Polished Alumina vs AlN Substrates for AI Server Power Modules: Thermal Performance, Flatness and Metallization - Shanghai Xinkehui New Materials Co., Ltd.

The rapid increase in AI computing density is changing the requirements placed on power conversion hardware.

AI servers increasingly require compact, high-efficiency power supplies, DC/DC converters, power stages and advanced thermal-management systems. As power density rises, the ceramic substrate underneath power semiconductor devices becomes increasingly important.

Two commonly considered materials are:

Polished alumina (Al₂O₃)
Azotek glinu (AlN)

Both provide electrical insulation, dimensional stability and compatibility with semiconductor packaging processes.

However, they behave very differently in terms of:

Przewodność cieplna
Coefficient of thermal expansion
Wykończenie powierzchni
Flatness
Metallization
Mechanical reliability
Koszt

Selecting between polished alumina and AlN should therefore be based on the actual thermal and packaging requirements of the power module rather than material reputation alone.

1. Why Ceramic Substrates Matter More in AI Server Power Systems

Power architecture for AI infrastructure is moving toward significantly higher power density.

Next-generation AI data centers are also moving toward higher-voltage DC distribution architectures, including 800 VDC systems, to reduce current, conversion stages and distribution losses.

This affects components such as:

Server power supply units
High-power DC/DC converters
Power distribution modules
Solid-state circuit protection
SiC power modules
GaN power stages
Auxiliary power converters

As semiconductor switching performance improves, more heat can be generated in a smaller footprint.

The ceramic substrate has to perform two apparently conflicting tasks:

electrically isolate the semiconductor while efficiently transferring heat away from it.

This is where the difference between alumina and AlN becomes important.

2. Thermal Conductivity: The Largest Difference

Thermal conductivity is usually the first specification engineers compare.

Typical values for electronic-grade substrates are approximately:

Własność	Alumina Al₂O₃	Aluminum Nitride AlN
Przewodność cieplna	~24 W/m·K	~170 W/m·K
CTE	~7–9 ×10⁻⁶/K	~5–6 ×10⁻⁶/K
Dielectric strength	≥15 kV/mm	≥15 kV/mm
Typical bending strength	≥500 MPa	≥450 MPa

Representative CeramTec substrate specifications list thermal conductivity of approximately 24 W/m·K for alumina and 170 W/m·K for AlN, illustrating why AlN is attractive for high-power-density electronics.

The thermal conductivity of AlN can therefore be several times higher than conventional alumina.

That difference becomes important when heat must move vertically from a semiconductor die through the substrate toward:

Copper layers
Baseplates
Radiatory
Cold plates
Liquid-cooling structures

3. When Does AlN Actually Make a Difference?

Higher thermal conductivity does not automatically mean every AI server power module should use AlN.

The benefit depends on where the dominant thermal resistance exists.

Consider the thermal path:

Semiconductor junction → die attach → metallization → ceramic → copper/baseplate → TIM → heat sink

If the ceramic layer represents a significant percentage of total thermal resistance, changing from alumina to AlN can provide meaningful improvement.

But if the main limitation is:

Poor die attach
Thick thermal interface material
Inadequate cooling
Small heat-sink area
Poor contact pressure

then replacing alumina with AlN may produce less improvement than expected.

Thermal simulation should therefore examine the complete stack-up.

4. AlN Is Particularly Attractive for High Heat-Flux Power Stages

AlN becomes especially interesting where several conditions occur simultaneously:

High semiconductor power density
Limited module footprint
High switching frequency
High junction temperature
SiC or GaN devices
Compact liquid cooling
High continuous load

Commercial power-electronics substrates based on AlN can provide thermal conductivity around 170 W/m·K, and AlN-based metallized substrates are used specifically for high-power-density applications.

For AI server power systems, these conditions can occur in high-power conversion stages where designers are attempting to reduce:

Module volume
Odporność termiczna
Junction temperature
Cooling-system size

5. Why Alumina Is Still Important

If AlN conducts heat much better, why is alumina still widely used?

Because thermal conductivity is only one design variable.

Polished alumina offers several important advantages.

Mature Manufacturing

Alumina substrate manufacturing is extremely mature.

