How to Design a Cold Plate for Liquid Cooling Systems

Liquid cooling adds complexity and cost. Use it when the benefits justify the investment.

Liquid Cooling is Justified When:

Power Density > 30-50 W/cm²: Air cooling becomes impractical at high power densities. Heat sinks grow unreasonably large or require excessive airflow.

Ambient Temperature > 40°C: High ambient reduces the temperature differential available for heat rejection. Liquid cooling maintains capacity at elevated ambient.

Sealed Enclosure Required: IP65+ enclosures prevent ventilation. Liquid cooling moves heat to external radiator or heat exchanger.

Acoustic Constraints: Liquid cooling can achieve equivalent thermal performance with much lower noise than air cooling with high-velocity fans.

Multi-Device Systems: Liquid loops can cool multiple components efficiently, simplifying thermal management.

Typical Performance Comparison:

Natural convection heatsink: 1-5 W/cm² Forced air heatsink: 5-20 W/cm² Liquid cold plate: 50-200+ W/cm²

Total Cost of Ownership:

Initial cost higher for liquid, but consider:

• Reduced heatsink size/weight
• Improved reliability (no fans)
• Higher power density = smaller enclosure
• Potential for waste heat recovery

Cold plates remove heat through forced convection of coolant through internal channels.

Key Performance Equation:

Q = ṁ × Cp × ΔT

Where:

• Q = Heat removed (W)
• ṁ = Mass flow rate (kg/s)
• Cp = Specific heat (J/kg·K)
• ΔT = Coolant temperature rise (°C)

Thermal Resistance Components:

Rth_total = Rth_spreading + Rth_conduction + Rth_convection

Rth_spreading: Heat spreads from source to channels Rth_conduction: Through cold plate material Rth_convection: From channel wall to coolant

Flow Rate Considerations:

Higher flow = Lower Rth (more turbulent, better convection) But: Higher flow = Higher pressure drop = Larger pump

Typical flow rates:

• Small cold plates: 0.5-2 LPM
• Power module cold plates: 2-8 LPM
• Large systems: 10-30 LPM

Pressure Drop:

ΔP ∝ L × v² / Dh

Where:

• L = Channel length
• v = Flow velocity
• Dh = Hydraulic diameter

Design target: 0.5-2 bar pressure drop Higher ΔP = Better cooling but requires larger pump

Coolant Selection:

Water:

• Best heat transfer properties
• Cp = 4180 J/kg·K
• Requires corrosion inhibitors
• Freeze protection needed below 0°C

Water-Glycol (50/50):

• Good freeze protection to -35°C
• Cp = 3300 J/kg·K (20% lower than water)
• Standard for automotive/outdoor

Dielectric Fluids (PAO, Fluorinert):

• Direct contact with electronics possible
• Much lower heat transfer (Cp = 1000-2000 J/kg·K)
• Required for immersion cooling

Channel geometry significantly impacts both thermal performance and manufacturability.

Parallel Channels:

Description: Straight channels running parallel across cold plate Advantages:

• Simple to manufacture (gun drilling, extrusion)
• Predictable flow distribution
• Low pressure drop

Disadvantages:

• Flow maldistribution between channels
• Requires careful manifold design

Best for: Large cold plates, moderate power density

Serpentine Channels:

Description: Single continuous channel winding back and forth Advantages:

• Equal flow through entire path
• No flow distribution issues
• Good for uneven heat sources

Disadvantages:

• Higher pressure drop
• Temperature gradient along flow path

Best for: Single/few heat sources, uniform cooling needed

Pin-Fin:

Description: Array of pins in flow cavity Advantages:

• Highest surface area
• Best thermal performance
• Good for very high power density

Disadvantages:

• Highest pressure drop
• More complex to manufacture
• Debris accumulation risk

Best for: High-performance applications, clean coolant systems

Micro-Channels:

Description: Many small channels (<1mm) Advantages:

• Very high heat transfer coefficient
• Compact size

Disadvantages:

• Very high pressure drop
• Clogging risk
• Manufacturing complexity

Best for: Specialized high-power-density applications

Jet Impingement:

Description: Coolant jets impact hot surface directly Advantages:

• Highest local heat transfer
• Excellent for hotspots

Disadvantages:

• Complex manifold design
• High flow rates needed

Best for: Extreme power density, spot cooling

Material choice affects thermal performance, weight, cost, and corrosion resistance.

