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Single-layer aluminum boards handle most LED lighting jobs without complaint. But once you’re designing automotive power modules, industrial motor drives, or dense solid-state lighting engines where routing complexity and thermal demand exist simultaneously, you need a different approach. That’s where multi-layer metal core PCB design comes in — and where material selection, specifically between products like Bergquist HT-09009 and CML-11006, becomes a genuinely consequential engineering decision rather than a checkbox on a BOM.

This guide is aimed squarely at engineers doing the actual design work: stack-up planning, dielectric selection, thermal budget calculation, and layout rules that keep fabricators and assemblers from sending your boards back.

Why Multi-Layer Metal Core PCB Design Exists

The honest answer is that most applications don’t need it. If your circuit is simple enough to route on a single layer and your thermal load is manageable, a standard single-layer MCPCB is cheaper, easier to fab, and easier to assemble. Multi-layer metal core PCB design makes sense when two problems collide on the same board:

Problem 1 — Routing density: Power stage components (IGBTs, MOSFETs, gate drivers, current sense resistors) plus control circuitry require more copper layers than a single-layer board can accommodate. Trying to squeeze everything onto one side often forces trace compromises that hurt EMI and current-handling simultaneously.

Problem 2 — Thermal load: The same board has components dissipating enough watts that FR4 with clip-on heatsinks is genuinely inadequate. You need the metal core’s thermal conductance path, not as an afterthought, but as a first-class design element.

When both problems are present, multi-layer MCPCB is usually the answer. Common real-world examples include EV onboard chargers, LED stadium lighting driver boards, industrial servo drives, and automotive LED headlight modules.

Multi-Layer MCPCB Stack-Up Configurations

Before selecting a dielectric material, you need to choose which stack-up architecture fits your application. The three most practical configurations are:

Two-Signal-Layer MCPCB (Same-Side Stack-Up)

Both copper circuit layers sit on the same side of the metal base, separated by a thermally conductive prepreg or Bond-Ply adhesive layer. The metal base is at the bottom. This is the most commonly fabricated multi-layer MCPCB configuration because:

• It avoids the need for PTH isolation from the metal core
• Thermal path from components to the base remains short
• Fabrication complexity is moderate compared to symmetric multilayer designs

Typical stack-up (top to bottom):

Layer	Material	Thickness
L1 — Signal/Power	Copper foil (1–3 oz)	35–105 μm
Dielectric 1	Bergquist HT or CML series	75–229 μm
L2 — Signal/Ground	Copper foil (1–2 oz)	35–70 μm
Dielectric 2 (to metal core)	Thermally conductive prepreg	75–150 μm
Metal Core	Aluminum 5052 or 6061	1.0–3.2 mm

Symmetric Multilayer MCPCB

Layers are distributed symmetrically above and below the metal core. A 4-layer symmetric design has 2 copper layers on top of the core and 2 on the bottom. This is the configuration Sierra Circuits and other fabricators reference when they specify that “the number of layers on top of the core should equal the number of layers on the bottom” — a rule that exists specifically to prevent warpage under thermal cycling.

This stack-up is used in high-power industrial modules and automotive ECUs where routing density is genuinely complex. The metal core sits at the center and acts as both a structural backbone and a ground/heat plane.

Symmetric 4-Layer MCPCB Stack-Up:

Layer	Material	Notes
L1 — Top Signal	1–2 oz copper	SMT component side
Prepreg 1	HT-09009 or CML-11006	High-k dielectric
L2 — Inner Ground	1 oz copper	EMI shield, thermal via landing
Metal Core	Aluminum / Copper	Heat spreader, structural
L3 — Inner Power	1 oz copper	Power distribution plane
Prepreg 2	HT-09009 or CML-11006	Mirror of Prepreg 1
L4 — Bottom Signal	1–2 oz copper	Secondary components or ground pour

Hybrid FR4/MCPCB Construction

Some applications need dense digital signal routing but only localized thermal management. A hybrid approach bonds a standard multilayer FR4 section to an aluminum base region. The FR4 section handles complex signal routing; the MCPCB section handles the power stage. This architecture is used in smart motor drives and EV battery management systems where a DSP or microcontroller sits next to power FETs on the same physical board.

