What is Aluminum Nitride (ALN) PCB ?

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Introduction

Aluminum nitride (AlN) PCBs are printed circuit boards made using aluminum nitride ceramics as the substrate material. Aluminum nitride is an advanced technical ceramic material that offers high thermal conductivity, electrical insulation, and good mechanical properties. Compared to standard FR-4 PCBs, AlN PCBs provide superior performance in high power, high frequency, and high temperature applications.

Some key properties and advantages of aluminum nitride PCBs:

High thermal conductivity

  • AlN has a thermal conductivity of 170-220 W/m-K, much higher than FR-4 (0.25 W/m-K)
  • Allows heat to dissipate quickly from components, improving reliability

Electrical insulation

  • High volume resistivity (>10^14 ohm-cm)
  • Low dielectric loss, high frequency insulation
  • Prevents current leakage between traces

Dimensional stability

  • Low thermal expansion coefficient
  • Maintains integrity at high temps
  • Reduces stresses on solder joints

Excellent mechanical strength

  • Hard and rigid substrate
  • Resists fracturing/chipping
  • Withstands vibration, shock, and flexing

Good chemical resistance

  • Inert surface
  • Resists corrosion, moisture absorption
  • Improved reliability in harsh environments

AlN PCBs are ideal for applications such as aerospace avionics, radio frequency (RF) communications, power electronics, and automotive electronics. They allow for smaller, lighter, and more powerful electronic systems.

Applications of Aluminum Nitride PCBs

Aluminum nitride PCBs are used in a variety of high performance and demanding applications where standard PCB materials fall short.

Power electronics

AlN’s high thermal conductivity enables efficient heat dissipation from high power components like IGBTs, MOSFETs, and power diodes. This allows packing more power into smaller volumes. AlN PCBs are used in power supplies, motor drives, converters, inverters, etc.

Radio frequency (RF) circuits

The low dielectric loss tangent of AlN results in low signal loss at high frequencies. This makes AlN suitable for RF circuits operating at MHz to GHz frequencies such as radar, satellites, 5G, and communications.

High temperature electronics

AlN PCBs can perform well over long durations at temperatures exceeding 250°C. This allows use in geothermal drilling, aerospace engines, industrial sensors, and other extreme environments.

LED lighting

High power LEDs generate significant heat which needs effective dissipation. AlN PCBs used in LED lighting fixtures allow packing LEDs more densely.

Automotive electronics

AlN PCBs withstand high temperatures and thermal cycling stresses in automotive under-the-hood applications such as engine control units, ABS, stability control, etc.

Aerospace and defense

AlN’s stable electrical and mechanical properties under radiation and at temperature extremes make it suitable for aerospace avionics and defense systems.

Properties of Aluminum Nitride

Aluminum nitride possesses several intrinsic properties that make it well suited as a PCB substrate material for high performance electronics.

Thermal conductivity

The thermal conductivity of AlN ranges from 170 – 220 W/m-K depending on purity levels. This is nearly 700 times higher than FR-4 (0.25 W/m-K). The high thermal conductivity allows AlN PCBs to dissipate heat rapidly from hot components to prevent failures.

Electrical insulation

AlN has a very high electrical resistivity (>10^14 ohm-cm) meaning it is an excellent electrical insulator. The dielectric strength of AlN is 20 kV/mm. These properties allow AlN PCBs to prevent current leakage and withstand high voltages.

Dielectric constant

AlN has a modest dielectric constant of 8.5. This allows signal integrity at higher frequencies compared to some ceramic substrates. The loss tangent of AlN is 0.0004, which is very low.

Coefficient of thermal expansion (CTE)

AlN has a CTE of 4.5 ppm/°C. This is very close to silicon (3 ppm/°C) reducing stresses on solder joints. The dimensional stability makes AlN suitable for advanced assembly techniques.

Mechanical strength

With a flexural strength above 300 MPa, AlN substrates are hard and rigid. They resist fracturing from shock, vibration, and flexing. The compressive strength exceeds 2000 MPa.

Chemical resistance

AlN offers excellent chemical resistance and stability. It is non-reactive with most chemicals and resists corrosion or absorption of moisture. This improves reliability in harsh environments.

Manufacturing Process of AlN PCBs

Aluminum nitride PCBs are fabricated using advanced ceramic manufacturing processes. The main steps are:

Raw material preparation

High purity (99%) aluminum nitride powder is used as the starting raw material.dopants are used to enhance electrical insulation. The powder is milled to achieve uniform particle sizes.

Binder mixing

The AlN powder is mixed with organic binders and solvents to create a slurry. The binder imparts plasticity for shaping the green ceramic. Common binders are polyvinyl alcohol (PVA), polyvinyl butyral (PVB), etc.

Tape casting

The slurry is cast onto a carrier film using a doctor blade to form a thin ceramic tape. The tape thickness can range from 50 to several hundred microns.

Via punching

Vias or holes are mechanically punched into the AlN green tape according to the circuit layout. This is done using a CNC machine or a hydraulic press.

Screen printing

Using screens and squeegees, conductive patterns are screen printed layer by layer on the AlN tapes using tungsten, molybdenum, or copper pastes.

Multilayer stacking

Individual AlN tape layers are precisely stacked and laminated under temperature and pressure to form a multilayer board. Alignment is critical.

Binder burnout and sintering

The binders are slowly burnt out from the laminated stack before sintering at over 1700°C in a controlled atmosphere. This densifies the AlN layers.

Metalization

External electrical connections are made by sputtering, electroplating, or vapor depositing metal layers onto the sintered AlN substrate.

