Metal Core PCB (MCPCB) Design Guide

Introduction: The Thermal Problem in Power Electronics

Every electronic component generates waste heat. For low-power circuits (microcontrollers, sensors, communication ICs), standard FR-4 PCBs dissipate heat adequately through copper planes and convection.

But high-power components create thermal densities that overwhelm organic substrates:

• High-brightness LEDs: 3–15 W per device, junction temperature must stay below 120°C for rated lifetime (50,000+ hours)
• Power MOSFETs/IGBTs: 10–500 W per device in motor drives and inverters
• Voltage regulators: 5–50 W switching converters in space-constrained designs
• RF power amplifiers: 10–100 W GaN/LDMOS devices for base stations

FR-4’s thermal conductivity is only 0.3 W/mK — acting as a thermal insulator rather than a heat path. Even with thermal vias (achieving equivalent conductivity of 2–4 W/mK in the via field), organic PCBs struggle to maintain acceptable junction temperatures for high-power devices.

Metal Core PCBs (MCPCBs) — also called Insulated Metal Substrates (IMS) — solve this by replacing the organic core with a thermally conductive metal plate, providing a direct low-resistance thermal path from the component to the heat sink.

MCPCB Construction

Single-Layer MCPCB (Most Common)

The standard MCPCB stackup from top to bottom:

1. Solder mask (25–35 μm): Standard LPI solder mask, white for LED applications
2. Copper circuit layer (35–105 μm / 1–3 oz): Standard etched copper traces
3. Dielectric insulation layer (75–200 μm): Thermally conductive, electrically insulating polymer-ceramic composite
4. Metal core (0.8–3.2 mm): Aluminum alloy or copper plate
5. Optional: back-side finish: Anodization, plating, or thermal interface material pre-applied

The Dielectric Layer: Critical Bottleneck

The dielectric layer is simultaneously the most critical and most challenging component of MCPCB design. It must provide:

• Electrical isolation: 2–8 kV/mm breakdown voltage (safety requirement)
• Thermal conduction: 1–7 W/mK (performance requirement)
• Adhesion: Bond strength to both copper foil and metal core
• CTE compliance: Accommodate differential expansion between copper and aluminum

Standard dielectric formulations use epoxy or polyimide matrices filled with thermally conductive ceramic particles (Al₂O₃, BN, or AlN). Higher filler loading increases thermal conductivity but reduces breakdown strength and adhesion.

Metal Core Options

Aluminum alloys (most common):

• 5052-H32: Good thermal conductivity (138 W/mK), excellent corrosion resistance, standard for outdoor LED fixtures
• 6061-T6: Higher strength (276 MPa yield), machinability for CNC heat sink integration, 167 W/mK
• 1060 pure aluminum: Maximum thermal conductivity (234 W/mK) but softer — used for indoor applications without mechanical stress

Copper alloys (high-performance):

• C110 (ETP copper): 385 W/mK, for maximum thermal performance in power modules
• C194 (copper-iron alloy): 260 W/mK with superior mechanical properties for automotive
• CPC (Copper-Polymer-Copper): Sandwich construction reducing weight while maintaining thermal path

Thermal Analysis and Calculation

Thermal Resistance Model

MCPCB thermal performance is characterized by junction-to-ambient thermal resistance (θ_JA):

θ_JA = θ_JC + θ_CS + θ_SA

Where:

• θ_JC = Junction-to-case resistance (component datasheet value)
• θ_CS = Case-to-sink resistance (MCPCB contribution)
• θ_SA = Sink-to-ambient resistance (heat sink + convection)

The MCPCB’s contribution (θ_CS) consists of:

θ_CS = t_Cu/(k_Cu × A) + t_diel/(k_diel × A) + t_core/(k_core × A)

Where t = thickness, k = thermal conductivity, A = heat spreading area

Practical Example: 10W LED on MCPCB

For a 10W LED (4×4 mm thermal pad) on standard 1.6mm aluminum MCPCB:

• Copper layer (35 μm): θ = 0.035/(386 × 16×10⁻⁶) = 0.006 °C/W (negligible)
• Dielectric (100 μm, 2 W/mK): θ = 0.1/(2 × 16×10⁻⁶) = 3.1 °C/W (dominant)
• Aluminum core (1.5 mm): θ = 1.5/(167 × 16×10⁻⁶) = 0.56 °C/W

Total θ_CS ≈ 3.7 °C/W → With 10W dissipation, ΔT = 37°C across the MCPCB

Compare to FR-4 with thermal vias:

• FR-4 (1.6mm, effective 2 W/mK with via field): θ = 1.6/(2 × 16×10⁻⁶) = 50 °C/W
• ΔT = 500°C — catastrophically impossible!

This 13× improvement in thermal resistance is why MCPCBs are mandatory for high-power LEDs.

Heat Spreading Benefit

The metal core provides lateral heat spreading that significantly increases the effective heat transfer area. For a 4×4 mm LED pad on 1.5 mm aluminum, the heat spreads outward at approximately 45° through the metal core, creating an effective transfer area of ~100 mm² at the bottom surface (vs. 16 mm² at the component) — a 6× area multiplication that further reduces θ_SA.

