Lesson 4/1331%
MODULE 04 — FUNDAMENTALS

Heatsink Fundamentals

Fin efficiency, fin density, base thickness, and aspect ratio determine how well a heatsink actually uses the material it's built from. This module builds the intuition and math behind heatsink geometry, then compares manufacturing methods.

Learning Objectives

Explain fin efficiency and calculate it from geometry
Understand fin density (FPI) trade-offs
Size base thickness for adequate heat spreading
Explain fin aspect ratio limits
Compare extrusion, skiving, bonded-fin, and stamping
Select a manufacturing method for a given design

A heat sink transfers thermal energy from a hot device to a cooler fluid (usually air) by increasing the surface area in contact with the cooling medium. A typical plate-fin heatsink has a base plate that spreads heat laterally and a fin array that exposes that heat to airflow.

Base Plate
Spreads heat from source to fins
Fin Array
Increases surface area exposed to air
Mounting Interface
Contacts device via TIM

A fin is not uniformly at the base temperature — the tip is cooler than the root. Fin efficiency quantifies how well the fin conducts heat relative to an ideal, fully isothermal fin.

ηfin = tanh(mL) / (mL)    where   m = √(2h / k·t)

h = convection coeff. (W/m²K) · k = fin conductivity (W/mK) · t = fin thickness (m) · L = fin height (m)

Guideline: Above 90% efficiency, fin material is well used. Below 60%, the outer portion of the fin contributes little — consider shorter fins. Aluminum (k = 200 W/mK) fins stay efficient up to ~50mm height in natural convection; copper (k = 390 W/mK) permits taller fins but adds weight and cost.

Fin Efficiency Calculator

Parameter m
—
m × L
—
Fin Efficiency η
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Assessment
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Fin Efficiency vs Fin Height

Fin density (FPI) is the number of fins per unit width. It balances surface area against airflow resistance.

Higher Fin Density
  • More surface area
  • Higher pressure drop
  • Better for forced convection
  • Prone to dust clogging
  • Typical: 10-20 FPI
Lower Fin Density
  • Less surface area
  • Lower pressure drop
  • Better for natural convection
  • Less dust accumulation
  • Typical: 4-8 FPI
Optimal spacing: sopt ≈ 2.71 × L / RaL0.25. For natural convection at sea level with 30mm fins, optimal spacing is typically 6-10mm (3-5 FPI). Going denser than optimal actually reduces total heat transfer because boundary layers overlap and choke airflow.

The base plate spreads heat laterally from the source to the fins. Too thin and heat concentrates under the source, leaving outer fins cold; too thick and it adds unnecessary weight and cost.

Rule of thumb: Base thickness should be roughly 1/4 to 1/3 of the heat source width, with a practical minimum of 3mm for extruded heatsinks. Localized heat sources benefit from thicker bases (5-10mm) for better spreading.
Spreading resistance: When the heat source is smaller than the base plate, an additional resistance Rsp ≈ 1 / (√π × k × as) adds to the thermal path, where as is the equivalent source radius. Copper base plates or vapor chambers dramatically reduce spreading resistance for small, high-power sources.

Aspect ratio (fin height / fin spacing) significantly affects both performance and manufacturability.

AR = Hfin / s
Low AR (< 3:1)
  • Easy to manufacture (extrusion)
  • Good airflow penetration
  • Lower thermal performance
  • Standard commodity heatsinks
High AR (> 8:1)
  • Difficult to extrude (limit ~10:1)
  • Requires skiving or bonding
  • Higher thermal performance
  • Risk of airflow bypass

Maximum Achievable Aspect Ratio by Method

Different manufacturing methods trade cost, performance, and geometric freedom differently.

Parameter Extruded Skived Bonded Fin Stamped
Relative Cost Low ($) Medium-High ($$$) Medium ($$) Very Low ($)
Tooling Cost $2k-$10k Low $500-$3k $5k-$50k
Max Aspect Ratio 8:1-10:1 20:1-40:1 ~60:1 2:1-4:1
Min Fin Thickness 1.0-1.5mm 0.2-0.5mm 0.1-0.3mm 0.5-1.0mm
Typical Material Al 6063 Cu or Al Cu fins + Al base Al or Steel
Thermal Performance Good Excellent Excellent Low-Moderate
Best For General purpose High-density server/telecom Custom high-performance Consumer/LED
Choosing a method: Start with extrusion for most projects up to ~100W with forced air. Move to skived or bonded fin when required RSA < 0.5 °C/W in a compact footprint, aspect ratio > 8:1 is needed, or copper fins are required for spreading. Use stamped heatsinks for very high volumes (>100k units/yr), low power (<5W), or combined shielding/heatsink parts.

Cost vs Thermal Performance

KNOWLEDGE CHECK

Module 4 Quiz

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