5.8 Mounting PCBAs, Displays and Modules

The installation of sensitive electronic components into the chassis is the point where electrical integrity and mechanical structure directly intersect. This process requires strict control over mechanical stress, ESD risks, thermal transfer, and front-panel alignment. Errors in mounting introduce latent failures, making disciplined adherence to specified procedures mandatory for product function and cosmetic integrity.

2.3.5.8.1 ESD and Handling Protocol Mandates

ESD and contamination control are mandatory during component installation.

A) ESD-Safe Handling

Mandate: ESD mat and wrist strap must be checked at the start of every shift. Handle boards only by the edges; use ESD-safe totes and racks (cardboard is prohibited).
Atmosphere: Humidity must be controlled between 40% – 60% RH. Avoid using materials (like specific plastics or films) that generate static near open boards.
Cleanliness: Only ESD-safe gloves or finger cots are permitted. Talc or silicone hand creams are prohibited near lenses or labels due to the risk of cosmetic failure (smears, label adhesion failure).

B) Protective Film Management

Keep protective films on lenses and displays until the last safe moment before sealing the enclosure.
Peel films low and slow, pulling away from the sensitive gasket lands to prevent static charge buildup or damage to the sealing surfaces.

2.3.5.8.2 PCBA and Module Mounting Sequence

Installation must follow a specific, documented sequence to prevent cross-threading and bowing.

A) Alignment and Fixturing

Pre-Alignment: Use dowel/locator pins or zero-point nests to set the board position; visual alignment ("eye-balling") is prohibited.
Blind Mates: For blind-mate connectors (backplanes, mezzanines), add guide pins or funnels to the chassis; rely on guide pins, not screws, to pull parts into alignment.

B) Installation Protocol

Prep: Verify standoff height and flatness on the chassis. Confirm screws/torque from the map.
Dry Fit: Lay the board on standoffs; confirm hole alignment with a plastic pick—forcing is prohibited.
Start by Hand: Insert all screws 2–3 turns by hand to prevent cross-threading.
Torque Pattern: Seat screws lightly corner-to-corner, then final torque in a cross-pattern sequence.
Verification: The finished board must not be bowed (bow ≤ 0.5 mm across the card). The gap over all standoffs must be even.

C) Connector Engagement

Mezzanines / Board-to-Board: Press straight down using thumb pads near the connector; avoid pressing at the board edge, which causes bowing.
FFC/FPC (ZIF/LIF): Open the latch fully, insert the cable to the depth mark, and close the latch evenly. Do not route the tail with a tight fold immediately at the latch.
Acceptance: After mating, listen/feel for the latch click; engage TPA/CPA where present.

2.3.5.8.3 Thermal Interface Management (TIM)

Thermal dissipation often relies on precise application of TIM. Surfaces must be perfectly clean.

Prep: Both surfaces must be cleaned with IPA immediately before TIM application.
Paste: Use a bead diameter gauge from the TIM kit. Apply the pattern per the Work Instruction (e.g., lines or X pattern). Torque the heat sink in a two-pass sequence (50% – 100%) using a cross-pattern.
Pads: Peel liners only at the moment of placement; place with tweezers; avoid stretching the pad.
Check: Verify light, even squeeze-out at edges; no paste on connectors; thermal sensors read sane after power-on.

~~2.3.~~5.8.4 Displays, Sealing, and Acceptance Cues

Installation of displays requires strict cosmetic and environmental sealing protocols.

A) Displays and Sealing Protocol

Cleaning Prep: Clean the window and lens with the OEM-approved wipe. IPA is prohibited if the anti-glare coating is sensitive.
Mounting: Align the display using a window template; tack the top edge first, then roll down to prevent air traps/bubbles.
Gasket Integrity: O-rings must be lightly oiled if specified; aim for 15% – 25% compression. Foam gaskets require 25% – 35% compression. Use a criss-cross torque sequence on multi-fastener sealed frames.
Final Check: Peel the outer film only after a dust check under diffuse light. Run a light leak test (dim room, max backlight): zero bright seams are permitted.

B) Quick Verification and Acceptance Cues

Final audit must ensure that structural and electrical integrity is maintained.

Feature
Acceptance (Target)
Defect (Reject)
PCBA Seating
All screws torqued; board flatness maintained ≤ 0.5 mm).
Bowed board, cracked boss, unseated standoff.
FFC/FPC
Tail at depth mark; latch level; no kink at exit.
Half-latched; crease at the latch or tight fold.
Display Fit
Centered in window; reveal uniform ± 0.3 mm.
Specks under lens; skewed reveal; pressure marks on display.
Harness Strain
First clamp ≤ 80 mm from backshell; no bend at solder cup.
Wire bending at solder cup; harness pulling on the connector header.
Earth Bond
PSU tray or board lug to chassis < 0.1 Ω .
High resistance or loose star washer.
Seals (IP/EMI)
Compression even; continuous contact verified.
Gasket gaps or crushed foam; high resistance across EMC seam.

Final Checklist

Mandate
Criteria
Verification Action
ESD Control
Wrist strap checked; humidity 40% – 60% RH; only ESD-safe gloves/cots.
Visual confirmation of no static-generating materials near the workstation.
Torque Sequence
Critical fasteners torqued in a cross-pattern sequence.
Tool log captures torque value per critical fastener.
PCBA Stress
Board flatness maintained (bow ≤ 0.5 mm); all mounting points secured.
Final visual audit to confirm uniform seating on standoffs.
Display Integrity
Panel centered; reveal uniform; zero dust/debris under the lens.
Cleanliness protocol followed (specific wipes); light leak test passed.
Thermal Bond
TIM applied with cross-pattern torque; surfaces cleaned with IPA.
Post-power-on thermal sensor readings verified sane.
Seating Verification
All latches engaged; TPA/CPA clicked; FFC/FPC seated to depth mark.
Tactile and visual confirmation of locking mechanisms.