrity long after it leaves the factory.