Bergquist Thermal Clad Dielectrics: HT-09009 vs CML-11006

Bergquist (now part of Henkel) produces the most widely specified MCPCB dielectric materials in the industry. Their Thermal Clad line covers four main families: HPL, MP, HT, and CML. For multi-layer metal core PCB design, HT-09009 and CML-11006 are the two materials that come up most often in demanding applications.

Bergquist HT-09009 — High Temperature, Maximum Thermal Conductivity

The HT (High Temperature) series is engineered for applications where operating temperatures push the limits of standard epoxy-based dielectrics. HT-09009 represents the highest-performance variant in the HT family.

HT-09009 Key Specifications:

Parameter	HT-09009 Value
Dielectric Thickness	9 mil / 229 μm
Thermal Conductivity	2.2 W/m·K
Thermal Resistance	0.90 °C·in²/W
Dielectric Breakdown (AC)	20 kV
Peel Strength	6 lb/in (1.1 N/mm)
UL RTI (Electrical/Mechanical)	150°C / 150°C
Volume Resistivity	>10⁹ MΩ·cm

The 9-mil (229 μm) dielectric thickness is the defining characteristic. Compared to thinner HT variants, this gives HT-09009 significantly higher dielectric breakdown voltage (20 kV AC) — critical for automotive applications where mains-referenced circuits or high-side gate drive networks must maintain isolation under transient overvoltage conditions.

The 2.2 W/m·K thermal conductivity is genuinely useful — roughly 7–8x higher than standard FR4 prepreg (0.3 W/m·K). The thicker dielectric does increase thermal resistance compared to thinner HT options, but the tradeoff is a board that won’t fail hi-pot testing during 1,000-hour thermal cycling in an automotive environment.

Where HT-09009 is the right choice: Automotive LED modules (headlights, DRL), power conversion boards with reinforced insulation requirements, high-voltage LED drivers (277V or 480V mains-referenced), industrial motor drives operating at elevated ambient temperatures (>85°C), and any application where UL or IEC certification requires 150°C continuous operating rating.

Bergquist CML-11006 — Cost-Optimized Multi-Layer Performance

CML (Ceramic-filled Multi-Layer) is a different beast from HT. It’s not designed for extreme temperature environments. Instead, it’s optimized for multi-layer assembly processes — specifically, it’s designed to bond reliably between copper layers in a multilayer stack without the processing complications of some higher-performance dielectrics.

CML-11006 Key Specifications:

Parameter	CML-11006 Value
Dielectric Thickness	6 mil / 152 μm
Thermal Conductivity	1.1 W/m·K
Thermal Resistance	1.10 °C·in²/W
Dielectric Breakdown (AC)	10 kV
Peel Strength	10 lb/in (1.8 N/mm)
UL RTI (Electrical/Mechanical)	130°C / 130°C
Volume Resistivity	>10⁹ MΩ·cm

The 1.1 W/m·K thermal conductivity is lower than HT-09009, but the CML-11006 compensates with significantly higher peel strength (10 lb/in vs 6 lb/in for HT-09009). In a multi-layer board that sees vibration, thermal cycling, and mechanical shock — a servo drive cabinet, for instance — the bond integrity between layers matters as much as the thermal conductivity number.

The 130°C/130°C UL RTI rating covers a large proportion of non-automotive industrial applications, and the 10 kV AC breakdown voltage is adequate for SELV circuits and Class II power supplies.

Where CML-11006 is the right choice: Industrial multi-layer MCPCB designs where routing density justifies multiple copper layers but the application doesn’t demand HT-level temperature ratings, two-layer MCPCB boards with high mechanical vibration requirements, power LED arrays in signage or commercial lighting, and designs where fabrication simplicity and layer-to-layer bond reliability are prioritized over maximum thermal conductivity.

