5.6 Chassis and Enclosure Preparation

Chassis and enclosure preparation is the mandatory foundation that dictates the success of final assembly. Failure at this stage — neglecting to verify electrical grounding paths or remove machining debris — introduces catastrophic latent failures like intermittent shorts and thermal failures. Prep must eliminate Foreign Object Debris (FOD) and ensure structural integrity against all defined tolerances before sensitive electronic sub-assemblies are installed.

5.6.1 Incoming Visual and Dimensional Integrity

All surfaces and critical dimensions must be verified against the design drawing and quality specification upon receipt.

Surface Grading and Acceptance

• Grade A (Customer-Facing): Bezels, lids, and primary label zones. Mandate: Color and gloss must be uniform; no scratches visible at arm's length under diffuse light.
• Grade B (Internal/Hidden): Bracket interiors and hidden sides. Minor cosmetic blemishes are allowed, provided they do not compromise function (e.g., no exposed base metal).

Material/Finish	Critical Inspection Points
Powder Coat/Wet Paint	Thickness at threads, mask lines on gasket/bond lands, runs/sags, chips at sharp edges.
Anodize (Aluminum)	Must verify if bond lands were masked, as anodize is insulating. Color variance lot-to-lot is common.
Plating (Zinc/Nickel)	Check for burn marks and thread growth (potential Go/No-Go tight spots).
Molded Plastic	Warp/flatness at gasket grooves, sink marks, and integrity of mounting bosses.

Dimensional Verification

• Flatness: Mating planes and gasket lands must be verified to be within the drawing tolerance (typically 0.3 – 0.5 mm across the span). Use feeler gauges and a flat plate for quick spot checks.
• Tolerance Check: Standoff heights for critical boards must be within tight limits (e.g., ± 0.1 to 0.2 mm). Record any consistent dimensional trend by vendor/lot.

5.6.2 Threads, Inserts, and Grounding Lands

The mechanical and electrical interfaces are defined by the fastener system and the bonding pads.

Thread and Fastener Preparation

• Thread Gauge Mandate: Thread gauges (Go/No-Go) must be used on a sample per lot and on all critical holes. Powder coat or paint must not choke threads.
• Insert Integrity: PEMs (self-clinching nuts) and rivnuts must be fully seated flush, show even press marks, and be verified for anti-spin capability. Studs must be visually checked for perpendicularity using a sight square.
• Debris Control: Prohibited: Oily taps that bleed into gasket lands. All blind holes must be chased and vacuumed to ensure zero internal chips (FOD risk).

Electrical Bonding and EMC Lands

• Paint-Scrape Pads: Grounding pads must be verified to be present, of the correct size, and clear of paint or anodize. Edges must be crisp (no overspray).
• Continuity Check: The shell-to-land continuity must be checked with a low-ohm meter. Mandate: Resistance must be less than 0.05 – 0.10 Ω to the chassis safety ground reference point.
• EMC Gaskets: Conductive foam or finger stock gaskets must be inspected for proper seating, alignment with fastener holes, and absence of compression damage prior to lid installation.

5.6.3 Sealing, Function, and Cleanliness

These steps prepare the enclosure for environmental protection and ensure the final functional usability of doors and access points.

IP Readiness and Seal Surfaces

• Gasket Preparation: Gasket lands must be clean and free of powder beads or nicks. Mandate: Wipe with lint-free cloth and approved solvent (e.g., IPA).
• Compression Targets: Compression must be predictable. For first articles, confirm compression targets (e.g., silicone foam 25% – 35% of thickness) using a gauge or tape test.
• Drain Paths: Drain paths or weep holes must be checked to be clear of paint clots or weld spatter.

Final Mechanical Function

• Door Alignment: Hinge lines must be straight, the lid must sit without rocking, and the reveal (gap) must be equal left and right.
• Latch Function: Latches and cams must engage smoothly. Check for rattle—a gentle shake must reveal no loose captive screws or washers. Foam bumpers must be present where specified to prevent buzz.

Class "Box Build Clean" Mandates

• Handling: Gloves are mandatory for handling Grade A surfaces. ESD straps are required when PCBs or sub-assemblies are brought near.
• Cleaning Sequence: Blow off with filtered air, wipe with lint-free cloth + IPA (or approved cleaner), and then dry.
• Prohibited Residue: Prohibited: Silicone residues unless explicitly called out, due to the high risk of label adhesion failure ("fish-eye") and paint contamination.

5.6.4 Data Capture and Rework Control

Traceability Data

The following data must be captured and stored per lot in the MES system:

• Enclosure PN/Rev, Vendor Lot, Finish Type/Color Code.
• Thread/insert audit results, bond resistance readings, flatness/standoff spot checks.
• Rework/touch-up actions with photos and location logs.

Rework Rules (Mandate vs. Prohibition)

Allowed (Record It):

• Minor edge deburr.
• Thread chase to clear coating (must not re-tap to a larger size without engineering release).
• Tiny paint touch-up on Grade B faces, provided it is documented.
• Clean/buff small scuffs on Grade A if they become invisible at arm's length.

Prohibited (Scrap or MRB):

• Paint or anodize on gasket or bond lands.
• Warped panels beyond flatness specification.
• Spun inserts or studs leaning out of perpendicularity.
• Deep gouges on Grade A surfaces or label zones that fail a tape-pull adhesion test.
• Any change that alters the EMC or IP (Ingress Protection) function without engineering release.

Final Checklist

Mandate	Criteria	Verification Action
Grounding Integrity	Paint/anodize removed; bond resistance verified < 0.1Ω.	Low-ohm meter audit using a star washer at the bond point.
Surface Flatness	Mating and gasket lands verified to be within 0.3 mm tolerance.	Feeler gauge check against a surface plate.
Thread/Insert Health	Threads clear; captive inserts/studs are flush and anti-spin.	Go/No-Go thread gauge check; tactile spin test on inserts.
Cleanliness/FOD	Surfaces wiped with IPA; no chips/debris in blind holes.	Magnet sweep conducted weekly; blind holes vacuumed.
Label Adhesion Zone	Surfaces cleaned; zone free of silicone/oil residues.	Adhesion verified with a documented tape-pull test if necessary.
Rework Restriction	No paint touch-up or material addition on Gasket/Bond lands.	Rework audit confirms only Grade B areas touched up; no warp or spun inserts allowed.