2.4 Brackets, shields and heat sinks

The installation of structural and thermal management components—brackets, shields, and heat sinks—is a critical step for ensuring mechanical robustness, electromagnetic compliance (EMC), and thermal stability. This process follows a controlled, logical sequence—mechanical, then electrical, then thermal—to prevent interference between stages. Adhering to this sequence is essential to avoid issues like ground loops, component overheating, and vibration-related failures in the field.

Structural brackets and geometric control

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

Positioning and tightening sequence

Datum-First Alignment: Begin with a dry fit of the bracket to confirm alignment. The bracket must be aligned with its primary datum feature, such as a locator pin or fixed standoff, before securing the secondary fastening points.
Torque Pattern: Secure the fasteners using a snug-all, then final-torque sequence. Apply the final torque in a cross-pattern to prevent distorting the bracket or the chassis base.
Stack Order: The standard fastening stack order is bracket, then washer (if specified in the diagram), then chassis. Do not substitute parts or use undocumented shims and washers that are not on the Bill of Materials (BOM).

Alignment and acceptance

Acceptance Criteria: Once installed, the bracket must sit flush against its mounting surface with no play or rocking motion. Mounting slots should not be bottomed out against one end. The clearance gap to neighboring components must be uniform, typically maintaining a constant ± 0.5 mm gap.
Keepout Zones: Verify that no bracket edge or lip will rub against a wire harness. If a cable route inherently passes close to a sharp sheet metal structure, approved edge guards must be added.
Witness Marks: Apply witness marks using an approved paint pen across critical fasteners immediately after reaching final torque. This provides a visual confirmation that the screw was properly torqued and has not loosened due to vibration.

EMI shields and grounding continuity

Establish EMI bonds and install shielding components early in the assembly flow. This prevents paint flakes, adhesive, or gasket debris from increasing the resistance of the electrical path.

Shield mounting protocol

Bond Lands: Grounding bond lands must be completely clean. Bare metal must be visible, with no paint, powder coating, or adhesive residue in the conductive path.
Seating: Shields must be seated squarely and passively. Do not “spring” or force a warped shield into place under tension, as this pre-loads the screws and increases the risk of vibration-induced failure.
Fastener Pattern: Secure shields using a fastener tightening pattern starting from the center and working outward. This technique ensures even compression across the entire conductive gasket surface.

Gasket compression and quality checks

Conductive Foams: Compress conductive EMI foam gaskets to a controlled 20% to 30% to achieve an effective environmental and electrical seal without damaging the cellular structure.
Fingerstock: Achieve uniform surface contact for beryllium copper or steel EMI fingerstock; avoid crushing it completely flat.
Continuity Check: Verify the final electrical integrity of the shield. As a guideline, shield-to-chassis or shield-to-shield resistance should measure < 0.1 Ω. Log this low-resistance meter measurement during the First Article Inspection (FAI).
Defect Rejection: Reject the assembly if the EMI gasket has gaps, is crushed flat, or if the grounding resistance exceeds the 0.1 Ω threshold.

Heat sinks and thermal management

Heat sink installation is a precise process that requires accuracy to ensure heat conduction is not impeded by thermal resistance from voids, debris, or excessive paste.

TIM choice and application

Pads and Gap Fillers: Place thermal pads using ESD-safe tweezers. Avoid stretching the pad material, as this alters its thickness and thermal properties. Verify the final compression, typically 10% to 30%, against the material’s durometer rating. Remember to peel the protective liners before mating the surfaces.
Pastes and Greases: Apply a controlled, even film. Use a bead diameter gauge or a volume-controlling stencil. Note: Do not slide or twist the heat sink laterally over the paste to spread it, as this can trap insulating air bubbles within the joint.
Standard TIM Patterns:
- Small Die (< 20 x 20 mm): Use a center pea or a very thin X pattern.
- Long Bar or VRM Strip: Apply three thin, parallel lines along the length.
- Large Plate: Use a cross-hatch pattern or a volume-controlled stencil to ensure even coverage, targeting 90% to 100% surface wetted area without significant overflow.

Clamping and torque sequence

Torque Guideline: Apply the exact specified torque to establish the necessary compressive clamping force without risking damage to the silicon die.
- Starter Torque Range (M3 hardware): Typically 0.6 to 1.0 Nm.
Torque Sequence: Use a two-pass sequence (first pass at 30–50% torque, second pass at 100% torque). Always use a cross-pattern to prevent uneven pressure and potential die damage and ensure perfectly parallel contact.
Squeeze-Out Verification: The acceptable target is a minimal, highly uniform perimeter line of Thermal Interface Material (TIM) squeeze-out at the mating edges. Avoid excessive paste overflow, as it can act as an insulator and may flow into nearby sensitive connectors.

Stack height and gap fillers

Gap Fillers: These materials are designed to be compressed in service to achieve target thermal conductivity. Use only compressible shims or thermal spacers that are strictly defined in the engineering BOM. Warning: Do not substitute standard metal washers ad hoc to adjust height.
Structural Board Check: The total accumulated spring force from multiple gap pads must not bow the PCB. If high compressive forces are required, use mid-board standoffs to prevent the printed circuit board from deforming under the thermal load.

Recap: Brackets, Shields and Heat Sinks

Component	Parameter	Requirement	Action / Inspection
Structural Bracket	Alignment & Gap	Flush mount; uniform gap ±0.5 mm to neighbors	Visual check; apply witness marks after final torque.
EMI Shield	Grounding Resistance	Shield-to-chassis resistance < 0.1 Ω	Measure with multimeter during FAI; log result.
EMI Gasket	Compression	Conductive foam: 20–30% compression	Visual inspection; reject if gapped, crushed flat, or resistance >0.1 Ω.
Heat Sink (TIM)	Application & Coverage	90–100% surface wetted area; minimal uniform squeeze-out	Apply per specified pattern; verify no sliding/twisting during mating.
Heat Sink (Mounting)	Torque Sequence	Two-pass cross-pattern (e.g., 30-50% then 100% of spec, M3: 0.6-1.0 Nm)	Use calibrated torque driver; follow sequence to prevent warping/damage.