Direct Comparison: HT-09009 vs CML-11006

Property	HT-09009	CML-11006	Advantage
Thermal conductivity	2.2 W/m·K	1.1 W/m·K	HT-09009 (2x)
Dielectric thickness	9 mil / 229 μm	6 mil / 152 μm	CML-11006 (thinner)
Thermal resistance	0.90 °C·in²/W	1.10 °C·in²/W	HT-09009
Breakdown voltage (AC)	20 kV	10 kV	HT-09009
Peel strength	6 lb/in	10 lb/in	CML-11006
UL RTI (Elec/Mech)	150°C / 150°C	130°C / 130°C	HT-09009
Primary application	Automotive, high-voltage	Industrial multilayer	—
Cost index	Higher	Lower	CML-11006

Neither material is universally better. HT-09009 wins on thermal conductivity, voltage isolation, and temperature rating. CML-11006 wins on peel strength, dielectric thinness, and cost. The selection depends entirely on your operating environment, isolation requirements, and mechanical stress profile.

Bergquist Thermal Clad Full Family Reference

To put HT-09009 and CML-11006 in context, here’s where they sit within the broader Bergquist Thermal Clad lineup:

Material	Thickness (mil/μm)	Thermal Conductivity	Breakdown	Max Temp (UL RTI)	Primary Use
HPL-03015	1.5 / 38	3.0 W/m·K	2.5 kV	—	High-power LED (thin, low isolation)
HT-04503	3 / 76	2.2 W/m·K	7 kV	140°C / 140°C	Auto, power electronics
HT-07006	6 / 152	2.2 W/m·K	11 kV	140°C / 140°C	Auto, HV power supply
HT-09009	9 / 229	2.2 W/m·K	20 kV	150°C / 150°C	Auto, high-voltage isolation
MP-06503	3 / 76	2.4 W/m·K	—	130°C / 140°C	General multi-purpose
CML-11006	6 / 152	1.1 W/m·K	10 kV	130°C / 130°C	Industrial multilayer

For specialty materials requiring extreme temperature cycling or unusual CTE matching — such as in aerospace or high-reliability military applications — Arlon PCB materials offer an alternative product family with different resin chemistry and performance characteristics.

Multi-Layer Metal Core PCB Design Rules

Getting a multi-layer MCPCB to behave thermally and electrically requires more careful layout discipline than either standard FR4 multilayer or single-layer MCPCB work. These are the rules that matter most in practice.

Stack-Up Symmetry Is Non-Negotiable

In any symmetric multilayer MCPCB, the copper weight and dielectric thickness above and below the metal core must mirror each other. Asymmetric copper distribution creates differential thermal expansion during lamination and reflow — the result is a warped board that will cause solder joint failures in the field and headaches in incoming inspection. For a 4-layer board with the metal core in the center, layers 1 and 4 should use the same copper weight, and the dielectric materials on both sides should be the same type and thickness.

Thermal Via Strategy

Unlike single-layer MCPCBs where the component sits directly over the dielectric and metal core, multi-layer boards add one or more copper layers between the heat source and the metal base. Every additional layer adds thermal resistance. Thermal vias are the solution — they create short, low-resistance thermal paths through the inner layers to the metal core.

Practical thermal via design for MCPCB multilayer:

Via Parameter	Recommended Value
Via diameter	0.3–0.5 mm (12–20 mil)
Via grid pitch	0.8–1.0 mm under thermal pads
Via fill	Resin fill + copper cap (for SMT pad placement)
Via-to-via spacing	≥ 0.25 mm edge-to-edge
Coverage under thermal pad	50–70% of pad area

Don’t overdo it. Beyond roughly 70% via coverage under a thermal pad, you start getting solder voiding issues during reflow because flux has nowhere to outgas. The 50–70% zone is well-validated empirically.