Dicing

The finished boards are diced using diamond wheel cutters into individual PCBs. Edge grinding and polishing may be done.

Electrical testing

Each AlN PCB undergoes final electrical testing and quality checks before being shipped to customers.

Design Considerations for AlN PCBs

Aluminum nitride PCBs have some unique design considerations compared to standard PCBs:

Thermal management

AlN PCBs must be paired with a suitable heat sink to dissipate heat from hot components. Thermal vias, metal cores, and interface materials should be considered.

Layer stack up

15-20 conductive layers can be screen printed on each AlN substrate considering its thickness and dielectric strength. Signal-power-ground layer sequencing is critical.

Component sizes

Components should have large thermal pads/regions to transfer heat efficiently to AlN substrate. Clearances must account for higher voltage isolation.

Board rigidity

AlN’s high rigidity requires use of solders/adhesives that withstand board flexing. Corner chamfers, edge profiles can help.

Coefficient of thermal expansion (CTE)

CTE mismatch between AlN substrate, conductors, and components should be minimized to reduce stresses.

Layout optimizations

Good grounding, decoupling capacitors, controlled impedances, and mirrored layouts maintain signal integrity at high frequencies.

Cost considerations

AlN PCBs cost more than standard PCBs. Designs should balance performance needs with cost by utilizing AlN judiciously.

Pros and Cons of Aluminum Nitride PCBs

Here are some key advantages and limitations of aluminum nitride PCBs:

Advantages:

  • Excellent thermal conductivity for heat dissipation
  • High frequency insulation due to low dielectric loss
  • Withstands high temperatures (>250°C)
  • Dimensionally and electrically stable at temperature extremes
  • Good mechanical strength and rigidity
  • Resists chemical corrosion and moisture absorption

Disadvantages:

  • Relatively expensive compared to FR-4 PCBs
  • Processing is complex requiring advanced ceramic equipment
  • Requires careful design considerations for thermal expansion, layer stack up, etc.
  • Limited board sizes and thickness constraints
  • Requires selective use for optimal cost-performance

Examples of AlN PCB Applications

Here are some examples highlighting the use of aluminum nitride PCBs in specialized applications:

  • AlN PCBs used in geothermal drilling equipment to measure wellbore temperatures up to 300°C.
  • High power RF amplifiers using AlN PCBs in radar and satellite communication systems.
  • Electric vehicle battery management systems using AlN PCBs to monitor cell voltages and temperatures.
  • LED lights for aerospace cabins using AlN PCBs to dissipate heat from high brightness LED arrays.
  • Military avionics systems using AlN PCBs due to its stability under shock, vibration and extreme temperatures.
  • IGBT power converter modules using AlN PCB direct bonded copper (DBC) substrates for superior heat dissipation.

Aluminum Nitride PCB Manufacturers

Some leading manufacturers providing aluminum nitride PCBs include:

  • Rogers Corporation (USA)
  • Maruwa Co Ltd (Japan)
  • Remtec (USA)
  • Zibo Special Ceramics Co (China)
  • Nanjing Zhongjiang New Material Science & Technology Co (China)
  • Ferrotec (USA)
  • Mitsubishi Chemical (Japan)
  • Toshiba Materials (Japan)

These companies offer a range of AlN PCB products for electronics applications requiring high thermal conductivity and electrical insulation. They use advanced in-house fabrication processes to manufacture high quality AlN PCBs.

Future Outlook for AlN PCBs

The global aluminum nitride PCB market is projected to grow at a CAGR of over 5% from 2022 to 2027. Key growth drivers include:

  • Rising adoption in electric vehicles, 5G infrastructure, avionics systems for improved thermal management.
  • Miniaturization of power electronics drives demand for AlN PCBs.
  • Growth in high temperature electronics for aerospace, automotive, and industrial use.
  • Innovation in fabrication methods aims to reduce cost and expand adoption.
  • New product development focuses on high thermal conductivity formulations.
  • Broader recognition of AlN PCB benefits compared to conventional PCBs.

Despite higher costs, aluminum nitride PCBs are likely to gain wider market acceptance given their enabling performance advantages for advanced electronics systems with extreme temperature, power, and frequency requirements.

Frequently Asked Questions (FAQs) on AlN PCBs

Here are some common questions about aluminum nitride PCBs:

Q: How is the thermal conductivity of AlN PCBs so high compared to FR-4 PCBs?

A: Aluminum nitride has an intrinsically high thermal conductivity because of strong covalent bonding between aluminum and nitrogen atoms. In contrast, FR-4 is a glass fabric reinforced epoxy composite with insulating properties.

Q: Are AlN PCBs electrically conductive or insulating?

A: AlN ceramics are highly electrically insulating with a resistivity exceeding 10^14 ohm-cm. On the other hand, the printed metallization on AlN PCBs is conductive to carry signals and power.

Q: Can AlN PCBs be used at very high frequencies beyond 10 GHz?

A: Yes, AlN PCBs can work well into the millimeter wave frequency range above 30 GHz owing to the low dielectric loss of AlN at high frequencies.

Q: What are thermal vias in AlN PCBs?

A: Thermal vias are holes filled with copper extending through the AlN PCB layers. These help conduct heat to bottom heatsinks and also reduce thermal stresses.

Q: Are AlN PCBs resistant to harsh environments and chemicals?

A: AlN offers excellent chemical inertness and stability in harsh environments including high humidity, salt spray, fuels, oils, acids etc. It also withstands high radiation.

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