Aluminum (6061-T6):

Thermal conductivity: 167 W/mK Density: 2.7 g/cm³ Cost: Lowest

Advantages:

• Lightweight
• Easy to machine
• Good for extrusion and casting
• Economical

Disadvantages:

• Galvanic corrosion with copper coolant fittings
• Requires corrosion inhibitors
• Lower conductivity than copper

Best for: General industrial applications, weight-sensitive designs

Copper (C110):

Thermal conductivity: 390 W/mK Density: 8.9 g/cm³ Cost: 3-5× aluminum

Advantages:

• Best thermal conductivity
• Excellent for brazing
• Good corrosion resistance with proper coolant

Disadvantages:

• Heavy
• Expensive
• More difficult to machine

Best for: Highest performance requirements, aerospace

Hybrid (Copper/Aluminum):

Construction: Copper inserts or plates in aluminum body Advantages:

• Copper where needed for spreading
• Aluminum for structure/channels
• Good cost/performance balance

Best for: High-power modules, cost-conscious high performance

Stainless Steel:

Thermal conductivity: 15 W/mK Density: 8.0 g/cm³

Advantages:

• Excellent corrosion resistance
• Compatible with aggressive coolants

Disadvantages:

• Poor thermal conductivity
• Heavy

Best for: Corrosive environments, chemical compatibility

Material Thickness Guidelines:

Base thickness (heat source side): 3-8mm

• Thicker = better spreading from small sources
• Thinner = lighter, faster response

Channel wall thickness: 1-3mm

• Must withstand coolant pressure
• Brazed/welded construction needs adequate material

Cover thickness: 2-4mm

• Structural support for fittings
• Access for manufacturing

CFD simulation enables optimization before prototype fabrication.

Step 1: Define Requirements

Inputs needed:

• Heat source locations and power
• Coolant type and properties
• Available flow rate and pressure
• Target thermal resistance
• Size constraints

Step 2: Initial Design

Start with proven geometry:

• Serpentine for single source
• Parallel channels for distributed sources
• Size channels for target pressure drop

Initial sizing:

• Channel width: 3-6mm
• Channel height: 3-10mm
• Wall thickness: 1.5-2mm
• Number of passes: Based on heat source size

Step 3: CFD Model Setup

Boundary conditions:

• Inlet: Mass flow rate and temperature
• Outlet: Pressure outlet
• Heat sources: Heat flux or power
• External: Adiabatic or specified

Mesh requirements:

• 3-5 cells across channel width
• Inflation layers on walls (y+ < 1 for accurate heat transfer)
• Mesh independence study essential

Turbulence model:

• k-epsilon or k-omega SST
• Adequate for most cold plate simulations

Step 4: Optimize

Parameters to vary:

• Channel geometry (width, height, count)
• Flow configuration (series, parallel, mixed)
• Inlet/outlet locations
• Material (if options available)

Optimization goals:

• Minimize maximum temperature
• Minimize temperature gradient
• Minimize pressure drop
• Meet thermal resistance target

Step 5: Validate

Compare simulation to:

• Thermal test data from prototype
• Pressure drop measurements
• Flow visualization if available

Adjust model parameters to match:

• Contact resistances
• Surface roughness
• Inlet/outlet loss coefficients

Cold plates must integrate with the complete liquid cooling system.

Pump Selection:

Requirements:

• Flow rate to achieve thermal target
• Pressure rise to overcome system losses
• Reliability for expected lifetime

Types:

• Centrifugal: High flow, moderate pressure
• Positive displacement: Precise flow, higher pressure
• Magnetic drive: Seal-less, improved reliability

Size for:

• Total system pressure drop + 20% margin
• Flow rate requirement
• Worst-case coolant viscosity (cold start)

Heat Rejection:

Options:

• Liquid-to-air radiator (fan-cooled)
• Liquid-to-liquid heat exchanger
• Chiller (active cooling below ambient)
• Dry cooler (outdoor installations)

Sizing:

• Must reject total heat load
• Size for worst-case ambient
• Include approach temperature in calculations

Reservoir/Expansion Tank:

Functions:

• Accommodate thermal expansion
• Remove air from system
• Provide coolant reserve

Sizing:

• 10-20% of total loop volume
• Allow for expansion at maximum temperature

Plumbing:

Considerations:

• Minimize pressure drop
• Avoid air traps
• Allow for thermal expansion
• Include drain points
• Use compatible materials

Fittings:

• Quick-disconnects for serviceability
• Compression fittings for permanent connections
• O-ring face seal for zero-leak

Controls:

Monitor:

• Inlet and outlet coolant temperatures
• Flow rate (optional)
• Coolant level/pressure

Control:

• Pump speed (for variable flow)
• Fan speed (for radiator)
• Heater (for cold start protection)

Protection:

• Over-temperature shutdown
• Low-flow alarm
• Leak detection

Free Resource: Download our Cold Plate Selection Guide — a 10-page guide on choosing the right cold plate for your application.