PTH Isolation from the Metal Core

Plated through-holes in a multilayer MCPCB present a short-circuit risk to the metal base. Every PTH that passes through the board must clear the metal core by 40–50 mils (1.0–1.25 mm) in each direction. The annular clearance is drilled oversize, then filled with non-conductive epoxy resin, then cured, then the surface is planarized before the via is plated. This adds process steps and cost — another reason to favor SMT-only or mixed SMT/PTH designs where PTH count is kept to an absolute minimum.

Copper Pour and Heat Spreading

Wide copper pours on the layer immediately above the dielectric help spread heat laterally before it passes down through the dielectric. This reduces peak heat flux density at any given point in the dielectric, which in turn reduces the probability of localized thermal runaway. As a rule of thumb, maximize copper pour coverage on the layer adjacent to the thermal dielectric, and use solid pours (not cross-hatch) for power and thermal planes in MCPCB designs.

Component Placement: Hot Devices Over the Core Center

Place the highest-dissipation components near the center of the board, not at the edges. The metal core acts as a heat spreader — heat flows from component → dielectric → core → heatsink/ambient. Components placed at board edges have less core area available behind them for lateral heat spreading before reaching the mounting surface.

Trace Width and Current Capacity

In multi-layer MCPCB, inner copper layers run hotter than in FR4 multilayer because the board has lower thermal resistance to the environment. This means the temperature coefficient of resistance effect is more significant — your copper resistance at operating temperature will be measurably higher than at 25°C. For current-carrying traces, use IPC-2221 as a starting point but add a 15–20% margin to account for the elevated steady-state temperature.

Thermal Resistance Calculation for Multi-Layer MCPCB

The total thermal resistance from junction to board mounting surface (θj-mb) in a multi-layer MCPCB design is the sum of each layer’s contribution:

θtotal = θpackage + θsolder + θcopper(L1) + θdielectric1 + θcopper(L2) + θdielectric2 + θmetal core

For a two-layer design using CML-11006 on 1.6mm aluminum:

Layer	Thickness	Conductivity	θ (°C·cm²/W)
Copper L1 (1 oz)	35 μm	390 W/m·K	0.009
CML-11006 dielectric	152 μm	1.1 W/m·K	1.38
Copper L2 (1 oz)	35 μm	390 W/m·K	0.009
Bonding dielectric	100 μm	1.1 W/m·K	0.91
Aluminum core (5052)	1,600 μm	138 W/m·K	0.116
Total			~2.42 °C·cm²/W

Replacing CML-11006 with HT-09009 increases both dielectric layers’ conductivity to 2.2 W/m·K:

Layer	θ with HT-09009 (°C·cm²/W)
HT-09009 dielectric 1	1.04
HT-09009 dielectric 2	0.50
Copper + aluminum	0.134
Total	~1.67 °C·cm²/W

The HT-09009 multilayer stack runs roughly 30% lower total thermal resistance compared to CML-11006 in this configuration. For a 10W component on a 1 cm² thermal pad, that’s a junction temperature difference of about 7.5°C — meaningful in a tight thermal budget.

Common Mistakes in Multi-Layer Metal Core PCB Design

Engineers who design multi-layer MCPCBs for the first time regularly make the same mistakes. Most of them are avoidable with some upfront DFM thinking.

Mistake	Consequence	Fix
Asymmetric copper distribution	Warpage during lamination and reflow	Mirror copper weight above and below metal core
PTH too close to metal core	Short circuit to ground plane	Maintain 40–50 mil clearance, fill with resin
No thermal vias under power devices	High junction temperature, premature failure	50–70% via coverage under thermal pads
Specifying HT-09009 for low-voltage industrial use	Unnecessary cost	Use CML-11006 below 10 kV isolation requirement
Specifying CML-11006 for 150°C ambient automotive	Dielectric degradation over time	Use HT-09009 for UL RTI 150°C applications
Solder paste under via-in-pad (unfilled)	Solder voiding, poor thermal contact	Specify resin-filled, copper-capped via-in-pad
Cross-hatch copper pours on thermal layers	Higher effective thermal resistance	Use solid pours on thermal plane layers