Large volumes can be produced with controlled:

Thickness
Flatness
Wykończenie powierzchni
Dimensional tolerance
Metallization

Cost Efficiency

Alumina is generally more economical than AlN.

For applications where heat flux remains moderate, the additional thermal performance of AlN may not justify the extra substrate cost.

Excellent Surface Quality

Fine-grain polished alumina can achieve extremely smooth surfaces suitable for precision metallization.

CeramTec specifically notes that fine alumina surfaces can support demanding thin-film processes, including sputtered metal layers and laser structuring.

This makes polished alumina attractive for applications requiring:

Thin-film circuits
Precision resistors
RF structures
Sensor circuits
Gate-drive circuits
Control electronics

6. Polished Alumina Is Different From Standard Alumina Substrate

The word polished is important.

Not every alumina substrate provides the same surface condition.

Standard as-fired ceramic may contain:

Surface texture
Grain-related topography
Local pits
Thickness variation

For many thick-film applications, this may be acceptable.

But thin-film metallization can require significantly better surfaces.

Precision polishing can improve:

Chropowatość powierzchni
Local flatness
Metal-film continuity
Photolithography consistency
Fine-line patterning

Therefore, an RFQ should not simply specify:

“96% alumina substrate.”

Surface requirements should also be defined.

7. Surface Roughness Becomes Important for Thin-Film Metallization

Thin-film processes may use materials such as:

These films may be deposited using:

Sputtering
Evaporation
PVD

When metallization thickness becomes small, the ceramic surface underneath strongly influences the resulting conductor.

Excessive roughness can contribute to:

Non-uniform film thickness
Fine-line variation
Poor adhesion consistency
Photolithography difficulties
Increased conductor defects

For these applications, polished alumina can remain highly competitive even when maximum thermal conductivity is not required.

8. AlN Can Also Be Polished

AlN should not be considered a rough, thermal-only substrate.

Precision AlN substrates can also be:

Ground
Lapped
Polished
Laser machined
Metallized

For high-end power semiconductor packaging, suppliers can control:

Thickness
TTV
Bow
Warp
Chropowatość powierzchni
Edge quality

However, producing high-flatness, highly polished AlN generally requires tighter process control and may increase cost.

This makes specification optimization important.

Do not request optical-level polishing if the metallization process does not require it.

9. Flatness Is Critical for Power Module Assembly

Thermal conductivity receives most of the attention, but flatness can be equally important during module manufacturing.

A substrate that is not sufficiently flat can create problems during:

Die attach
Spiekanie srebra
Soldering
Baseplate attachment
Heat-sink mounting

Excessive bow may produce uneven bond-line thickness.

That can lead to:

Thermal hot spots
Bonding voids
Local stress
Poor contact
Assembly yield loss

Rogers notes that substrate bending is a critical parameter during die assembly and later attachment to baseplates or heat sinks. Bending is affected by substrate dimensions, aspect ratio, copper layout and ceramic/copper thickness.

10. Flatness and Bow Are Not Only Ceramic Problems

When investigating substrate warpage, engineers should not automatically blame the ceramic.

The finished assembly is a composite structure.

Na przykład:

Copper / Ceramic / Copper

Each layer has different:

Thickness
Elastic modulus
Rozszerzalność cieplna
Residual stress

An asymmetric copper pattern can cause the complete substrate to bend.

Factors affecting finished-substrate bow include:

Ceramic thickness
Copper thickness
Copper coverage
Circuit symmetry
Substrate length
Substrate width
Metallization process
Thermal history

Therefore, ceramic flatness should be specified both before and, where necessary, after metallization.

11. CTE Is Another Advantage of AlN

Coefficient of thermal expansion becomes increasingly important when mounting semiconductor dies directly onto ceramic-based substrates.

Representative substrate data shows approximately:

Tlenek glinu: 7–9 ppm/K
AlN: 5–6 ppm/K

AlN therefore has a thermal expansion behavior closer to many semiconductor materials.

Rogers similarly reports AlN DBC CTE around 4.7 ppm/K in one commercial substrate system.

A closer CTE match may help reduce thermo-mechanical stress in:

Solder layers
Sintered silver
Semiconductor dies
Metallization interfaces

This becomes particularly valuable during repeated power cycling.

