Introduction

Metal core printed circuit boards (MCPCB) provide enhanced thermal management capabilities critical for high power electronics devices and modules. Multilayer metal core boards allow mounting components on both sides of the PCB while effectively conducting heat away through inner copper layers bonded to the metal base plate.

This article provides an in-depth understanding of the manufacturing process steps involved in fabricating multilayer metal core PCBs.

Overview of Multilayer Metal Core PCBs

Multilayer metal core PCBs contain two or more signal layers separated by dielectric insulating substrates which are bonded to a metal base acting as a heat spreader.

Some key characteristics of multilayer metal core PCBs:

• Aluminum, copper or alloy base plates for heat spreading
• Dielectric insulation layers between metal and signal layers
• Multiple alternating copper signal and dielectric layers
• Plated through holes interconnect various conductive layers
• Components can be mounted on both sides for density
• Used in LED lights, power modules, motor drives etc.

Multilayer construction provides layout flexibility along with thermal and mechanical advantages.

Multilayer Metal Core PCB Manufacturing Process Steps

Fabricating a multilayer metal core printed circuit board involves several complex steps to create the layered arrangement and bond together under high temperature and pressure.

1. Base Plate Preparation

The base metal plate material made of aluminum, copper or alloys is machined to the required PCB dimensions and holes are punched as needed. The plate is cleaned to remove any oxidation or surface contamination.

2. Dielectric Layer Coating

Liquid dielectric materials like epoxy or polyimide resin are roller coated or laminated onto the base plate to create an insulating layer over the metal plate surface. The coated metal plate is then pre-baked.

3. First Layer Imaging

Photoresist dry film is then laminated on the dielectric layer and the circuit pattern is created using photolithography process. Unexposed photoresist is developed away and the dielectric layer is etched to create the circuit traces.

4. Build Up Layer Stacking

Alternating layers of dielectric substrates and copper foils are stacked up layer by layer on top of the base plate using epoxy or prepreg bonding films. Alignment is critical.

5. Lamination

The multilayer metal core board stack up goes through a high temperature and pressure lamination process to firmly bond the layers together into a finished core board.

6. Plated Through Holes

Holes are drilled through the multilayer board and copper is plated onto the hole walls to form plated through holes interconnecting the conductive layers.

7. Outer Layer Imaging

Similar to the first layer, the outermost layers are processed by using photolithography to define the final solder pad circuit pattern on the PCB surface.

8. Finishing

The panels go through various finishing steps like hole masking, soldermask coating, surface metal plating, legend printing and Singulation.

9. Testing

Final finished boards are electrically tested and inspected before shipment to customers. Testing validates continuity, isolation and functional performance.

Key Fabrication Processes for Multilayer Metal Core PCBs

Some of the critical processes involved in creating multilayer metal core PCBs include:

Photolithography

High precision imaging is required to define ultra fine circuit features on each copper layer. Photolithography employs UV light through a mask to create circuit patterns.

Sequential Lamination

Individual layers are stacked and bonded in a controlled sequential order using high temperatures >180°C and pressures >1MPa in a vacuum press.

Plated Through Holes

Holes are mechanically drilled and plated with copper to form vertical connections between the conductor layers through the dielectric.

Layer Alignment

Precise alignment during lay up is critical otherwise plated through hole walls may not intersect inner layer pads causing defects.

Copper Plating

Electrolytic copper is plated to desired thicknesses on through holes, circuits and as finished plating on external layers. Nickel/Gold may be used.

Hole Masking

The plated through hole walls are masked with epoxy or dry film soldermask to prevent wicking of solder into the holes during component assembly.

CTE Management

The coefficient of thermal expansion between metal, dielectric layers, copper and components needs to be closely matched to avoid thermal stresses.

Multilayer Metal Core PCB Design Guidelines

Here are some key design considerations for multilayer metal core PCBs:

• Minimize the number of drilled holes penetrating the base plate to limit thermal leakage paths.
• Position heat generating components over the base plate to transfer heat efficiently into the spreader.
• Incorporate thermal vias with metallized walls interconnecting layers to dissipate heat.
• Manage CTE mismatches between metal, dielectrics and copper using compliant interface layers if needed.
• Take into account the dielectric material properties like thermal conductivity, breakdown voltage, Tg when selecting insulators.
• Use thicker 2 oz copper Signal layers for high current carrying capacity.
• Incorporate large copper heat dissipation islands/planes connected to the base plate.
• Plan adequate cutouts, mouse bites, clearances for thermal expansion of the metal base plate.
• Model thermal performance under temperature transients and power cycling conditions.
• Assess mechanical stresses on solder joints due to bending, vibration and shock.

Multilayer MCPCB Materials

The materials chosen for fabricating a multilayer metal core PCB significantly impact its electrical, thermal and mechanical performance.

