Flip-chip technology is an advanced semiconductor packaging and assembly method that involves directly mounting the semiconductor chip onto a substrate or PCB with the circuitry facing downward. This FAQ will convey the basic idea of flip-chip technology and how it differs from conventional wire bonding technology.
A simple view of the flip-chip technology
A simplistic understanding of flip-chip technology is illustrated as a four-step process in Figure 1. Each of these steps is as follows:
- Semiconductor chip — the initial step involves preparing a semiconductor chip or IC after fabrication.
- Bumping — small solder balls (bumps) are deposited onto the chip’s bonding pads. These solder bumps serve as electrical and mechanical connection points between the chip and the PCB or substrate.
- Flip — the bumped chip is flipped upside down (thus the term “flip-chip”) so that the solder bumps face downward towards the PCB.
- Packaging — the flipped chip with solder bumps is aligned with the corresponding pads on the PCB. Heat is applied to melt the solder, creating a robust electrical and mechanical connection between the chip and the PCB.
How is flip-chip technology different from wire bonding technology?
Flip-chip technology is an alternative to the traditional wire bonding technology in IC packaging. Figure 2 illustrates how these two packaging technologies differ.
Wire bonding technology
In wire bonding, individual semiconductor chips are separated from a wafer through a process called singulation (Figure 2). Once the chip is separated, it is attached to a substrate or PCB using a die attach adhesive material. Electrical connections between the chip and substrate or PCB are made using thin wires (wire bonds). An over-mold compound is then applied to protect the chip and wires. Figure 2 presents two wire bonding configurations:
- Chip-on-Board (COB): Chip directly bonded to PCB with wire bonds.
- Chip-in-Package (CIP): Chip placed onto a substrate, wire bonded, encapsulated, and then mounted on a PCB.
Flip-chip Technology
Flip-chip packaging involves a different approach where the device wafer undergoes wafer bumping before singulation. Solder bumps are formed on the chip surface, which serve as electrical and mechanical connection points. After bumping, the wafer is singulated into individual chips, which are then flipped and directly mounted onto a substrate or PCB. The chip is bonded through solder reflow, creating strong electrical and mechanical connections. Underfill material is introduced beneath the chip to reinforce the connection and ensure reliability. Here, two flip-chip configurations are shown for illustration:
- Chip-in-Package (CIP): Flip-chip mounted on a substrate, which is then attached to a PCB.
- Direct-Chip-Attach (DCA): Flip-chip directly attached to the PCB without an intermediate substrate.
Wire bonding is more conventional, simpler, and less expensive, but has longer electrical paths (wire loops), which affects high-frequency performance. Flip-chip technology is more compact and leads to efficient heat dissipation. It has shorter electrical paths, provides better electrical performance, and is suitable for high-speed and high-density electronic applications, but at a higher cost.
Flip-chip technology in 3D integration
The flip-chip approach enables 3D integration, which allows quantum processors to overcome the limitations of planar architectures. Figure 3 demonstrates this phenomenon by showing both standard and flip-chip configurations.
Figure 3 shows how the flip-chip design separates qubits (on the top chip) from the control and readout elements (on the bottom chip). This separation has the following benefits as it allows for:
- An independent fabrication process for each chip.
- Optimization of the qubit chip without compromising it with complex control circuitry.
- Protection of the sensitive qubits from possible decoherence sources.
One of the most important findings demonstrated with the setup in Figure 3 is that high qubit coherence times (T₁, T₂ > 20 μs) are maintained despite the proximity of another chip. This proves that the 3D integration approach does not degrade qubit performance.
A primary benefit is solving the interconnect crowding problem. Additionally, lateral addressing of qubits from the perimeter becomes impractical when scaling to larger qubit arrays. The flip-chip approach elegantly solves this by utilizing the third dimension for routing control and readout lines.
The flip-chip configuration provides a larger mode volume for qubit electromagnetic fields, reducing surface participation effects that can limit qubit coherence.
Case study – Samsung’s LED flip-chip design
LEDs traditionally incorporate a chip that emits monochromatic light at specific voltages. Conventional designs install these chips in packages with gold wire bonding connecting them to contacts. However, this approach presents notable limitations: the delicate gold wires are susceptible to breakage under minimal stress, and package reflections reduce overall efficiency.
Samsung has advanced this concept with its innovative flip-chip LED package, as shown in Figure 4. Their approach involves inverting blue LED chips and applying phosphor film directly to each unit. Unlike traditional packages requiring phosphor dispensing and plastic molding, Samsung’s technology achieves chip-scale packaging without molds, enabling more compact lighting fixture designs.
This implementation creates the shortest possible distance from the junction to the package base while eliminating wire bonding requirements. These engineering refinements deliver approximately five percent temperature reduction per watt within the optimal 25°C to 85°C operating range.
Summary
Flip-chip technology has revolutionized semiconductor packaging by offering superior electrical performance and thermal management. Its applications span multiple industries, and ongoing advancements continue to enhance its capabilities. While there are only a few applications mentioned in this FAQ, this technology applies to areas where wire bonding technology can be replaced.
References
Samsung Electronics Introduces New Flip Chip LED Package with Wide Range of Operating Currents, Samsung
AN4196 Application note – Flip Chip package description and recommendations for use, STMicroelectronics
Samsung Introduces Versatile New Flip Chip LED Packages and Modules, Samsung
Flip Chip Assembly – Process, Applications, and Benefits, RoodMicrotec GmbH
3D integrated superconducting qubits, Springer Nature
Flip Chip Attach Techniques, ACI Technologies
What is Flip Chip technology?, Techovedas
Samsung LM301B LED Chip, Crescience
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