Advanced Packaging Capacity Crisis: CoWoS AI Chip Hidden Gate, Full Analysis of US Stock Testing Industry Investment Opportunities

When the market is still debating the shortage of HBM memory, optical interconnects, and liquid cooling bottlenecks, the next "choke point" in the AI computing supply chain is emerging: advanced packaging. A modern AI accelerator is essentially a precision assembly of a compute die and multiple HBM stacks, and the key process that connects these components with extremely high precision is advanced packaging technology, represented by TSMC's CoWoS 2.5D/3D packaging technology. As AI chip orders continue to extend into the future, advanced packaging capacity is shifting from a question of "enough" to "how much more is needed," making the packaging stage an invisible gate determining the actual shipment volume of AI chips. ## Why Advanced Packaging Has Become a Bottleneck To understand why packaging is tight, first we need to understand the structural changes in AI chips. Traditional chips place logic, cache, and interfaces on the same die; packaging is merely "boxing." Today's AI accelerators generally follow the Chiplet route: the compute die pursues maximum area, while HBM is responsible for bandwidth, with the two connected by thousands of fine lines on a silicon interposer. This structure leads to two direct consequences: first, the area of the silicon interposer continuously increases with chip iteration, meaning less silicon can be cut for the interposer; second, the number of HBM stacks increases with each generation, simultaneously raising the demands for packaging precision and yield. In other words, even if the process node foundry pumps out horsepower, if CoWoS capacity cannot keep up, the foundry can only queue up in the warehouse waiting for packaging. This is why leading cloud vendors, while locking in GPU orders, often need to commit to additional packaging capacity upstream—packaging has evolved from a "low-margin process" at the end of the supply chain to one of the three scarce resources alongside advanced process nodes and HBM. ## Capacity Race: Who is Expanding, Who Benefits Most Currently, 2.5D packaging capacity is highly concentrated in TSMC, whose CoWoS platform handles the packaging orders for most mainstream AI accelerators on the market. Facing supply shortages, TSMC has generally maintained a doubling capacity expansion pace annually in recent years while incorporating 3D stacking technologies like SoIC into its roadmap to meet the integration demands of the tera-watt era. Meanwhile, leading customers, prioritizing supply chain security, are seeking "second sources," which presents a historical opportunity for testing houses: * Amkor: The largest testing house in the US, is building an advanced packaging line in Arizona, deeply binding to TSMC's foundry ecosystem; it is the most direct "pure packaging" target for US stock investors. * ASE Technology Holding (ASE): Global testing leader, forming a 2.5D capacity synergy with TSMC through its silicon products, with the AI business share rapidly increasing, showing significant elasticity. * Samsung and Intel: Each enters with its proprietary advanced packaging platforms, competing for second supplier share; Intel also competes as a foundry, making the situation nuanced. * Mature Process Manufacturers like ASE: Shifting to silicon interposer manufacturing, turning surplus mature capacity into a "shovel seller" business for packaging upstream. Equipment and materials are equally noteworthy. Advanced packaging imposes new requirements on photolithography, electroplating, cutting, and grinding equipment; major US equipment giants like Applied Materials, GlobalFoundries, and KLA consider advanced packaging a high-growth business line. Meanwhile, new technology routes like hybrid bonding and glass substrates may reshape the entire competitive landscape in the long term. ## Three Main Lines for US Stock Investors

• Main Line One: Platform Leader. TSMC is a leader in leading-edge process technology and the dominant player in advanced packaging capacity. Packaging business is one of the fastest-growing segments in its revenue structure, suitable for allocation as a base position.
• Main Line Two: High-Elasticity Targets. Companies like Anchor and Sunlight have relatively smaller market caps and high business purity. Once supply orders increase, their earnings elasticity is greater than that of leaders, but caution is needed regarding customer concentration and yield ramp-up risks.
• Main Line Three: Equipment Seller Logic. Equipment and materials companies benefit from industry expansion. Regardless of which packaging company ultimately wins, equipment orders will be placed, offering higher certainty.

Risks and Variables: The Sword of Damocles in Oversupply

Any narrative of "shortage" must be viewed dialectically. The packaging industry has repeatedly cycled through "shortage—expansion—oversupply." If AI capital expenditure growth slows in two years, advanced packaging capacity expanded today might be phased oversupply. Furthermore, three variables need attention: First, as customer self-developed ASICs increase, packaging demand structure may shift from "small quantities of large chips" to "large quantities of medium chips," benefiting independent foundries with larger capacity scales. Second, if new routes like glass substrates and panel-level packaging accelerate, technological barriers in silicon interlayers may be partially bypassed. Third, driven by geopolitical factors, regional diversification of the packaging segment is accelerating, with the US, Japan, and Southeast Asia competing for capacity placement. This brings both capital expenditure opportunities and higher manufacturing costs for the entire industry.

For investors, advanced packaging is essentially a manifestation of the "certainty premium" in AI computing power: the companies closer to the scarce bottlenecks have stronger bargaining power when training and inference demand continues to exceed expectations; conversely, if demand proves false, the pullback in high-valuation segments will be more severe. When including these in a portfolio, it is advisable to compare them with previously analyzed "bottleneck series" such as HBM, liquid cooling, and optical interconnect CPO, to diversify single-point risks from a supply chain perspective and avoid betting positions on a single segment.

Conclusion: The AI computing power race is underway, shifting from "whether there are chips" to "whether there is capacity for every process step." Advanced packaging, once an inconspicuous segment, is becoming the key variable determining the actual delivery progress of AI chips. For US investors, understanding packaging means understanding the next source of alpha in the AI computing supply chain.