Comprehensive Guide to QFN Packages: Advantages, Types, and Applications in Electronics

Quad flat no-lead (QFN) packages are a type of IC package that are small, lightweight, and have a thin profile. They are also known as chip-scale packages because the lead can be seen and contacted even after assembly. They have electrode pads at the bottom of the package instead of leads, and a thermal pad that provides good thermal performance.

QFN packages are used in a variety of industries, including mobile devices and automotive electronics. Among the many important choices, QFN packaging has always been a popular choice. What makes this type of package so popular? Should you also use it in your project? This guide provides a clear and comprehensive study of it.

What is a QFN package?

QFN stands for square flat without lead. The QFN package connects a silicon chip (ASIC) to a printed circuit board (PCB). It is achieved using surface mount technology. As the name suggests, this package does not include classic clues that have existed in the past. The square flat leadless package does not have the usual leads but has edge pads with open pads below. This structure can improve electrical and thermal performance, which is why QFN packages are popular with users.

QFN Packaging and Parts:

A QFN package is a leadless package of surface mounting technology. A QFN package usually consists of the following basic components：

Lead frame: This part is very critical to determine the performance of the IC. It basically acts as package support.

Single or multiple chips: These are actually silicon chips in the package and are mounted to the circuit board using surface mount technology.

Welding wire: usually made of copper or gold. These wires form the necessary connection between the lead frame and the chip.

Molding plastic: This material surrounds and protects the internal components. It provides electrical insulation, prevents corrosion, and enhances the durability and reliability of the package.

Advantages of QFN packages:

QFN packages offer several advantages that make them highly desirable in various electronic applications. They excel in thermal performance due to the exposed thermal pad on the bottom. Their compact size and absence of leads make them space-efficient, ideal for space-constrained devices. QFN packages also exhibit improved electrical performance with shorter electrical paths and lower inductance. Additionally, they comply with lead-free regulations, and are easy to handle during assembly, streamlining the manufacturing process, Overall we can say QFN has:

Low cost

Good electrical performance

Good thermal performance and heat dissipation

Small form factor and lightweight

Short bond wires connecting the die and lead frame

Low lead inductance due to short bond wires

QFN Marking Specification:

The QFN package is relatively small and therefore does now allow much space for legible marking. A 5mm x 5mm QFN can have up to 5 or 6 characters in one line; 3 or 4 lines are possible.

Wire Bonding:

Gold wire bonds were the default material for many years. They are still available but are being replaced by copper. Copper wire bonds have lower costs and offer better conductivity. Yet copper wires require more force to bond the wire to the pad. Many semiconductor foundries supply IO pad cells designed to support copper bonding as a thicker pad is usually needed.

Die Attach:

This is the epoxy material that fixes the die to the lead frame pad. Two main types are used, conductive and non-conductive, depending on the system's electrical requirements; electrically conductive materials (i.e. silver-loaded epoxy) tend also to have better thermal conductive properties.

Assembly of QFN packages:

1) Clean PCB & Apply Solder Paste: Clean the board, then stencil solder paste onto the pads.

2) Place QFN: Align and place the QFN package on the solder-pasted pads.

3) Reflow Soldering: Use a reflow oven with the correct temperature profile to solder the QFN.

4) Inspect: Perform visual and X-ray inspection for alignment and solder quality.

5) Rework (if needed): Use hot air rework to fix any defects.

6) Final Test: Conduct electrical and functional testing to ensure proper assembly.

Application of QFN packaging:

QFN packages are particularly popular in areas where space-saving and top-of-the-line performance are critical. QFN is used in the following areas：

Consumer electronics products: Square flat leadless packages are usually used in smartphones and tablets. Their general purpose is to occupy a small footprint and have excellent thermal management capabilities.

Automotive systems: The high performance of the QFN package makes it a device used in important modules such as engine control units.

Communication equipment: QFN can be applied to high-speed network equipment, where fast signal processing is essential.

QFP and QFN: What is the difference? How to choose between them?

QFP and QFN are the two most common integrated circuit packages. Although their names differ by only one letter, the QFP package has gull wing leads protruding from the package body. This is very helpful for inspection or rework, and at the same time, it is very compact.

If space on the PCB is limited, and compact size is essential, QFN packages might be preferred due to their absence of leads and smaller footprint. On the other hand, if the component requires a higher pin count and wider lead spacing, QFP packages would be more suitable. Thermal considerations, soldering techniques, and assembly processes also play a role in determining the optimal package for a particular application.

Punching type QFN and Sawing type QFN:

According to the manufacturing process, QFN packaging can be divided into two main types. The naming is based on the singulation method, where a punched QFN is separated by a punch tool, and a sawn QFN is separated by sawing a large amount of packaging into single units.

1) Punch type QFN: This type is produced with a mold cavity. After the molding process is over, special tools are used to stamp out each individual package from the molded substrate. This method is very efficient for large-scale production and usually produces a clean and sharp cutting effect.

2) Sawing type QFN: On the other hand, sawing type QFN is produced by the mold array process. This involves the process of using a saw to cut a large piece of molded packaging into individual units. This technology is very effective in managing large amounts of data.

Higher volume production tends to support sawn-type QFN, whereas punch type is often seen with lower volume products. Both have very similar electrical and thermal properties. The drawing below shows the package structure difference between a punched and sawn QFN.

Other popular type of QFN Packages:

QFN packages are available in different types. Here are mentioned some popular ones:

Air-cavity QFN: Consists of a plastic or ceramic lid, copper lead frame, and an open-without-seal-plastic molded body. Air cavity QFN packages are used in microwave systems with frequencies ranging from 20 to 25 GHz.

Plastic molded QFN: Plastic molded QFNs are cheaper than air-cavity QFNs. They consist of a plastic compound and copper lead frame. This type of QFN package is used in 2-3 GHz frequency applications. There is no lid on plastic molded QFN packages.

QFN with wettable flanks: This type of QFN helps designers visually check that the pad is mounted to the PCB through the elevation provided by the wettable flanks.

Flip-chip QFN: A cheap modeled package offered by flip-chip QFNs. This package uses flip-chip interconnection to establish electrical connections.

Wire bond QFN: In this package, wires are used to connect the PCB to the chip terminal.