12. Metallization Options for Alumina

Alumina supports a wide range of metallization technologies.

Depending on circuit requirements, these may include:

Thick-Film Metallization

Screen-printed conductive pastes can be fired directly onto alumina.

Typowe zastosowania obejmują:

Hybrid circuits
Resistors
Sensors
Control modules

Thin-Film Metallization

Polished alumina is particularly suitable for sputtered thin-film systems.

This enables:

Fine conductor lines
Precision circuits
High-density interconnects

DBC

Direct Bond Copper is widely used when thick copper conductors and high current capability are required.

DBC forms a strong copper-to-ceramic interface and is an established technology for power electronics.

13. Metallization Options for AlN

AlN can also support multiple metallization technologies.

These include:

Thin-film metallization
Thick-film systems designed for AlN
DBC
AMB

However, bonding copper directly to non-oxide ceramics requires different interface chemistry.

Rogers notes that conventional DBC naturally works with oxide ceramics such as alumina, while AlN requires surface treatment/oxidation for DBC or can be joined using Active Metal Brazing.

This distinction is important when comparing manufacturing cost and process complexity.

14. DBC vs AMB for AlN

Two common solutions for high-current power modules are:

AlN DBC

oraz

AlN AMB

DBC uses a high-temperature bonding process to create a copper/ceramic structure.

AMB uses an active brazing system that enables bonding to non-oxide ceramics.

The ideal choice depends on:

Copper thickness
Ceramic thickness
Thermal cycling requirements
Circuit geometry
Reliability requirements
Supplier process capability

The metallization technology should therefore be specified together with the ceramic material.

15. Copper Thickness Also Changes Thermal Performance

When discussing ceramic thermal conductivity, engineers sometimes overlook the copper layer.

Power substrates may contain relatively thick copper to provide:

High current capacity
Heat spreading
Mechanical interconnection

Commercial ceramic power substrates can use copper layers in the hundreds-of-micrometers range.

This means overall thermal performance depends on the full structure:

Copper thickness + ceramic thickness + interface quality

not simply the ceramic thermal conductivity listed in the material datasheet.

16. Metallization Adhesion Should Be Qualified

For AI server power modules that experience repeated temperature changes, metallization reliability is critical.

Qualification may evaluate:

Peel strength
Interface voids
Cykl termiczny
Metallization thickness
Pattern dimensional accuracy
Zanieczyszczenie powierzchni
Edge condition

A substrate with excellent thermal conductivity provides little value if the copper-to-ceramic interface fails during service.

17. Thermal Cycling Can Change the Material Decision

AlN has outstanding thermal conductivity, but material selection must consider mechanical reliability as well.

Ceramic/copper assemblies experience stress during repeated heating and cooling because copper and ceramic expand differently.

Published power-electronics substrate studies and manufacturer guidance emphasize that thermal cycling performance depends not only on the ceramic material but also on copper thickness, ceramic thickness, geometry and bonding technology.

This is another reason not to select a substrate based on thermal conductivity alone.

18. Alumina May Be the Better Choice When

Polished alumina can be a good choice when:

Heat generation is moderate
Substrate cost is sensitive
Thin-film metallization is required
Extremely smooth surfaces are important
The substrate contains control or sensing circuitry
Mature high-volume sourcing is preferred
Thermal resistance elsewhere dominates the system

Typical applications may include:

Gate-drive modules
Monitoring circuits
Sensor electronics
Auxiliary power circuits
Lower-power converters

19. AlN May Be the Better Choice When

AlN becomes more attractive when:

Heat flux is high
Power density is increasing
Semiconductor junction temperature is critical
SiC or GaN devices are used
Module size must be reduced
Cooling area is limited
Ceramic thermal resistance is a bottleneck
CTE matching is important

Potential applications include:

High-power DC/DC modules
High-density server PSUs
SiC power stages
High-frequency power conversion
Compact liquid-cooled power modules

20. Do Not Select the Material Before Defining the Stack-Up

For custom power electronics substrates, the RFQ should ideally define the complete structure.