Metal Base Plate Materials

• Aluminum – Offers best thermal performance. Most widely used.
• Copper – Higher thermal and electrical conductivity but heavier and prone to oxidation.
• Alloy variants – Combinations of Al, Cu, Mg, Fe to optimize properties.

Dielectric Layer Materials

• Standard FR-4 glass epoxy – Good high temperature performance but lower thermal conductivity.
• High Tg hydrocarbon resin composites – Used for high temp stability applications.
• Ceramics like Alumina and Aluminum nitride – Provide very high thermal conductivity.
• PTFE composites – Offers high frequency insulation.
• Polyimide films – Withstands high temperatures with flexibility.

Copper Types

• Rolled annealed copper foils – Most common type for inner layers.
• Electrodeposited copper – Used for majority of outer circuit layers.
• Thick copper up to 3 oz – Used when high current carrying capacity needed.

Applications of Multilayer Metal Core PCBs

Some major application areas where multilayer MCPCBs provide unique benefits include:

High Power LED Lighting

• LED luminaires, horticulture lighting, stadium lights

Power Converters and Inverters

• PV solar inverters, motor drives, UPS systems

Electric Vehicle/Hybrids

• Battery management systems, traction inverters

Telecom/Server Racks

• Base stations, blade servers, data centers

Defense and Avionics

• Radars, mission computers, flight control systems

Medical

• MRI machines, diagnostic imaging systems

Industrial

• Robots, programmable automation controllers

In all these applications, multilayer MCPCBs can effectively manage high power densities while providing complex circuit routing capabilities.

Thermal Management Schemes Used with MCPCBs

Multilayer metal core boards dissipate heat using various aids:

• Heat Sinks – Attach large heat sinks directly over the metal plate for forced air cooling.
• Fins/Heat Pipes – Incorporate fins or heat pipes into enclosure for extended surfaces.
• Liquid Cooling – Liquid cold plates making direct contact with the metal core quickly transfer heat.
• Fans/Blowers – Use fans or blower motors directed over the board for forced convection.
• Thermal Interface – Fill air gaps between board and enclosure using thermal interface materials.
• Thermal Vias – Strategically place plated through hole vias in the board allowing vertical heat transfer.

Key Manufacturers of Multilayer Metal Core PCBs

Some of the leading companies having expertise in mass producing complex, high reliability multilayer metal core PCBs include:

• AT&S (Austria)
• TTM Technology (US)
• Ibiden (Japan)
• Tripod Technology (Taiwan)
• Unimicron (Taiwan)
• HannSTAR (Taiwan)
• CMK (Japan)
• NCAB Group (Sweden)
• Thermal Clad (US)
• Ventec (US)

These manufacturers offer end-to-end solutions right from design, simulation, fabrication through testing of application specific multilayer MCPCBs.

Future Outlook for Multilayer Metal Core PCBs

The global multilayer MCPCB market is estimated to grow steadily at 6% CAGR over the next 5 years owing to:

• Surging demand for higher power density electronics across industries.
• Heat management challenges associated with emerging applications like EVs, 5G telecom.
• Expanding power electronics driven by industrial automation.
• Technology innovations enabling manufacture of thinner, larger and more reliable metal core boards.
• Growth in high power semiconductor adoption driving thermal design considerations.

Developments in materials technology combined with advanced manufacturing processes will enable more sophisticated implementations of multilayer MCPCBs in the future.

Comparison of Multilayer Metal Core PCBs vs Thermal Clad

Parameter	Multilayer MCPCBs	Thermal Clad PCBs
Structure	Multiple conductive and dielectric layers	Signal layers bonded directly over spreader
Trace layers	Multiple signal layers possible	Typically single or double layer
Materials	Cu, Al base plates with resin dielectric films	Dielectric less Cu cladding over Al or Cu spreader
Thermal vias	Required for vertical heat transfer	Not needed without dielectric
Routing	Complex routing capability between layers	Simpler routing over directly bonded copper
Applications	High power LEDs, power modules	Appliances, industrial controls
Cost	Higher due to multi-layer process	Lower cost without laminations

Frequently Asked Questions

Q: What are some key benefits of multilayer metal core PCBs?

A: They enable thermal management of heat generating components while providing for routing complexity necessary in high density PCBs.

Q: What dielectric materials are commonly used in MCPCBs?

A: FR-4, polyimide films, ceramic compounds and thermally conductive resin composites like hydrocarbon are typically used.

Q: How many conductive layers are generally present?

A: Multilayer metal core PCBs usually have 2 to 12 conductive signal layers separated by insulating dielectric substrates.

Q: What are thermal clad PCBs?

A: Thermal clad PCBs have the copper layer directly bonded to the metal plate eliminating the need for separate dielectric layers.

Q: What are some key benefits of using a metal core PCB base?

A: The metal base acts as an efficient heat spreader conducting heat laterally away from components mounted above. This enables effective cooling.