Hardware

Finding the Perfect Fit: Board Outline Iteration in Constrained Spaces

Vinayak M K

16 Dec 2025 — 7 min read

Introduction: Where Electrical Meets Mechanical

To the untrained eye, it is merely a rectangle—a boundary line. But to us, it is the negotiated truce between electrical ambition and mechanical reality. This outline is not drawn at a whim; it is forged through a rigorous collision of constraints. Component density demands real estate, while enclosure envelopes enforce rigid boundaries. Mounting points anchor the design, while connector placement dictates the flow of the entire harness ecosystem.

For our vBus modular products, this optimization is not just a step—it is the strategy. These modules must inhabit compact enclosures and interface seamlessly through standardized vBus connectors, all while maintaining superior thermal performance. Achieving this balance requires more than just drafting; it demands structured iteration.

Today, I want to share our methodology—the systematic process that transforms rough dimensional estimates into precision-engineered, production-ready architectures.

Initial Constraints: The Design Envelope

Enclosure-Driven Dimensions

Board outline iteration starts with the enclosure:

Maximum PCB Size: Enclosure internal dimensions minus clearances:

Typical clearance: 2-3mm per side (tolerance stack-up, assembly alignment)
Example: 100mm × 80mm enclosure interior → ~95mm × 75mm max PCB

Height Restrictions: Vertical space limits component selection:

Enclosure internal height minus connector protrusion
Typical consumer products: 10-15mm available height
Determines component packages: QFN over DIP, SMD over through-hole

Wall Interference Zones: Enclosure features constrain PCB shape:

Boss posts for screw mounting
Internal ribs for structural strength
Component clearances (avoid interference with enclosure details)

Keep-Out Definition: Early mechanical CAD defines prohibited zones:

Import enclosure 3D model into PCB tool
Overlay PCB layout area
Identify interference regions
Define initial keep-out boundaries

Mounting Point Requirements

Screw Hole Positions: Dictated by enclosure mounting bosses:

Hole diameter: Typically 3.2mm for M3 screws
Clearance around hole: 6-8mm diameter copper-free zone
Position tolerance: ±0.5mm relative to board edges

Mounting Hole Quantity: Balance mechanical stability with board area:

Minimum: 3 holes (triangular pattern—prevents rotation)
Typical: 4 holes (corners—maximizes stability)
Large boards (>100mm): 6+ holes (prevents board flexing)

Standoff Height: Vertical spacing between PCB and enclosure bottom:

Determines bottom-side component clearance
Affects thermal contact with enclosure
Typical: 3-5mm standoffs

Connector Placement Constraints

vBus Connector Position: Standardized location for ecosystem compatibility:

Edge-mounted on specific board edge
Height above bottom surface: Fixed by mating connector in enclosure
Alignment tolerance: ±0.2mm (tight for reliable connection)

External Connectors: User-facing ports aligned with enclosure openings:

USB port aligned with enclosure cutout
Power jack centered in opening
Tolerance: ±0.5mm (accounts for connector and enclosure tolerances)

Connector Accessibility: Space around connectors for:

Mating connector insertion angle
Cable bend radius
Strain relief clearance

Electrical Requirements Drive Area

Component Footprint Analysis

Mandatory Components: Fixed area requirements:

Main processor: 10mm × 10mm (QFN64) to 14mm × 14mm (LQFP100)
Power regulators: 3-5 per board, each 5mm × 5mm typical
Passives: Hundreds of 0402/0603 components

Initial Area Estimate:

Sum component footprints
Add 40-60% for routing space (traces, vias, clearances)
Result: Minimum theoretical board area

Example Calculation:
Major ICs: 200 mm²
Power components: 80 mm²
Passives: 150 mm²
Connectors: 100 mm²
─────────────────────
Component total: 530 mm²
Routing factor: ×1.5 (50% overhead)
Estimated area: 795 mm²
→ Approximate dimensions: 35mm × 23mm

Routing Density Considerations

Trace Width and Spacing: Affects achievable density:

Fine-pitch routing: 6 mil trace/space enables tighter packing
Standard routing: 8-10 mil trace/space needs more area
Power traces: Wider traces (20+ mil) require dedicated routing channels

Via Count: High via density indicates complex routing:

Blind/buried vias: Enable denser routing (but increase cost)
Through vias: Simpler manufacturing, need more space

