No matter how smart your AI chip is, it still needs low CTE to stay in shape.

 

As AI chip performance advances, maintaining packaging stability has become a major challenge. By focusing on the coefficient of thermal expansion (CTE), we can explore how materials precisely control dimensional changes to ensure long-term, reliable system operation.

 

 

(source: pexel.com)

 

With AI computing power surging and chip sizes along with packaging layers expanding, the thermal stress on packaging structures rises significantly.

 

 

Whether it is an AI accelerator delivering hundreds of TOPS or a high-frequency, low-latency network switch, if the packaging deforms, warps, or fails due to temperature variations, even the most advanced algorithms cannot perform optimally.

 

 

Packaging stability has thus become the hidden gatekeeper of reliable AI computation. Among the key factors,

 

 

the material's coefficient of thermal expansion (CTE)

 

 

stands out as a core determinant influencing warpage and stress control.

 

 

What is CTE?

 

 

CTE describes a material's capability to expand or contract in response to temperature changes. As the temperature rises, the vibrations of the material's molecular chains increase, causing its volume and dimensions to expand. The higher the CTE value, the faster the material expands when heated.

 

 

Conversely, the lower the CTE, the better the dimensional stability. In multi-layer packaging structures, excessive differences in CTE between materials can easily generate thermal stress, leading to warpage, delamination, cracking, and other reliability issues. Therefore, designing material systems with well-matched CTE and coordinated thermal expansion has become a core challenge in packaging material design.

 

 

Key factors affecting CTE

 

 

The CTE of a material is not a single property but the result of interactions among multiple structural and compositional factors. First of all, one of the factors lies the molecular structure; for example, the rigid aromatic rings in epoxy resins or the flexible Si–O bonds in organosilicon resins significantly influence the thermal expansion behavior of molecular chains.

 

 

Second, crosslinking density is also an important variable. The higher the degree of crosslinking, the more restricted the mobility of molecular chains, and the CTE generally decreases accordingly.

 

 

Third, the filler system exerts a more direct influence on CTE. Incorporating low-CTE inorganic fillers such as silica (SiO₂), alumina (Al₂O₃), or silicon nitride (Si₃N₄) can substantially reduce the overall thermal expansion of the material.

 

 

In addition, interfacial adhesion, curing shrinkage, moisture absorption, and even filler particle size and distribution uniformity can all fine-tune the CTE performance. Therefore, from molecular design to filler engineering, every step matters in the design of low-CTE material.

 

 

Table 1. CTE of Common Materials

Category	Main Material	Typical CTE (ppm/°C)	Applications
Epoxy-modified systems	Bisphenol-F epoxy	30–50	Underfill, molding compounds
Benzoxazine systems	Benzoxazine	20–30	IC substrates, high-frequency boards
Polyimide	Polyimide	10–30	COF, FPC
Polyether systems	Aromatic polyether (e.g., ELPAC HC-G)	~20	Servers, high-frequency communication boards
Silica-filled	SiO₂ filler	0.5	Underfill, encapsulants
Aluminum nitride	AlN	4–5	Heat-dissipation substrates, IGBT
Silicon nitride	Si₃N₄	2–3	High-power packaging
Silicon carbide	SiC	4–5	High-temperature, high-voltage modules
Molybdenum	Mo	4–5	Thermal buffer layers

 

 

Application Trends and Key Design Considerations for Low-CTE Materials

 

 

(source: pexel.com)

 

 

1. Low Warpage Resin for 5G/AI Packaging (Stress Compensation within Multilayer Composite Adhesives)

 

 

Currently, in 5G packaging and AI high-performance computing packaging, to suppress warpage of large chip modules during thermal cycling, resin systems typically employ rigid-ring-structured modified epoxy combined with a high loading of silica or silicon nitride fillers. Furthermore, through multilayer composite stacking designs, the differing thermal expansion behaviors between layers can compensate for each other, thereby enhancing the overall structural stability.

 

 

2. Optical Adhesives for Display Panels (Dimensional Stability Control)

 

 

For optical adhesives used in display panels, the material must simultaneously exhibit low CTE, high transparency, and excellent yellowing resistance. Systems often employ cycloolefin structures or low-polarity polyether-modified formulations to prevent long-term dimensional drift that could compromise optical alignment precision.

 

 

3. Low-Deformation Resins for Advanced Substrates (Fan-Out, Chiplet, etc.)

 

 

In the field of advanced substrate applications (such as Fan-Out and Chiplet integration), development focuses on thin resin systems with low deformation and high dimensional control precision. Resin formulations typically incorporate high crosslinking density and narrow particle-size-distributed inorganic fillers to further suppress overall thermal expansion, while maintaining good flowability and processing stability. These characteristics support the stable operation of future multi-chip integration architectures.

 

 

Mismatched CTE? Say Goodbye to Your Signals
 

 

In AI and high-frequency applications, signal stability and accuracy (Signal Integrity, SI) are especially critical. Even a deformation of several tens of micrometers in the package can alter the length of signal lines, affecting transmission timing and impedance matching.