Important information includes:

Ceramic material
Ceramic purity or grade
Substrate length and width
Thickness
Thickness tolerance
TTV
Flatness
Bow
Warp
Chropowatość powierzchni
Metallization material
Metallization thickness
Circuit pattern
Minimum line width
Minimum spacing
Edge metallization
Via requirements
Operating voltage
Temperatura pracy
Power dissipation
Cooling method

Without this information, comparing substrate quotations can become misleading.

21. Suggested Incoming Inspection

For precision AI power-module substrates, incoming inspection may include:

Ceramic Geometry

Measure:

Length
Width
Thickness
TTV
Flatness
Bow
Warp

Surface Quality

Inspect:

Scratches
Chips
Edge defects
Zanieczyszczenie powierzchni
Polishing marks

Metallization

Verify:

Metal thickness
Pattern dimensions
Registration
Adhesion
Surface condition

For metallized power substrates, additional inspection may include interface quality and thermal-performance evaluation.

22. Alumina vs AlN: Practical Selection Table

Requirement	Polished Alumina	AlN
Przewodność cieplna	Umiarkowany	Bardzo wysoki
Polerowanie powierzchni	Doskonały	Excellent with proper processing
Thin-film compatibility	Doskonały	Doskonały
Thick-film compatibility	Mature	Dostępne
DBC compatibility	Doskonały	Available with suitable process
AMB compatibility	Dostępne	Doskonały
CTE match to semiconductor	Umiarkowany	Lepiej
High heat-flux capability	Umiarkowany	Doskonały
Koszt	Niższy	Wyższy
High-volume availability	Doskonały	Dobry
Typical use	Cost-sensitive / moderate power	High-power-density modules

The table should be treated as a starting point rather than a universal material ranking.

23. Why AI Power Electronics May Increase AlN Demand

The important industry change is not simply that AI servers consume more power.

The architecture of power delivery itself is changing.

Higher rack power, higher-voltage DC distribution, increased use of wide-bandgap semiconductors and more aggressive liquid cooling all encourage designers to reduce thermal resistance and increase power density.

That environment strengthens the case for materials such as AlN.

But it does not eliminate alumina.

Instead, ceramic substrates are likely to become more application-specific:

Polished alumina for cost-sensitive, precision-metallized and moderate-thermal-load circuits

oraz

AlN for high-heat-flux, high-density power conversion.

24. Information to Provide When Requesting a Custom Ceramic Substrate

To obtain a useful quotation, provide:

Zastosowanie
Alumina or AlN preference
Substrate dimensions
Thickness
Ilość
Required thermal conductivity
Chropowatość powierzchni
Flatness
TTV
Metallization system
Metal thickness
Pattern drawing
Operating voltage
Temperatura pracy
Semiconductor type
Cooling configuration

If the material has not yet been selected, thermal-load information can help the ceramic supplier evaluate both alumina and AlN options.

Wnioski

For AI server power modules, the choice between polished alumina and aluminum nitride should not be reduced to a single thermal-conductivity number.

AlN offers a major thermal advantage and a CTE closer to semiconductor materials, making it particularly attractive for compact, high-power-density SiC and GaN power stages.

Polished alumina remains highly competitive when cost, surface quality, fine metallization and manufacturing maturity are more important than maximum heat conduction.

The correct substrate therefore depends on the complete package:

Thermal load → Ceramic thickness → Flatness → Metallization → Cooling → Reliability

As AI power density continues to increase, substrate suppliers will increasingly need to control not only ceramic composition but also polishing, dimensional tolerances, metallization and finished-substrate warpage.

FAQ

Is AlN always better than alumina for AI server power modules?

No. AlN provides much higher thermal conductivity, but polished alumina can be more economical and is highly suitable for precision thin-film metallization and moderate-power applications. The best material depends on total thermal resistance and module design.

How much higher is the thermal conductivity of AlN than alumina?

Representative commercial electronic ceramic data lists approximately 170 W/m·K for AlN compared with around 24 W/m·K for alumina, although actual values depend on ceramic grade and manufacturing process.

Can AlN substrates be metallized with copper?

Yes. AlN can be used with DBC under suitable processing conditions and is also compatible with Active Metal Brazing (AMB). The correct method depends on copper thickness, reliability and module design requirements.