Fastening technologies play a critical role in battery pack assembly, ensuring structural integrity, electrical connectivity, and thermal management while accommodating the unique challenges of battery systems. Unlike welding, which is covered separately, mechanical fastening and adhesive bonding offer distinct advantages in terms of modularity, repairability, and compatibility with diverse materials. This review examines key fastening methods, their applications, and performance considerations in battery pack manufacturing.

### Mechanical Fastening Methods

Mechanical fasteners provide reliable, reversible connections suitable for high-volume production and maintenance. Common techniques include bolts, screws, clips, and self-piercing rivets (SPRs).

**Self-Piercing Rivets (SPRs)**
SPRs are widely used in battery pack assembly due to their ability to join dissimilar materials without pre-drilled holes. The process involves a semi-tubular rivet that pierces the top layer of material and flares into the bottom layer under pressure, forming a mechanical interlock. SPRs are particularly effective for aluminum-to-aluminum or aluminum-to-steel joints, common in battery enclosures and busbar connections.

Advantages:
- No heat input, avoiding thermal distortion or damage to sensitive battery components.
- High joint strength and fatigue resistance, critical for automotive applications subjected to vibration.
- Compatibility with automated assembly lines, enabling high throughput.

Limitations:
- Requires access to both sides of the joint, which may complicate pack design.
- Limited to ductile materials; brittle substrates like some composites may crack.

**Threaded Fasteners**
Bolts and screws are used for modular pack designs where disassembly for maintenance or recycling is necessary. They are common in busbar connections, module-to-pack integration, and enclosure sealing.

Advantages:
- Adjustable clamping force, ensuring consistent electrical contact resistance.
- Ease of disassembly for repair or end-of-life recycling.

Limitations:
- Higher assembly time compared to rivets or adhesives.
- Risk of loosening under vibration unless paired with locking mechanisms (e.g., thread-locking adhesives).

**Clips and Snap-Fits**
Plastic or metal clips are used for lightweight, low-stress applications such as mounting battery management systems (BMS) or securing cooling plates. Snap-fit designs reduce part count and simplify assembly.

Advantages:
- Tool-less assembly, reducing production complexity.
- Cost-effective for high-volume manufacturing.

Limitations:
- Lower mechanical strength compared to rivets or bolts.
- Limited to non-structural applications.

### Adhesive Bonding

Adhesive bonding is increasingly adopted in battery packs for its ability to distribute stress evenly, seal joints, and isolate vibrations. Key adhesive types include structural epoxies, conductive adhesives, and thermally conductive formulations.

**Structural Adhesives**
Epoxy and acrylic adhesives bond enclosure panels, module frames, and cooling components. They provide continuous load transfer, reducing stress concentrations that could lead to fatigue failure.

Advantages:
- Uniform stress distribution, improving durability under mechanical loads.
- Sealing properties, preventing moisture or contaminant ingress.
- Compatibility with dissimilar materials (e.g., metal-to-composite).

Limitations:
- Cure time can slow production; UV or heat-curing formulations mitigate this.
- Surface preparation (cleaning, priming) is critical for bond reliability.

**Electrically Conductive Adhesives (ECAs)**
ECAs replace soldering or welding in some busbar and cell-connection applications. They contain conductive fillers (e.g., silver, carbon) to ensure low-resistance paths while providing mechanical adhesion.

Advantages:
- No thermal stress, avoiding damage to heat-sensitive battery materials.
- Fine-pitch applicability, useful for compact pack designs.

Limitations:
- Higher resistivity compared to welded joints, potentially increasing energy loss.
- Long-term reliability under thermal cycling requires careful formulation selection.

**Thermally Conductive Adhesives**
These adhesives bond cooling plates or heat sinks to battery modules while enhancing thermal transfer. Filled with ceramics or graphite, they bridge microscopic gaps to improve heat dissipation.

Advantages:
- Eliminates the need for mechanical fasteners in thermal interfaces.
- Fills irregularities, reducing thermal contact resistance.

Limitations:
- Bond line thickness must be controlled to optimize thermal performance.
- Cure shrinkage can induce stresses if not properly managed.

### Hybrid Fastening Systems

Combining mechanical fasteners with adhesives leverages the strengths of both methods. For example, a structural adhesive may provide sealing and vibration damping, while SPRs or bolts ensure immediate fixture strength during adhesive curing.

Applications:
- Battery enclosure assembly: Adhesives seal seams, while rivets provide instant handling strength.
- Busbar connections: Conductive adhesives supplement bolted joints to reduce contact resistance and prevent loosening.

Advantages:
- Reduces reliance on precise fastener placement, as adhesives accommodate minor misalignment.
- Enhances long-term reliability by mitigating vibration-induced fastener fatigue.

Limitations:
- Increased process complexity due to multiple joining steps.
- Adhesive cure time may still bottleneck production without optimized workflows.

### Performance Considerations

**Vibration and Fatigue Resistance**
Battery packs in electric vehicles face constant vibration, necessitating fasteners with high fatigue resistance. SPRs and adhesives outperform threaded fasteners in cyclic loading tests, with hybrid systems showing the best results.

**Thermal Expansion Mismatch**
Dissimilar materials in packs (e.g., aluminum enclosures, steel brackets) expand at different rates. Adhesives with flexible formulations accommodate this mismatch better than rigid fasteners alone.

**Electrical Contact Resistance**
For busbar connections, maintaining low resistance is critical. Conductive adhesives combined with bolts ensure stable contact pressure, preventing oxidation-induced resistance increases over time.

**Repairability and Recycling**
Mechanical fasteners allow easier disassembly for repair or material recovery. Adhesives complicate recycling unless designed for debonding (e.g., thermally reversible adhesives).

### Industry Trends

Automakers and battery manufacturers are increasingly adopting SPRs and structural adhesives to reduce weight and improve pack energy density. The shift toward modular designs also favors threaded fasteners for serviceability. Meanwhile, advancements in cure-on-demand adhesives aim to address production speed limitations.

In summary, the choice of fastening technology depends on trade-offs between strength, speed, cost, and repairability. While no single method is universally optimal, hybrid approaches and material-specific solutions are driving innovation in battery pack assembly.