 

 

More seriously, in multi-layer package architectures (such as 2.5D, Fan-Out, or Chiplet), any warpage in a single layer can lead to poor solder joint contact, signal misalignment, or layer skipping. Selecting low-CTE adhesives is therefore not just about crack resistance—it serves as the first line of defense for maintaining signal stability.

 

 

From Low CTE to Negative CTE: The Next Frontier in Precision Thermal Management

 

 

As AI chip performance increases and package complexity grows, simple low-CTE materials are no longer sufficient. With multi-chip integration, multi-layer stacking, and localized high-temperature zones becoming more common, thermal expansion compensation is moving into the realm of negative CTE materials.

 

 

Special fillers such as zirconium tungstate (ZrW₂O₈) and lithium aluminosilicate (β-eucryptite) exhibit slight contraction upon heating. When combined with conventional resins, they yield more stable and balanced composite adhesives, thereby further minimizing overall dimensional changes in the package.

 

 

Table 2. Negative CTE Materials and Their Application Potential

Material	Negative CTE Range (ppm/°C)	Description	Application Potential
ZrW₂O₈ (Zirconium Tungstate)	-9 ~ -12	Stable negative CTE from room to high temperature; mechanism based on lattice “scaffold effect”	Composite materials compensation, precision packaging
HfW₂O₈ (Hafnium Tungstate)	-5 ~ -10	Structurally similar to ZrW₂O₈, but with higher stability	High-end electronic materials
β-Eucryptite (LiAlSiO₄)	-1 ~ -2	Ceramic system; negative expansion due to lithium ion displacement within structure	Glass-ceramic substrates, optical packaging
ScF₃ (Scandium Fluoride)	-10 ~ -13	Non-oxide system, yet exhibits stable negative CTE	Under research; limited current applications
Certain intermetallic compounds	Varies with composition	e.g., Invar alloys (not negative CTE, but extremely low)	Precision machinery, packaging

 

 

 

 

Table 3. Possible Mechanisms of Negative CTE Material                                                                                                                                                                                                                                                                                                                                           

Mechanism	Description	Representative Materials
Rigid Unit Modes (RUMs)	Rigid polyhedra within the crystal (e.g., oxygen octahedra) tilt and rotate upon temperature changes, causing overall structural contraction	ZrW₂O₈, HfW₂O₈
Phase-Transition-Induced Contraction	Material undergoes a phase transition at specific temperatures, accompanied by volume shrinkage	β-Eucryptite
Phonon Mode Contribution	Specific lattice vibration modes lead to shortening of average bond lengths	ScF₃
Porous Structure Collapse	Micropores or interlayer voids shrink as temperature rises	Certain MOFs (Metal-Organic Frameworks)

 

 

In practical applications, negative CTE materials alone cannot fully solve dimensional stability issues; precise coordination through overall material design is required.

 

 

Considering processability, interfacial stability, and overall thermo-mechanical balance, negative CTE materials are typically combined with other low-CTE fillers and incorporated using appropriate dispersion techniques and localized stress-buffering designs. This approach allows their thermal expansion compensating properties to be fully utilized, enhancing both long-term reliability and dimensional stability of the package.

 

 

The following are some possible design strategies:

 

• Composite Filler Compensation Design

Negative CTE fillers (e.g., ZrW₂O₈, β-eucryptite) are mixed with conventional low-CTE fillers (e.g., SiO₂, Al₂O₃). By carefully adjusting the ratios, the overall thermal expansion behavior can be balanced.

 

• Filler Content Control

Negative CTE fillers are typically used in controlled amounts (e.g., 5–20 wt%) to avoid excessive loading, which can deteriorate rheology, complicate processing, or weaken the filler–matrix interface.

 

 

• 
  Surface Modification for Enhanced Dispersion

Negative CTE fillers are often treated with coupling agents (e.g., silane treatments) to improve dispersion within the resin system and enhance interfacial adhesion, ensuring stable long-term performance.

 

 

• Localized Stress-Buffer Layer Design

In large package substrates, interposer layers, or regions with concentrated stress, localized negative CTE functional layers can be incorporated to absorb dimensional mismatches, improving overall package lifetime.

 

 

• Multilevel Material Integration Strategy

By combining rigid resin structures, high crosslink density, and negative CTE compensation, an integrated material system can be created with stable thermo-mechanical behavior for advanced packaging applications.

 

 

Conclusion: Stability Is the Key

 

AI chips may run fast, but only with strong packaging can systems remain stable over the long term.

 

The future of packaging materials isn't about chasing a single performance limit, but about orchestrating harmony—precisely aligning low-CTE and negative-CTE materials to support a high-efficiency, stable AI computing structure.

 

A low CTE has never been just a nice number in materials engineering; it's the foundation that must be safeguarded from the very start of design.

 

As we push the limits of computing power, let's not forget:

 

Sometimes, stability matters more than speed.