Signal Integrity: High-speed signals constrain layout:

Differential pairs require matched length (serpentine routing consumes area)
Clock signals need isolation (spacing from other traces)

Iteration Cycle: Refining the Outline

Iteration 1: Initial Layout Attempt

Starting Point: Maximum available area from enclosure:

Use full enclosure interior dimensions
Place mounting holes at corners
Position connectors at designated edges

Component Placement:

Critical components first (processor, power regulators)
High-pin-count components near connectors
Passives grouped near associated ICs

Routing Attempt:

Route critical nets (power, clocks, high-speed data)
Route general signals
Assess routing completion: 70-80% typical on first attempt

Outcome: Identify problematic areas:

Congested regions (too many signals competing for space)
Unreachable pins (routing blocked by other components)
Thermal hotspots (components too close together)

Mechanical Feedback Integration:

Mechanical team reviews initial layout, identifies issues:

Mounting Hole Conflicts:

"Hole at corner interferes with enclosure boss—move 2mm inboard"
Adjust board outline to accommodate boss position
Re-center mounting hole in new location

Component Height Issues:

"Connector exceeds available height—switch to lower-profile variant"
Select alternate connector (2mm lower profile)
Update layout with new footprint

Harness Routing Concerns:

"Cable clearance insufficient—need 8mm radius bend"
Add keep-out zone around connector (8mm radius)
Relocate nearby components outside keep-out

Board Outline Adjustments:

Trim corners where components don't reach (saves material cost)
Extend specific edges where routing congestion identified
Result: Irregular outline optimized for actual component distribution

Iteration 3: Harness Integration Validation

3D Harness Routing Simulation:

Mechanical CAD includes cable assemblies:

Connector-to-Connector Paths: Model actual harness routing:

vBus connector to external ports
Power input to power distribution
Sensor connectors to peripheral interfaces

Bend Radius Verification: Cables follow minimum bend radius:

Minimum radius: Typically 6-10× cable diameter
Tighter bends damage cable over time
Insufficient radius indicates need for board outline adjustment

Clearance Verification: Harnesses don't interfere with:

Other harnesses (avoid bundled cables crossing)
Enclosure internal features (ribs, supports)
Heat-generating components (prevent cable insulation degradation)

Identified Issues:

Connector Too Close to Corner:

Harness bend radius requires 12mm clearance
Only 8mm available with current board position
Solution: Shift board 4mm away from corner

Harness Crosses Hot Zone:

Power regulator generates 5W heat
Harness routed directly above (thermal stress)
Solution: Move regulator to opposite board side, or reroute harness

Connector Orientation Wrong:

Harness exit direction conflicts with enclosure geometry
Cable would need unrealistic 90° bend in constrained space
Solution: Rotate connector 180° (requires PCB routing changes)

Iteration 4: Manufacturing Optimization

DFM Feedback from Fabrication Partner:

PCB manufacturer reviews outline for manufacturability:

Outline Complexity: Non-rectangular outlines increase cost:

Straight edges: Standard routing operation
Curved edges: Requires multiple tool passes
Internal cutouts: Additional routing steps
Optimization: Simplify curves to straight segments where functionally acceptable

Panel Utilization: Maximize PCBs per manufacturing panel:

Standard panel: 18" × 24" (457mm × 610mm)
Board dimensions affect how many fit per panel
Example: 50mm × 40mm fits 60 per panel; 52mm × 41mm fits only 45
Optimization: Trim 2mm from non-critical edges → 25% more boards per panel

Mounting Hole Position: Ensure compatibility with test fixtures:

Test fixture uses standard hole patterns for board registration
Non-standard hole positions require custom fixture (cost and lead time)
Optimization: Shift holes to align with standard fixture grid

Board Edge Clearance: Components near board edge risk damage during handling:

Minimum component-to-edge: 3-5mm recommended
Edge-mounted connectors exempt (intentionally at edge)
Optimization: Pull components inward or extend board edge slightly

Iteration 5: Thermal Validation

Thermal Simulation with Actual Outline:

Run thermal FEA with final board shape and component positions:

Thermal Distribution: Identify hotspots:

Processor temperature: 75°C (within limits)
Power regulator: 82°C (acceptable but high)
Ambient to hotspot gradient: 35°C

Thermal Path Analysis: Heat flow to enclosure:

Processor has thermal pad to enclosure (good—effective cooling)
Power regulator isolated from enclosure (poor—limited cooling)

Outline Adjustment for Thermal:

Extend board edge 3mm to position regulator near enclosure wall
Add thermal pad contact point to enclosure
Result: Regulator temperature drops to 70°C

Thermal Via Density: Ensure adequate heat transfer through PCB:

High-power components require dense via arrays
Board area sufficient for 30+ thermal vias under each hot component

Final Outline: The Optimized Solution

Dimensional Summary

Final Board Dimensions: After 5 iterations:

Overall size: 48mm × 38mm (vs. initial estimate 35mm × 23mm)
Area increase: +56% over initial estimate (routing complexity underestimated initially)
Irregular outline: Three corners chamfered (avoids enclosure bosses)
Two mounting holes: 3.2mm diameter, positioned per mechanical requirements

Component Placement Results:

All mandatory components are placed
Thermal hotspots are managed within limits
Routing completion: 100% (all nets successfully routed)

Connector Positions:

vBus connector: Edge-mounted, aligned with mating connector in enclosure (±0.1mm achieved)
USB port: Centred in enclosure cutout (±0.3mm tolerance)
Power input: Positioned for optimal harness routing

Key Learnings Captured

Routing Density Reality: Initial estimates typically underestimate space needs:

Rule of thumb refined: Add 60-80% routing overhead for dense designs (not 40-60%)
High-speed signals (USB, Ethernet) require even more overhead (differential pairs, length matching)

Mechanical Coordination Critical: Early and frequent mechanical review prevents late changes:

Review after initial placement (before routing begins)
Review after routing completion (before finalising)
3D interference checking mandatory (catches issues 2D review misses)

Harness Routing Drives Outline: Cable bend radius is non-negotiable:

Insufficient clearance identified late requires an expensive redesign
Simulate harness routing in 3D CAD before finalising PCB outline

Panel Efficiency Matters: Small dimensional changes have large cost impacts:

2-3mm reduction in non-critical dimension can increase the panel yield 20-30%
Always ask: "Can this be 1mm smaller without compromising function?"

Design Process Integration

Outline Review Gates

Formal review checkpoints prevent late surprises:

Gate 1 - Initial Outline (Pre-Layout):

Participants: Electrical, Mechanical, Manufacturing
Deliverables: Enclosure CAD, mounting requirements, and connector positions
Approval criteria: All mechanical constraints documented, initial outline agreed

Gate 2 - Post-Placement (Pre-Routing):

Participants: Electrical lead, Mechanical lead
Deliverables: Component placement screenshot, 3D assembly view
Approval criteria: No mechanical interferences, thermal paths viable

Gate 3 - Post-Routing (Pre-Release):

Participants: Full cross-functional team
Deliverables: Complete layout, 3D assembly with harnesses, DFM report
Approval criteria: All routing complete, all clearances verified, manufacturing feedback addressed

Documentation Standards

Board Outline Drawing: Mechanical drawing specifies:

Dimensions with tolerances (±0.2mm typical)
Mounting hole positions and diameters
Critical component positions (connectors, thermal pads)
Keep-out zones and restricted areas
Reference to the enclosure CAD file

Revision Control: Outline changes tracked rigorously:

Major revision (A→B): Dimensional changes affecting enclosure or mounting
Minor revision (B1→B2): Non-critical changes (internal component shifts)

Conclusion: Iteration Enables Excellence

Board outline optimization isn't a single design decision—it's a collaborative, iterative process where electrical needs, mechanical constraints, harness routing realities, and manufacturing efficiency converge into an optimized solution. Each iteration refines the shape, each review catches issues earlier, and each lesson learned informs future designs.

At Hoomanely, we've embraced this structured iteration as an essential discipline. Our vBus modules achieve remarkable density and functionality in compact form factors not through heroic design effort, but through systematic refinement where multiple perspectives shape the final result. The board outline that emerges isn't a compromise—it's an optimized synthesis that delivers electrical performance, mechanical fit, harness accessibility, and manufacturing efficiency simultaneously.

For tech enthusiasts developing their own designs: embrace iteration. Your first board outline won't be optimal—and that's expected. But through systematic review, cross-functional collaboration, and willingness to adjust based on feedback, you'll converge on designs that fit perfectly, work reliably, and manufacture efficiently. That's the difference between designs that work and designs that excel.