2.4 Brackets, shields and heat sinks

The installation of structural and thermal management components—brackets, shields, and heat sinks—is mandatory for mechanical robustness, electromagnetic compliance (EMC), and thermal stability. This process dictates a controlled, logical sequence (mechanical, electrical, then thermal) to ensure zero interference between stages. Failure to adhere to these sequences risks ground loops, component overheating, and catastrophic vibration 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: A dry fit of the bracket must be performed first to confirm alignment. The bracket must be seated squarely against its primary datum (such as a locator pin or fixed standoff) before working outward to the secondary fastening points.
• Torque Pattern: The fasteners must be secured using a snug-all, then final-torque sequence. Final torque must be applied in a cross-pattern to avoid warping the bracket or the chassis base.
• Stack Order: The standard fastening stack order is bracket, then washer (if specified in the diagram), then chassis. Substituting parts or utilizing undocumented shims and washers apart from the BOM is prohibited.

Alignment and acceptance

• Acceptance Criteria: The installed bracket must sit flush against its mounting surface with zero rocking. Mounting slots must not be bottomed out against one end. The clearance reveal to neighboring components must be uniform (e.g. maintaining a constant ± 0.5 mm gap).
• Keepout Zones: It must be confirmed that no bracket edge or lip will rub against a routing wire harness. Approved edge guards must be added if a cable route inherently passes close to a sharp sheet metal structure.
• Witness Marks: Witness marks must be applied using an approved paint pen across critical fasteners immediately after reaching final torque. This provides a visual confirmation that the screw was torqued and has not backed out due to vibration.

EMI shields and grounding continuity

Establish EMI bonds and shielding components early in the assembly flow to prevent paint flakes, adhesive, or gasket debris from compromising the low-ohm electrical path.

Shield mounting protocol

• Bond Lands: Grounding bond lands must be completely clean. Bare metal must be visible, with absolutely zero paint, powder coating, or adhesive residue in the conductive path.
• Seating: Shields must be seated squarely and passively. “Springing” or forcing a warped shield into place under tension is strictly prohibited, as this pre-loads the screws and increases the risk of vibration-induced failure.
• Fastener Pattern: Shields must be secured using a fastener tightening pattern that works from the shortest path to the longest path. This technique enforces 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 destroying the cellular structure.
• Fingerstock: Achieve uniform surface contact for beryllium copper or steel EMI fingerstock; never crush it completely flat.
• Continuity Check: Verify the final electrical integrity of the shield. Guideline: Seam-to-seam or shield-to-chassis resistance must measure < 0.1 Ω. Log a low-ohm meter measurement during the First Article Inspection (FAI).
• Defect Rejection: Reject the assembly if the EMI gasket exhibits 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 assembly process demanding absolute accuracy to guarantee heat conduction is not impeded by introduced thermal resistance from voids, debris, or excessive paste.

TIM choice and application

• Pads and Gap Fillers: Thermal pads must be placed using ESD-safe tweezers. Stretching the pad material must be avoided, as this alters its thickness and thermal properties. The final compression (e.g. 10% to 30%) must be verified against the material’s durometer rating. The protective liners must be peeled before mating.
• Pastes and Greases: A controlled, even film must be applied. A bead diameter gauge or a volume-controlling stencil must be utilized. Prohibited: Sliding or twisting the heat sink laterally over the paste to spread it, as this traps insulating air bubbles within the joint.
• Standard TIM Patterns:
  • Small Die (< 20 x 20 mm): Center pea or a very thin X pattern.
  • Long Bar or VRM Strip: Three thin, parallel lines along the length.
  • Large Plate: A cross-hatch pattern or a volume-controlled stencil to ensure even coverage (targeting 90% to 100% surface wetted area without massive overflow).

Clamping and torque sequence

• Torque Guideline: Exact torque must be applied to establish the compressive clamping force without cracking the silicon die.
  • Starter Torque Range (M3 hardware): Typically 0.6 to 1.0 Nm.
• Torque Sequence: A two-pass sequence must be utilized (first pass at 30–50% torque, second pass at 100% torque). A cross-pattern must always be used to prevent angular die damage and guarantee perfectly parallel contact.
• Squeeze-Out Verification: A minimal, highly uniform perimeter line of TIM squeeze-out at the mating edges is the acceptable target. Excessive paste overflow is prohibited; it acts as an insulator block and can flow into nearby sensitive connectors.

Stack height and gap fillers

• Gap Fillers: Actively compress these materials while in service to achieve target thermal conductivity. Compressible shims or thermal spacers must be strictly defined in the engineering BOM. Substituting standard metal washers ad hoc to adjust height is explicitly prohibited.
• Structural Board Check: The total accumulated spring force from multiple gap pads must not bow the PCB. If high compressive forces are required, utilize mid-board standoffs to prevent the printed circuit board from deforming under the thermal load.

Final Checkout: Brackets, shields and heat sinks

Parameter	Engineering Criteria	Verification Action
Grounding Continuity	Shielding and ground bond joints measure < 0.1 Ω to the main chassis ground.	A low-ohm meter audit is performed and officially logged on the first article assembly.
Thermal Clamping	Heat sink fasteners are secured using a definitive two-pass, cross-pattern torque sequence.	QA audit visually confirms the torque sequence and ensures fasteners are set exactly to the specified Nm.
TIM Application	Mating surfaces are clean. Paste is applied evenly without trapped air voids or external contamination.	Final inspection confirms a minimal, perfectly uniform paste squeeze-out around the full perimeter of the heat sink edges.
EMI Sealing	Conductive foam gaskets are visibly compressed by 20% to 30% with zero gaps or crushing.	Screw torque is verified to ensure it provides sufficient pressure without destroying the conductive cellular material.
Structural Alignment	All brackets sit flush. Alignment is verified with tooling jigs. No components are “sprung” or forced into place under tension.	A visual gauge check confirms a consistent reveal (typically ± 0.5 mm) relative to all neighboring structural parts.
Shield Cable Terminations	Harness braid shielding straps are secured using proper 360-degree clamps. Exposed pigtails are ≤ 10 mm in length.	The QA audit confirms the correct length and the prescribed mechanical method used for the shield termination.