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QFN Package Dimensions

Published Date
Author OrCAD PCB Solutions

Key Takeaways

  • Quad flat no-lead (QFN) packages are lead frames used for connecting ICs to printed circuit boards.

  • QFN package dimensions are designed to give a small form factor that provides a thin profile.

  • The PCB footprint design should take into account the dimensional tolerances associated with the QFN package and assembly factors.

QFN Package Dimensions

PCB footprints need to be designed with respect to QFN package dimensions

QFN packages are often used in consumer, automotive, industrial, or power applications. QFN packages are one of the popular packaging styles in surface-mount ICS. Package dimensions are designed to give a small form factor that provides a thin profile. PCB footprints should be designed with respect to the QFN package. 

QFN Packages

Quad flat no-lead (QFN) packages are lead frames used for connecting ICs to printed circuit boards. QFN is a surface-mount technology-based IC, also known as a chip scale package (CSP). In a QFN package, the lead can be seen and touched even after assembly. 

QFN Configuration Based on Rows

A QFN package can either have single-pin rows or multiple-pin rows.

Single Row QFNs

 The two processes followed for the formation of single-row QFN packages are:

  1. Saw singulation process

In sawn-type QFN technology, a process called the mold array process (MAP) is used for molding. In the MAP process, small parts are cut from a massive box set. Then the sawn-type QFN packages are created.

  1. Punch singulation process

In punch-type QFN technology, the package is molded into a single mold cavity set-up. The punch tool is used for splitting the molded cavity.

Multiple Row QFNs

In multiple-row QFNs, a copper etching process is used to produce the number of rows and pins. Once the copper etching process is over, a row is used to singulate the rows and pins and the multiple-row QFN is ready to use.

Types of QFN Packages

There are other types of QFN packages available: 

  1. Plastic molded QFN - The plastic molded QFN consists of two sections: the upper lead frame and the plastic composite compound. The plastic molded QFNs are some of the cheapest QFN packages and are used in the range of 2 to 3 GHz frequency.

  1. Air-cavity QFN - There is an air cavity in air cavity QFN packages. The parts of an air cavity QFN are a copper lead frame, a plastic or ceramic lid, and a plastic molded body. Frequency of operation around 20-25 GHz is possible with air cavity QFN packages.

  1. Wettable flanks QFN - An elevation is provided in wettable flanks QFN  for reflecting the solder wetting. A visual check is possible to check if the pads are mounted to the PCB.

  1. Flip-chip QFN - In a flip-chip QFN package, flip-chip interconnectivity is utilized. The flip-chip QFN offers shorter electrical paths.

  1. Wire bond QFN - Wires are used to connect semiconductor PCB tracks to chip terminals.

QFN Package Dimensions 

The recommended soldering process for surface mount technology ICs is to print solder paste on the PCB and reflow it after placing the component. When mounting QFN packages using this method, the following factors need to be given consideration:

To achieve enhanced electrical, thermal, and board-level performance, the pads at the bottom of the QFN package need to be soldered to the PCB. Usually, thermal vias are incorporated for proper heat dissipation.

The PCB footprint design should take into account the dimensional tolerances associated with the QFN package and assembly factors. For the PCB footprint design, the QFN case or package outline drawing should be referred to first. Each QFN configuration is specified with nominal package footprint dimensions. 

PCB Footprint Dimensions

The PCB footprint or mounting pads should be designed larger than the package dimensions. The center pad width in the PCB can be the same size as the QFN package center pad. The peripheral pad length should extend beyond the package perimeter dimension. The peripheral pad width can be approximately equal to the QFN package dimensions. Bridging is avoided by designing the spacing between the PCB peripheral pads to be less than required. Enough clearance is to be provided between the center pad's outer boundary and the inner boundary of the peripheral pads to avoid shorts.

Providing the appropriate PCB footprint dimensions is critical for soldering QFN packages to the board. Cadence’s suite of analysis tools can help you design PCBs with QFN packages. Leading electronics providers rely on Cadence products to optimize power, space, and energy needs for a wide variety of market applications. If you’re looking to learn more about our innovative solutions, talk to our team of experts or subscribe to our YouTube channel.