Useful Resources for Multi-Layer MCPCB Engineers

Resource	What You’ll Find	Link
Bergquist / Henkel Thermal Clad Selection Guide	Full dielectric specs including HT-09009, CML-11006, and Bond-Ply series	henkel-adhesives.com
IPC-2221B	Generic PCB design standard; trace width/current tables, via design rules	ipc.org
IPC-6012E	Qualification and performance specification for rigid PCBs including MCPCB	ipc.org
IEC 62758	Test methods for MCPCB thermal resistance measurement	iec.ch
Ansys Icepak (thermal simulation)	Predict junction temperatures, verify thermal via designs before fabrication	ansys.com
Saturn PCB Design Toolkit	Free tool for trace width, thermal resistance, via current calculations	saturnpcb.com
Digikey Bergquist ThermalClad Datasheet PDF	Original Bergquist selection guide with full material matrix	media.digikey.com

Frequently Asked Questions About Multi-Layer Metal Core PCB Design

Q1: Can I use standard FR4 prepreg between layers in a multi-layer MCPCB?

Technically yes, but it defeats much of the purpose. Standard FR4 prepreg has thermal conductivity of about 0.3 W/m·K, which creates a high-resistance thermal barrier between your top copper layers and the metal core. For any inter-layer dielectric that sits in the thermal path between components and the metal base, you should be using a thermally conductive material like HT-09009 or CML-11006. FR4 prepreg can be used in hybrid stack-ups where signal layers that are genuinely far from the thermal path need standard dielectric properties — but those are specialized designs.

Q2: What’s the maximum number of layers practical in a multi-layer MCPCB?

Most fabricators can produce 4-layer MCPCBs routinely, and some can go to 6 layers with the right process controls. Beyond 6 layers, the thermal resistance added by each additional dielectric layer starts to undermine the point of using a metal core in the first place. If you genuinely need more than 6 routing layers with embedded thermal management, a hybrid design (FR4 multilayer bonded to aluminum base) often makes more engineering and manufacturing sense than pushing a pure multilayer MCPCB to extreme layer counts.

Q3: Does HT-09009 work with copper core as well as aluminum?

Yes. The Bergquist HT and CML dielectrics are available for use with both aluminum and copper metal cores. The dielectric lamination chemistry doesn’t change based on the metal substrate — what changes is the surface pre-treatment on the metal core itself (copper requires different chemical pre-treatment for adhesion than aluminum). Confirm with your fabricator that they have a validated process for copper-core lamination if you’re going that route, since copper core boards are less common and not all shops have validated press profiles for them.

Q4: How do I verify thermal performance of a multi-layer MCPCB design before committing to fabrication?

Use a combination of analytical calculation (as shown in the thermal resistance table above) for a first-pass estimate, then validate with simulation. Ansys Icepak and SolidWorks Flow Simulation both handle MCPCB structures well if you enter the layer-by-layer thermal conductivities correctly. For the dielectric, use the manufacturer-specified value (2.2 W/m·K for HT-09009, 1.1 W/m·K for CML-11006) — don’t use generic PCB dielectric values. After prototype fabrication, measure thermal resistance per IEC 62758 using a thermal test vehicle to close the loop between simulation and reality.

Q5: What surface finish is recommended for multi-layer MCPCB with fine-pitch power components?

ENIG (Electroless Nickel Immersion Gold) is the standard recommendation for multi-layer MCPCB designs with fine-pitch packages, thermal pads, or any component that requires a flat, consistent soldering surface. The planarity advantage of ENIG over HASL becomes especially important when you have 0.5mm-pitch QFN packages next to 5mm² thermal pads — HASL’s uneven solder coating creates height variation that causes bridging with the former and voids with the latter. For designs where wire bonding is part of the assembly (COB LED integration), ENIG is mandatory.

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