Extruded vs Bonded Fin vs Skived Heat Sinks: Cost and Performance Comparison

Understanding how heatsinks are made reveals their inherent capabilities and limitations.

Extruded Heatsinks:

Process:

1. Aluminum billet heated to ~500°C
2. Forced through shaped die under high pressure
3. Emerges as continuous profile
4. Cut to length, machined as needed

Characteristics:

• Single-piece construction
• No joints or interfaces
• Limited by die capabilities
• Economical for standard profiles

Typical materials:

• 6063-T5 aluminum (most common)
• 6061-T6 (higher strength)
• Rarely copper (difficult to extrude)

Achievable geometries:

• Fin height: Up to 75mm
• Fin thickness: 0.8mm minimum
• Fin spacing: 3mm minimum
• Aspect ratio (height/thickness): Up to 20:1

Bonded Fin Heatsinks:

Process:

1. Base plate manufactured (cast, machined, or extruded)
2. Individual fins fabricated (stamped, machined)
3. Fins attached to base (brazing, epoxy, or friction welding)
4. Finish machining if needed

Characteristics:

• Two-piece construction (base + fins)
• Interface between fins and base
• Greater design flexibility
• Higher cost than extrusion

Attachment methods:

• Brazing: Best thermal, most expensive
• Epoxy: Economical, lower thermal performance
• Press-fit: Good thermal, production challenges
• Swaging: Deformation locks fins in base

Achievable geometries:

• Fin height: Virtually unlimited (>150mm)
• Fin thickness: 0.3mm possible
• Fin spacing: 1mm possible
• Aspect ratio: 50:1 or higher

Performance differences stem from geometric capabilities and construction method.

Base Spreading:

Extruded:

• Base is monolithic with fins
• No interface resistance in base
• Thermal conductivity: ~200 W/mK uniform

Bonded:

• Base can be copper for better spreading
• Copper base: ~390 W/mK
• Useful for small, concentrated heat sources

Fin Efficiency:

Higher aspect ratio fins (bonded advantage):

• More surface area per footprint
• Thinner fins possible (0.3mm vs 0.8mm)
• More fins per unit width

But diminishing returns:

• Very thin fins have lower efficiency
• Optimal thickness depends on airflow

Interface Resistance:

Extruded: None Bonded:

• Brazed: 0.01-0.02°C/W per cm²
• Epoxy: 0.1-0.2°C/W per cm²

For high-power applications, interface adds measurable resistance.

Comparative Performance:

Same footprint, same airflow:

• Optimized bonded fin: 10-30% better than extrusion
• Improvement from higher fin density
• But interface partially offsets gains

Real-world examples (50×50mm, 2 m/s airflow):

Extruded (10 fins, 3mm spacing, 1mm thick, 40mm tall):

• Rth ≈ 1.2°C/W

Bonded (20 fins, 1.5mm spacing, 0.5mm thick, 40mm tall):

• Rth ≈ 0.9°C/W (brazed)
• Rth ≈ 1.0°C/W (epoxy)

~20% improvement with brazed bonded-fin construction.

Cost structure differs significantly between manufacturing methods.

Tooling Costs:

Extrusion die:

• Simple profiles: $2,000-5,000
• Complex profiles: $5,000-15,000
• Amortized over high volume

Bonded fin tooling:

• Stamping die for fins: $5,000-20,000
• Base machining fixtures: $1,000-5,000
• Higher total tooling investment

Per-Unit Costs:

Extruded (commodity profile):

• Material: $5-15/kg
• Processing: Minimal
• Example: 100×100×25mm → $5-10

Custom extruded:

• Similar material cost
• Tooling amortization adds $1-5
• Example: 100×100×25mm → $7-15

Bonded fin:

• Material: Higher (more fins)
• Assembly: Significant labor/process
• Example: 100×100×25mm → $20-50

Cost Comparison (100×100×40mm heatsink):

| Method | Qty 100 | Qty 1000 | Qty 10000 | |--------|---------|----------|-----------| | Stock extrusion | $8 | $6 | $4 | | Custom extrusion | $15 | $8 | $5 | | Bonded (epoxy) | $40 | $25 | $18 | | Bonded (brazed) | $60 | $35 | $25 |

Bonded fins cost 3-5× more than extrusion.

Break-Even Analysis:

When does bonded-fin justify cost premium?

• 20% thermal improvement worth 3× cost?
• Consider: Size reduction, smaller enclosure
• Or: Power increase without redesign
• Or: Premium product differentiation

Generally justified when:

• Space is at premium
• Maximum performance required
• Cost is secondary concern

Design constraints differ significantly between methods.

