2.4 Brackets, Shields & Heat Sinks

Brackets,The installation of structural and thermal management components—brackets, shields, and heat sinkssinks—is formcritical thefor structural,mechanical electrical,robustness, electromagnetic compliance (EMC), and thermal backbonestability. ofThis anprocess enclosure,requires anda theirspecific, installationcontrolled sequence decides(mechanical whether– theelectrical system– holds together cleanly or fights itself at every screw. Brackets define the geometry, locking in datums and clearances so that everything else aligns. Shields then establish low-resistance paths that keep electromagnetic noise under control, provided that conductive gaskets and bond lands are seated properly. Only after these foundations are secure can thermal hardware be installed, with TIMs and torque patterns tunedthermal) to spread heat without bending boards. Following this order ensures that mechanics, EMC, and cooling reinforce rather than undermine one another.

2.4.1 The idea (in one line)

Set mechanical datums first (brackets), close EMC paths second (shields/bonds), then lock thermal paths (heat sinks/TIMs)—in that order—soensure 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

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bracketry

2.4.2 Sequencing that avoids rework

1. Bracketry & frames: createestablishes the square, repeatable geometry that defines the restinternal depends on.
2. EMC bonds & shields: establish low-Ω paths before paint flakes or gaskets get trapped wrong.
3. Thermal hardware (heat sinks/spreaders/pipes): apply TIMslayout and torquecomponent once,clearances.
   with

   A) final alignment set.

4. Dress & verify: continuity (EMC), compression (seals),Positioning and tempsTightening (sanity).

If two parts compete for the same screw, the mechanical locator (bracket) wins the first pass; the shield or sink stacks under/over it per drawing with spacers/washers as designed.

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2.4.3 Brackets & frames (make the chassis true)

Purpose: set position, carry load, define clearances.

Sequence

• Datum-First Mandate: Dry fit first:the holesbracket align without force; slots centered when possible.
• Datum-first tightening:to seatconfirm alignment. Seat the bracket at the primary datum (pin/locator pin or fixed standoff), then work outward.
• PatternTorque &Pattern: Secure fasteners using a snug all – final torque: snug all → final torquesequence in a cross cross-pattern (23.2).
• Stack orderOrder:: The standard stack order is bracket →– washer (if spec)specified) →– chassis;chassis. avoidAvoid “creative”using shims.creative shims or washers not defined in the BOM.

B) Alignment and Acceptance

• KeepoutsAcceptance Cues:: confirm noThe bracket lipmust will rub a harness; add edge guards if route passes near.
• Witness marks on critical fasteners after torque.
  

Acceptance cues

• Bracket sitssit flush (no rocking); slots must not be hard against one end; and the reveal to neighborsneighboring 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.

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2.4.42 EMI Shields &and EMIGrounding gaskets (conductivity before cosmetics)Continuity

TypesEMI you’llbonds see:and fingerstock,shields conductivemust foam,be stampedestablished cans,early lidin bonds,the braidassembly straps.flow to prevent paint flakes or gasket debris from compromising the low-ohm path.

A) Shield Mounting Protocol

• Bond lands cleanLands: (23.1):Ground bond lands must be clean—bare metal visible, with no paint or powder residue in the path.
• Seat shields squareSeating: Shields must be seated squarely; noprohibited “springing”is "springing" the shield into placeplace, thatwhich preloads screws.
• Gasketthe compression:screws conductiveand foamsrisks wantvibration 20–30%; fingerstock wants contact, not flattening.failure.
• Fastener patternPattern:: short-to-longShields must be secured using a fastener pattern that works the shield from the short path to herdthe long path, ensuring even compression across the gasket;conductive finalgasket.

pass

B) atGasket specCompression torque.and Check

• Conductive Foams: Conductive foams require 20% – 30% compression for effective sealing.
• Fingerstock: Fingerstock requires uniform contact, but not crushing flat.
• Continuity checkCheck:: seam-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).
• Straps: 360° clamps on braid, or solder sleeves; pigtails ≤ 10 mmDefect: if unavoidable.

Reject is mandated if the gasket has gaps,gaps, is crushed flat,flat, or the bond Ωresistance is high.

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2.4.53 Heat sinksSinks &and spreadersThermal (the clamp that carries heat)Management

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

A) TIM choiceChoice &and handling

Application

• Pads/gapGap fillersFillers: (silicone/graphite/PCM):Pads easy,must sizedbe pieces;placed using tweezers; avoid stretching; compressverify 10–30% depending on durometer.
• Pastes/greases (thermal compound): thin, even film; bead size per kit; avoid pump-out paths.
• Phase-change films: place cool; activate/hot torque once; watch for reflow temperature in SWI.
• Insulating pads (mica/Kapton/ceramic): required where electrical isolation + conduction needed—don’t skip bushings on screws.

Mounting steps

1. Dry align the sink/spreader to confirm standoff heights.
2. Apply TIM: pads last-minute; paste with stencil/dispense (pattern = lines/X); keep off connectors.
3. Set sink straight down; no slide on paste if avoidable.
4. Torque in pattern: 30–50% pass → 100% pass; use spring screws if spec’d.
5. Squeeze-out sanity: a minimal, uniform line at edges is good; puddles are not.

Starter torque (validate on your joint)

• M2: 0.2–0.3 N·m; M2.5: 0.35–0.6 N·m; M3: 0.6–1.0 N·m (23.2.5).
• For spring-loaded posts, run to stop height or angle spec per vendor.
  

