The Semiconductor Back-End Process Explained: A Complete Picture of Product Packaging

Semiconductors are the backbone of our digital society. Packaging technology—known as the back-end process—is attracting unprecedented attention. While the miniaturization of transistors and interconnects has long driven performance gains, physical and cost limits are emerging, making advanced packaging the key to future progress.

Semiconductor manufacturing is broadly divided into front-end and back-end processes. After circuits are formed on wafers in the front-end, the chips are not yet complete. In the back-end process, chips are separated, electrically connected to substrates, packaged and inspected to become finished products ready for integration into electronic devices. This article explains, step-by-step, the journey by which semiconductors become usable products, revealing the technical depth and importance of the back-end process.

Semiconductor Back-End Process Overview

In the front-end process, microscopic circuits such as integrated circuits are fabricated on silicon wafers with nanometer (nm)-level precision, building in vast numbers of transistors.

In the back-end process, the individual chips fabricated on the wafer must be separated and electrically connected to a substrate. Whereas wire bonding—using fine metal wires—was once common, flip-chip mounting that uses protruding connection terminals called bumps has now become mainstream.

In flip-chip mounting, bumps are first formed on the chip electrodes while the wafer is still intact. The wafer is then diced to singulate the chips. Next, each chip is flipped and bonded onto the substrate. After bonding, an underfill material is injected into the gap between the chip and substrate, and finally the entire assembly is encapsulated with resin molding. The products then undergo burn-in, electrical characteristic testing and reliability testing to screen out defects. Good products receive laser marking and are shipped as finished devices.

Many semiconductor manufacturers conduct some back-end manufacturing in-house, but—just as much as front-end manufacturing is handled by foundries—the back-end is often performed by specialized outsourced semiconductor assembly and test (OSAT) companies. Through this division of labor—foundries for front-end, OSATs for back-end—each party leverages specialized knowledge to produce semiconductors efficiently.

Major Back-End Processes

The back-end process is a continuous series of steps that enable chips to go from wafer into product-grade devices. The main processes are as follows:

Bump Formation

Before singulating the chips, bumps—microscopic protruding connection terminals—are formed on each chip's electrode pads at the wafer level. Formation methods include electroplating, ball placement and screen printing. Precise control of bump height and shape are essential; uniform bump size reduces defects in subsequent mounting.

Dicing

The silicon wafer is precisely cut and divided into individual chips, also called dies. A dicer cuts out chips with micrometer-level accuracy. Chips are separated using diamond blades, known as saw blades, or lasers. Proper cutting conditions and a clean environment are critical to prevent chipping or micro-damage.

Bump Bonding (Flip-Chip Bonding)

Each chip is rotated 180 degrees, aligned to the substrate's connection pads and bonded. Bonding establishes both electrical and mechanical connection using heat, pressure and/or ultrasonic energy.

Underfill Application

After bump bonding, underfill is dispensed and drawn by capillary action into the narrow gap between the chip and substrate. This greatly enhances the reliability of the chip–substrate interconnection by distributing mechanical stress and improving thermal cycling endurance.

Molding

The entire assembly is encapsulated with resin. Using specialized molding equipment, epoxy resin or similar materials are injected to protect the chip and interconnects from the external environment. The cured resin shields against impact, moisture and temperature changes, ensuring long-term stable operation. Resins with appropriate heat resistance and insulation are selected by application, and quality is ensured through uniform molding and void elimination.

Final Testing

Finished chips undergo comprehensive performance and reliability verification through the following tests and inspections:

• Burn-in testing: Devices are powered at elevated temperature and voltage to accelerate the detection and removal of early failures. Dynamic burn-in, which applies signals close to real operation, further reduces post-shipment failure risk.
• Reliability testing: Environmental stresses such as temperature cycling and high-temperature/high-humidity bias is applied to evaluate long-term stability and degradation mechanisms. The resulting data supports lifetime prediction and failure analysis.
• Electrical characteristic testing: Operating voltage and signal characteristics are measured at room and high temperatures to verify changes before/after burn-in and confirm specification compliance.

These three test types complement one another and are indispensable for ensuring product quality and reliability. In addition, visual inspection checks for package cracks and defects are completed, and only devices that pass all tests are shipped.

Chiplet Packaging Technology at Rapidus

Rapidus is building a cleanroom at the Seiko Epson Chitose facility in Chitose City, Hokkaido, and launching a back-end R&D hub called Rapidus Chiplet Solutions (RCS). There, Rapidus will enable 2-nm-generation semiconductor chiplet package design and manufacturing technology development, including mass-production technologies for chiplet packaging with automation. RCS will install pilot lines supporting flip-chip ball grid array (FCBGA), Si interposer, RDL and hybrid bonding processes, in addition to conducting R&D on mass-production methods including equipment automation.

Chiplet technology—integrating multiple chips within a single package—is also drawing strong interest. Stacking and 2.5D/3D integration can offset the limits of transistor scaling while delivering higher functionality and lower power. In this approach, interposer performance—which carries chip-to-chip interconnects—is crucial; as requirements rise, interposers are growing larger. Rapidus is developing 600-mm square panel-level packaging as an industry-leading carrier-substrate format for batch production of multiple packages, aiming for interposer manufacturing with high throughput, lower cost and high reliability.

Key Points in Back-End Processing

Final Assurance of Product Quality and Reliability

Though often behind the scenes, the back-end process serves as invisible quality assurance that underpins modern digital life. Because packaged chips are encapsulated and hidden inside products, their internals are rarely seen—yet the packaging protects against shock, humidity and temperature changes, enabling stable long-term operation. In this sense, the back-end process acts as a gatekeeper that ultimately guarantees device quality and reliability.

Contribution to Cost Reduction and Efficiency

The back-end process also contributes significantly to production efficiency and cost control. In today's division-of-labor model, foundries specialize in front-end and OSATs in back-end, minimizing capital outlays and managing manufacturing costs. Moreover, with chiplet designs, smaller dies have a lower probability of defect impact than a single large die at the same defect density, improving yield. Better yield reduces scrap and directly lowers cost, so innovations in the back-end carry substantial economic value.

Evolution of Packaging and Response to Diverse Needs

What once merely protected and connected chips has become central to heterogeneous integration—integrating multiple dies with different processes and functions in one package. For example, combining a cutting-edge logic die with memory or analog dies fabricated on different processes can pack the required functionality while controlling cost. Together with 3D packaging that stacks chips vertically as well as laterally, chiplet-based heterogeneous integration is becoming a major driver of next-generation semiconductor performance.

Summary

The semiconductor back-end process is a critical stage that finishes wafer-level chips into final products. Each step—bump formation, dicing, flip-chip bonding, underfill, molding and testing—determines quality and reliability. With advances in chiplet-oriented advanced packaging, the back-end now decisively shapes both performance and cost. By compensating for scaling limits, heterogeneous integration expands what devices can do, and the back-end is rapidly evolving into an innovation engine for the semiconductor industry.