Extruded Heatsink Constraints:

Profile limitations:

• Constant cross-section along length
• Limited fin aspect ratio (~20:1)
• Minimum feature sizes (0.8mm fins, 3mm gaps)
• Die-imposed constraints

Length flexibility:

• Cut to any length from stock
• No additional tooling for length changes
• Secondary machining for features

Width constraints:

• Maximum width depends on press size
• Typically up to 300mm
• Wider requires multiple sections

Height limits:

• Maximum ~75mm for high aspect ratio
• Can go taller with lower aspect ratio
• Collapse risk limits fin height

Bonded Fin Flexibility:

Geometric freedom:

• Non-uniform fin distribution possible
• Variable fin height across heatsink
• Custom base thickness profiles
• Copper base, aluminum fins (hybrid)

Size capability:

• Virtually unlimited dimensions
• Very tall fins possible (>150mm)
• Wide heatsinks straightforward
• Modular construction for huge assemblies

Complex features:

• Integrated mounting features
• Multiple fin patterns on one base
• Heat pipes embedded in base
• Cross-flow optimization

Design Examples:

Application: High-power IGBT module

• Heat source: 60×120mm
• Power: 500W

Extruded approach:

• 150×150×60mm profile
• 12 fins, 50mm tall
• Rth ≈ 0.25°C/W

Bonded approach:

• 140×140×80mm
• 25 fins, 70mm tall
• Rth ≈ 0.18°C/W
• 30% better, slightly smaller footprint

Matching technology to application requirements optimizes cost and performance.

Use Extruded Heatsinks When:

Standard performance adequate:

• Thermal resistance target achievable
• Space available for larger heatsink
• Cost optimization priority

High volume production:

• Tooling cost amortized
• Per-unit cost advantage
• Standard profiles reduce lead time

Simple geometry sufficient:

• Rectangular footprint
• Uniform heat distribution
• Standard mounting

Quick turnaround needed:

• Stock profiles available
• Custom dies take 4-8 weeks
• Cut-to-length from stock

Use Bonded Fin Heatsinks When:

Maximum performance required:

• Every fraction of °C/W matters
• Cannot increase heatsink size
• Premium application justifies cost

Space constrained:

• Need taller fins than extrusion allows
• Higher fin density required
• Compact form factor mandatory

Special requirements:

• Non-uniform fin layout
• Copper base needed
• Integrated heat pipes
• Complex mounting features

Low to medium volume:

• Tooling cost less prohibitive
• Per-unit cost acceptable
• Custom design worthwhile

Special Considerations:

Natural convection:

• Extrusion adequate (wide spacing works)
• Bonded fin overkill for most applications

Forced air (high velocity):

• Bonded fin advantages more pronounced
• Tighter fin spacing beneficial
• Worth cost premium for performance

Cross-flow applications:

• Bonded fin can optimize for cross-flow
• Extrusion requires external ducting

Hybrid solutions:

• Extruded base with bonded fins
• Copper inserts in aluminum extrusion
• Combine advantages

A systematic approach ensures optimal heatsink selection.

Step 1: Define Requirements

Thermal:

• Power dissipation (W)
• Maximum junction temperature (°C)
• Ambient temperature (°C)
• Airflow conditions

Mechanical:

• Available envelope (L×W×H)
• Weight limit (kg)
• Mounting method

Commercial:

• Target cost ($)
• Volume (units/year)
• Lead time requirements

Step 2: Calculate Thermal Requirement

Required Rth = (Tj_max - Ta) / P

Example:

• Tj_max = 125°C
• Ta = 50°C
• P = 200W
• Rth_required = 0.375°C/W

Step 3: Check Standard Extrusions

Search supplier catalogs:

• Filter by footprint and height
• Compare Rth at your airflow
• Note: Catalog values are often optimistic

If standard extrusion meets needs: → Select and proceed (lowest cost)

Step 4: Evaluate Custom Extrusion

If standard insufficient:

• Can custom profile achieve Rth?
• Account for tooling cost/time
• Consider volume (amortization)

Custom extrusion viable if:

• Volume > 500-1000 units
• Tooling lead time acceptable
• Geometry achievable in extrusion

Step 5: Consider Bonded Fin

If extrusion cannot meet requirements:

• Request bonded-fin quotes
• Compare total cost (including development)
• Verify lead time

Bonded fin when:

• Extrusion geometrically limited
• Performance gap significant
• Cost premium acceptable

Quick Reference:

| Requirement | First Choice | Alternative | |-------------|--------------|-------------| | Lowest cost | Stock extrusion | Custom extrusion | | Fastest delivery | Stock extrusion | Bonded from standard parts | | Maximum performance | Bonded (brazed) | Custom extrusion | | Copper base needed | Bonded | Hybrid | | Very tall fins | Bonded | Stacked extrusions | | High volume | Custom extrusion | Bonded | | Low volume | Stock extrusion | Bonded |