2.4 Brackets, Shields & Heat Sinks

Brackets, shields, and heat sinks form the structural, electrical, and thermal backbone of an enclosure, and their installation sequence decides whether the 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 tuned 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—so nothing fights the next step.

2.4.2 Sequencing that avoids rework

Bracketry & frames: create the square, repeatable geometry the rest depends on.
EMC bonds & shields: establish low-Ω paths before paint flakes or gaskets get trapped wrong.
Thermal hardware (heat sinks/spreaders/pipes): apply TIMs and torque once, with final alignment set.
Dress & verify: continuity (EMC), compression (seals), and temps (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.

2.4.3 Brackets & frames (make the chassis true)

Purpose: set position, carry load, define clearances.

Dry fit first: holes align without force; slots centered when possible.
Datum-first tightening: seat the bracket at the primary datum (pin/standoff), then work outward.
Pattern & torque: snug all → final torque in cross pattern (23.2).
Stack order: bracket → washer (if spec) → chassis; avoid “creative” shims.
Keepouts: confirm no bracket lip will rub a harness; add edge guards if route passes near.
Witness marks on critical fasteners after torque.

Acceptance cues

Bracket sits flush (no rocking); slots not hard against one end; reveal to neighbors consistent ±0.5 mm.

2.4.4 Shields & EMI gaskets (conductivity before cosmetics)

Types you’ll see: fingerstock, conductive foam, stamped cans, lid bonds, braid straps.

Bond lands clean (23.1): bare metal visible, no powder in path.
Seat shields square; no “springing” into place that preloads screws.
Gasket compression: conductive foams want 20–30%; fingerstock wants contact, not flattening.
Fastener pattern: short-to-long path to herd the gasket; final pass at spec torque.
Continuity check: seam-to-seam or shield-to-chassis < 0.1 Ω (log on first article).
Straps: 360° clamps on braid, or solder sleeves; pigtails ≤ 10 mm if unavoidable.

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

2.4.5 Heat sinks & spreaders (the clamp that carries heat)

TIM choice & handling

Pads/gap fillers (silicone/graphite/PCM): easy, sized pieces; avoid stretching; compress 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

Dry align the sink/spreader to confirm standoff heights.
Apply TIM: pads last-minute; paste with stencil/dispense (pattern = lines/X); keep off connectors.
Set sink straight down; no slide on paste if avoidable.
Torque in pattern: 30–50% pass → 100% pass; use spring screws if spec’d.
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.

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

Pick thickness so compressed in service hits target (e.g., 20%).
If multiple pads in one stack, their total spring must not bow the PCB; add mid-standoffs if needed.
Shims/spacers are part of the BOM—never substitute washers ad hoc.

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

2.4.7 TIM “recipes” (patterns that work)

Surface	Pattern	Why
Small die < 20×20 mm	Pea or thin X	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-out and no voids over hotspots.

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

A) RF lid with shield + sink on CPU

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

B) Power tray with spreader + braid bond

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

2.4.9 Quick verifications (cheap, powerful)

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

2.4.10 Acceptance cues (what good looks like)

Item	Accept	Reject
Bracket fit	Flush, no rock; centered in slots	Forcing to align; slot maxed at one end
Shield/gasket	Continuous contact, corners tight	Gaps, crushed foam, peeling fingerstock
Bond	<0.1 Ω seam/strap	Paint under washer; high Ω
Heat sink	Even clamp; pattern torque done	One corner high; skew from sliding
TIM	Even thin spread; tiny edge bead	Dry spots; pumps onto connectors
PCB stress	No visible bow/whine	Bowed board; fan/pipe fouls parts

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

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

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.