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2.4.6 Gap fillers & stack height (avoid bow and tilt)

• Pick thickness so compressed in service hits targetcompression (e.g., 20%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:If 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 in one stack, their total spring must not bow the PCB; adduse mid-standoffs if needed.
• Shims/spacersnecessary areto partprevent ofboard the BOM—never substitute washers ad hoc.deformation.

Quick

Final gauge: caliper pad pre vs post thickness or witness tape stack at first article.

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2.4.7 TIM “recipes” (patterns that work)Checklist

SurfaceMandate	PatternCriteria	WhyVerification Action
SmallGrounding die < 20×20 mm	Pea or thin XContinuity	Fast, avoids edge bleed
Long bar/VRM strip	Three parallel lines	Fills between components; reduces pump-out
Large plate	Cross-hatch or stencil	Controls volume; even coverage

Coverage target: 90–100% of mating area after clamp, with ≤1 mm squeeze-outShield and no voids over hotspots.

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2.4.8 Order-of-operations examples (two common builds)

A) RF lid with shield + sink on CPU

1. Install board brackets (standoffs/frame) → torque.
2. Place EMI gasket around RF bay → drop RF lid → torque; verify <0.1 Ω.
3. Apply CPU paste → mount CPU sink with cross torque; confirm minimal squeeze-out.
4. Fit duct/air guide and fan bracket; spin test.

B) Power tray with spreader + braid bond

1. Mount tray bracket to chassis; earth lug with serrated washer → <0.1 Ω.
2. Lay gap filler pads on MOSFET line; place spreader; spring-screw torque in pattern.
3. Install braid strap 360° at gland; bond tojoints tray; verify Ω.
4. Close top shield over supplies; check clearances to harness.

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2.4.9 Quick verifications (cheap, powerful)

• Board bow near sink: ≤ 0.5 mm across card; visual straightedge.
• EMC seam Ω:verified < 0.1 Ω across shield joints; record first article.
• TIM witness: small, even bead around sink; peel-check one NPI unit to confirmchassis spread pattern.
• Fan/air path: no interference from brackets or shields; airflow arrow respected.
• Screw head height: all seated; witness marks drawn.
  

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2.4.10 Acceptance cues (what good looks like)

Item	Accept	Reject
Bracket fitground.	Flush,Low-ohm nometer rock;audit centeredlogged inon slots	Forcingfirst to align; slot maxed at one endarticle.
Thermal Clamp	Shield/gasketHeat sink fasteners secured using a cross-pattern torque sequence.	ContinuousAudit contact,confirms cornerstwo-pass tight	Gaps,torque crushedsequence foam,applied; peelingfasteners fingerstockset to spec Nm.
BondTIM Application	Surfaces clean; paste applied evenly; <0.1no Ωvoids or contamination seam/strap.	PaintInspection underconfirms washer;minimal, highuniform Ωsqueeze-out at sink edges.
EMI Seal	HeatGaskets sink(conductive foam) compressed 20% – 30%; no gaps or crushing.	Even clamp; patternFastener torque done	Oneverified cornerto high;prevent skewcrushing fromthe slidingconductive material.
Structural Alignment	TIMBrackets sit flush; alignment verified with jigs; no "springing" components into place.	EvenVisual thincheck spread;confirms tinyconsistent edgereveal bead(	Dry± spots;0.5 pumpsmm) ontoto connectorsneighboring parts.
Fatigue Prevention	PCBHarness stressbraid straps secured with 360˚ clamps; pigtails ≤ 10 mm.	NoAudit visibleconfirms bow/whine	Bowedcorrect board;length fan/pipeand foulsmethod partsused for shield termination.

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2.4.11 Common traps → smallest reliable fix

Trap	Symptom	First move
Sink used to “straighten” a warped board	Intermittents, cracked vias	Fix standoff heights; add mid support; then mount
Over-compressed foam gasket under lid	IP fails later, bent lid	Torque in two passes; verify 25–35% compression
Paint under star washer	High earth Ω	Scrape to bare, re-torque, measure
Too-thick gap filler	PCB bow, poor screw start	Drop one thickness grade; confirm compression target
Sliding sink on paste	Voids, misaligned holes	Place straight down; use alignment pins; minimal reposition
Shared screw stack in wrong order	Rattles, poor EMC/thermal	Follow BOM stack; bracket → shield → sink only if drawing says so
Spring screws torqued like rigid	Uneven pressure	Tighten to stop/angle spec; not N·m guess

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2.4.12 Pocket checklists

Before install

• Bracket holes free; standoffs at height; edges safe
• Bond pads clean; gaskets cut to length; straps ready
• TIM kit (pads/paste) at station; torque bits staged

Brackets & shields

• Brackets datum-seated; cross-pattern torque; witness marks
• Gasket in place; shield seated; seam <0.1 Ω measured

Heat sinks/spreaders

• Pad/paste applied to recipe; no smear on connectors
• Cross-pattern torque in two passes; spring screws by spec
• Board bow within limit; small uniform squeeze-out

Final checks

• Air paths clear; fans spin; no rubs
• Harness clears edges; first clamp before first bend
• Log Ω, torque tool IDs, and any pad thickness calls
  

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By sequencing carefully and verifying fit, continuity, and compression, assemblies avoid warped boards, leaky shields, and overheated components. The result is hardware that stays square, quiet, and thermally stable, passing tests with fewer surprises and reworks.