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Improve Heat Dissipation With Flip-Chip QFN Packages

Published Date
Author Cadence PCB Solutions

Key Takeaways

  • The process of attaching the wire bond to the semiconductor can be classified based on the application of force, ultrasonic energy, and temperature. 

  • The flip-chip technology uses small metal balls, called bumps, to make connections.

  •  In a flip-chip QFN package, flip-chip interconnections are integrated into the QFN body.

Flip-Chip QFN Package

ICs based on flip-chip QFN package technology are utilized for maintaining performance without the ill effects of wire bonds

In certain semiconductor product applications, wire bonding is impractical due to a large number of interconnections or the adverse effect of wire bonds on product performance. In most cases, ICs based on flip-chip QFN package technology are utilized for maintaining performance without the ill effects of wire bonds. Even though the cost required for introducing flip-chip QFN package technology is 1.5 times higher than the cost for wire bond packaging, the former finds applications in consumer high-frequency RF products. 

Wire Bonding and Chip Packaging

It is important to establish reliable internal connections or connections between semiconductor and silicon chips in ICs, and wire bonding can be used to do this. In wire bonding, electrical interconnections between semiconductors, other ICs, and silicon chips are established using wire bonds. Wire bonds are fine wires made of copper, silver, gold, or aluminum.

Wire Bonding Mechanisms

The process of attaching the wire bond to the semiconductor can be classified based on the application of force, ultrasonic energy, and temperature. Standard wire bonding methods can be classified based on the input energy.

Wire Bonding Mechanism

Type of Force and Energy

Ultrasonic bonding

Mechanical force and ultrasonic energy

Thermosonic bonding

Mechanical force, high thermal energy, and ultrasonic energy

Thermocompression bonding

Mechanical force and high thermal energy

Types of Wire Bonding

Wire bonding is bifurcated to:

Ball bonding - Gold wires are usually used. The gold wire is melted to form a ball, which is placed into the contact using force and the energy that corresponds to the wire bonding mechanism used. A metallurgical weld is formed between the ball and the contact pad.

Wedge bonding - The wire bond material typically used is aluminum. The aluminum wire is brought into contact with the contact pad and the energy required for the wire bonding is applied. The application of energy forms the wedge bond between the wire and the bond pad.

Flip-Chip Technology

Power management is a concern when choosing IC packaging styles in product design. The power and thermal energy increase significantly with increased semiconductor device integration density and component size reduction. The wire-bonded QFN package as well as wafer-level chip scale package (WCSP) configurations fail to meet the thermal performance requirements of the product due to the limited direct thermal path from the die to the board. The flip-chip QFN package is preferable for meeting the thermal energy dissipation requirements of high-speed or high-power density product design.

As aforementioned, high-speed and high-performance applications are adversely affected by the wire bond interconnections with wire bond connections. The device speed, power and ground distribution, reliability, etc. are compromised due to non-idealities in the wire bonds. To overcome the shortfalls of wire bond connections, flip-chip bonding technology can be used.

Flip-chip technology is a method used to interconnect semiconductor dies to the substrate. This technology uses small metal balls, called bumps, to make connections. Generally, the balls used are made of silver, lead, tin, etc. The metal bumps are electroplated directly on the metal pads of the integrated chip. The chips are then flipped and bonded to the substrate.

Advantages of Flip-Chip Technology

  • Excellent assembly dynamics

  • Good power and ground distribution

  • Large number of interconnections or high pin count

  • High signal density and signal integrity

  • Improved power dissipation

  • High-speed interfacing is possible

  • Short-distance interconnections are possible

  • Low signal inductance

  • Reliability

  • Reduced form factor

The Flip-Chip QFN Package

Quad-flat no-lead IC packaging that uses flip-chip technology for electrical interconnections is generally called a flip-chip QFN package or FC QFN package. In a flip-chip QFN package, flip-chip interconnections are integrated into the QFN body. In the flip-chip QFN package, the connection between the lead frame and chip is created using flip-chip technology, which is packaged on a QFN body. The exposed thermal pad in the flip-chip QFN package improves heat transfer and provides low inductance ground connection. The thermal efficiency of the QFN package as well as the electrical efficiency of the flip-chip interconnect is blended into the single IC of the flip-chip QFN package.

Advantages of the Flip-Chip QFN Package

  • Good electrical efficiency

  • Good thermal performance

  • Shorter assembly cycles

  • Small chip-to-package ratio

  • Multiple lead rows are possible

The flip-chip QFN package finds applications in cellular phones, digital signal processors, microcontrollers, USB controllers, wireless LANs, etc. Cadence can help you design flip-chip QFN packages for various applications such as DC-DC conversion, signal processing, etc.

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