Design Guidelines for MCPCB

Copper Circuit Layer Rules

1. Minimum trace width: 150 μm (6 mil) standard — wider than typical FR-4 minimum due to etching on thick copper
2. Minimum spacing: 200 μm (8 mil) for 1 oz copper; increase to 300 μm for 2–3 oz
3. Thermal pad design: Maximize pad area under power components; solder mask opening = component pad + 0.1 mm per side
4. No through-holes to metal core: Standard MCPCBs are single-layer; if you need grounding to core, use dedicated “thermal via” structures (dielectric removed locally)

Dielectric Design Considerations

• Uniform thickness: Variation >±10% creates thermal hotspots and voltage withstand weak points
• Thermal via option: Some manufacturers offer “direct thermal path” technology — local removal of dielectric under thermal pads, with a thin metallization barrier providing minimal electrical isolation while maximizing thermal transfer
• Voltage isolation: For SELV (Safety Extra Low Voltage) circuits (<60V), standard 75 μm dielectric is adequate. For mains-connected circuits (>250V), specify 150+ μm dielectric with 4 kV Hi-pot test

Metal Core Thickness Selection

DFM Best Practices

1. Score-line routing preferred: V-score depaneling is standard for MCPCB arrays (tab routing generates aluminum burrs that cause electrical shorts)
2. Board outline tolerances: ±0.2 mm (aluminum CNC routing), tighter than standard FR-4 (±0.15 mm)
3. No plated through-holes: Standard single-layer MCPCB has no PTH capability. If through-board connections are needed, use pressed-in pins or rivets.
4. Surface finish: HASL or OSP for LED (reflectivity matters); ENIG for fine-pitch power QFN packages
5. Solder mask color: White for LED applications (95%+ reflectance improves luminaire efficiency by 3–5%)

Multilayer MCPCB Construction

For designs requiring signal routing complexity beyond single-layer capability, multilayer MCPCBs combine standard PCB layers with a metal core base:

2-Layer MCPCB:

• Layer 1: Signal/component copper (35 μm)
• Prepreg (standard FR-4, 100–200 μm)
• Layer 2: Ground/power plane (35 μm)
• Thermal dielectric (75–100 μm)
• Metal core (1.0–2.0 mm)

4-Layer MCPCB:

• Layer 1–4: Standard multilayer PCB stackup
• Thermal dielectric
• Metal core

The thermal penalty of multilayer construction: each additional prepreg/copper layer adds ~50 μm of FR-4-equivalent thermal resistance between power components and the metal core. Thermal vias through the organic layers partially mitigate this — achieving effective through-board conductivity of 10–20 W/mK in the via field.

Application Case Studies

High-Bay LED Luminaire (200W)

A typical 200W industrial high-bay light uses:

• 2.0 mm aluminum core MCPCB (6061-T6)
• 3 W/mK dielectric layer (100 μm)
• 50× 4W LED devices in 5×10 array
• White solder mask for maximum reflectance
• Mounting directly to die-cast aluminum heat sink housing

Thermal analysis shows junction temperature of 85°C at 50°C ambient — well within the LED manufacturer’s 105°C maximum for 50,000-hour lifetime rating.

Automotive IGBT Gate Driver (500V/50A)

Half-bridge motor drive for EV traction inverter:

• 3.2 mm copper core MCPCB (C110)
• 5 W/mK dielectric (150 μm for 2kV isolation)
• Power MOSFETs with exposed drain pad soldered directly to MCPCB
• 2-layer construction for gate drive routing on top + ground plane
• Operating temperature range: -40°C to +150°C

UV LED Curing System (395nm)

Industrial UV curing array for printing/coating:

• 1.6 mm aluminum MCPCB with mirror-finish white mask
• 36× UV LED devices (1W each, 395nm wavelength)
• Narrow 75 μm dielectric for minimum thermal resistance (UV LEDs are highly temperature-sensitive)
• Custom board outline matching reflector optics geometry

Testing and Quality Assurance

Standard MCPCB Tests

1. Thermal resistance measurement (ASTM D5470): Verify dielectric thermal conductivity meets specification
2. Dielectric breakdown voltage (IPC-TM-650 2.5.6): Hi-pot test at 2× working voltage + 1000V for 60 seconds
3. Peel strength (IPC-TM-650 2.4.8): Copper-to-dielectric bond ≥1.0 N/mm (6 lb/in)
4. Thermal cycling (IPC-TM-650 2.6.7): -40°C to +150°C, 1000 cycles, no delamination
5. Solder float (IPC-TM-650 2.4.13): 288°C for 10 seconds, no blistering or separation

Common Failure Modes

• Dielectric delamination: Caused by moisture absorption + thermal shock; ensure dry-pack storage and pre-bake before assembly
• Copper lifting at pad edges: Over-etching undercuts the circuit layer; specify etch factor ≤3:1
• Core warpage: Asymmetric copper pattern creates CTE mismatch stress; balance copper area on board front/back

Further Reading

• Thermal Via PCB Heat Dissipation — Via-based thermal management for standard PCBs
• Heavy Copper PCB Design for Power Electronics — High-current PCB alternatives
• PCB Copper Weight and Thickness Guide — Copper thickness selection for current capacity
• PCB Reflow Soldering Profile Optimization — Assembly considerations for MCPCB

Ready to design your next MCPCB? Get a quote from AtlasPCB — we manufacture aluminum and copper core PCBs from prototype through high-volume production with thermal conductivity options from 1.0 to 7.0 W/mK.