2.4 Brackets, Shields & Heat Sinks

The installation of structural and thermal management components—brackets, shields, and heat sinks—is critical for mechanical robustness, electromagnetic compliance (EMC), and thermal stability. This process requires a specific, controlled sequence (mechanical – electrical – thermal) to ensure nothing fights the next step. Failure risks intermittent ground loops, component overheating, and catastrophic vibration failures.

2.4.1 Structural Brackets and Geometric Control

Structural bracketry establishes the square, repeatable geometry that defines the internal layout and component clearances.

A) Positioning and Tightening Sequence

• Datum-First Mandate: Dry fit the bracket first to confirm alignment. Seat the bracket at the primary datum (locator pin or fixed standoff), then work outward.
• Torque Pattern: Secure fasteners using a snug all – final torque sequence in a cross-pattern.
• Stack Order: The standard stack order is bracket – washer (if specified) – chassis. Avoid using creative shims or washers not defined in the BOM.

B) Alignment and Acceptance

• Acceptance Cues: The bracket must sit flush (no rocking); slots must not be hard against one end; and the reveal to neighboring parts must be consistent (e.g., ± 0.5 mm).
• Keepouts: Confirm no bracket lip will rub a wire harness; add edge guards if a route passes near the structure.
• Witness Marks: Apply witness marks (paint pen) on critical fasteners after final torque to provide visual evidence that the screw has not backed out.

2.4.2 EMI Shields and Grounding Continuity

EMI bonds and shields must be established early in the assembly flow to prevent paint flakes or gasket debris from compromising the low-ohm path.

A) Shield Mounting Protocol

• Bond Lands: Ground bond lands must be clean—bare metal visible, with no paint or powder residue in the path.
• Seating: Shields must be seated squarely; prohibited is "springing" the shield into place, which preloads the screws and risks vibration failure.
• Fastener Pattern: Shields must be secured using a fastener pattern that works the shield from the short path to the long path, ensuring even compression across the conductive gasket.

B) Gasket Compression and Check

• Conductive Foams: Conductive foams require 20% – 30% compression for effective sealing.
• Fingerstock: Fingerstock requires uniform contact, but not crushing flat.
• Continuity Check: The final electrical integrity must be verified. Mandate: Seam-to-seam or shield-to-chassis resistance must be < 0.1 Ω (low-ohm meter log required on the first article).
• Defect: Reject is mandated if the gasket has gaps, is crushed flat, or the bond resistance is high.

2.4.3 Heat Sinks and Thermal Management

Heat sink installation is a metallurgical process demanding high precision to ensure heat conduction is not impeded by thermal resistance.

A) TIM Choice and Application

• Pads/Gap Fillers: Pads must be placed using tweezers; avoid stretching; verify compression (e.g., 10% – 30%) against the durometer. Peel liners only at the last minute.
• Pastes/Greases: Apply a thin, even film. Use a bead diameter gauge or stencil. Prohibited: Sliding the sink on paste, as this introduces air bubbles.
• TIM Recipes (Patterns):
  • Small Die (< 20 x 20 mm): Pea or thin X pattern.
  • Long Bar/VRM Strip: Three parallel lines.
  • Large Plate: Cross-hatch or stencil to control volume and ensure even coverage (target 90% – 100% coverage).

B) Clamping and Torque Sequence

• Torque Mandate: Torque is applied to set the clamping force.
  • Starter Torque Range (M3): 0.6 – 1.0 Nm.
• Torque Sequence: Use a two-pass sequence (30-50% pass –100% pass) using a cross-pattern to prevent component die damage and ensure even contact.
• Squeeze-Out Sanity: A minimal, uniform line of TIM at the edges is acceptable. Excessive paste is prohibited as it creates thermal resistance and can contaminate nearby connectors.

C) Stack Height and Gap Fillers

• Gap Fillers: Must be compressed in service to achieve the target thermal conductivity. Shims or spacers must be defined in the BOM; substituting washers ad hoc is prohibited.
• Structural Check: The total spring force from multiple gap pads must not bow the PCB; use mid-standoffs if necessary to prevent board deformation.

Final Checklist

Mandate	Criteria	Verification Action
Grounding Continuity	Shield and bond joints verified < 0.1 Ω to chassis ground.	Low-ohm meter audit logged on first article.
Thermal Clamp	Heat sink fasteners secured using a cross-pattern torque sequence.	Audit confirms two-pass torque sequence applied; fasteners set to spec Nm.
TIM Application	Surfaces clean; paste applied evenly; no voids or contamination.	Inspection confirms minimal, uniform squeeze-out at sink edges.
EMI Seal	Gaskets (conductive foam) compressed 20% – 30%; no gaps or crushing.	Fastener torque verified to prevent crushing the conductive material.
Structural Alignment	Brackets sit flush; alignment verified with jigs; no "springing" components into place.	Visual check confirms consistent reveal (± 0.5 mm) to neighboring parts.
Fatigue Prevention	Harness braid straps secured with 360˚ clamps; pigtails ≤ 10 mm.	Audit confirms correct length and method used for shield termination.