Quad Flat No-leads package

28-pin QFN, upside down to show contacts and thermal/ground pad

Flat no-leads packages such as quad-flat no-leads (QFN) and dual-flat no-leads (DFN) physically and electrically connect integrated circuits to printed circuit boards. Flat no-leads, also known as micro leadframe (MLF) and SON (small-outline no leads), is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Flat no-lead is a near chip scale package plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB.^[1] Flat no-lead packages include an exposed thermal pad to improve heat transfer out of the IC (into the PCB). Heat transfer can be further facilitated by metal vias in the thermal pad.^[2] The QFN package is similar to the quad-flat package, and a ball grid array.

Flat no lead cross section

QFN sideview.

The figure shows the cross section of a Flat No lead package with a lead frame and wire bonding. There are two types of body designs, punch singulation and saw singulation.^[3] Saw singulation cuts a large set of packages in parts. In punch singulation, a single package is moulded into shape. The cross section shows a saw-singulated body with an attached thermal head pad. The lead frame is made of copper alloy and a thermally conductive adhesive is used for attaching the silicon die to the thermal pad. The silicon die is electrically connected to the lead frame by 1-2 mil diameter gold wires.

The pads of a saw-singulated package can either be completely under the package, or they can fold around the edge of the package.

Different Types of QFNs

Two types of QFN packages are common: air-cavity QFNs, with an air cavity designed into the package, and plastic-moulded QFNs with air in the package minimized.

Less-expensive plastic-moulded QFNs usually limited to applications up to ~2–3 GHz. It is usually composed of just 2 parts, a plastic compound and copper lead frame, and does not come with a lid.

In contrast, the air-cavity QFN is usually made up of three parts; a copper leadframe, plastic-moulded body (open, and not sealed), and either a ceramic or plastic lid. It is usually more expensive due to its construction, and can be used for microwave applications up to 20–25 GHz.

QFN packages can have a single row of contacts or a double row of contacts.

Advantages

This package offers a variety of benefits including reduced lead inductance, a small sized "near chip scale" footprint, thin profile and low weight. It also uses perimeter I/O pads to ease PCB trace routing, and the exposed copper die-pad technology offers good thermal and electrical performance. These features make the QFN an ideal choice for many new applications where size, weight, and thermal and electrical performance are important.

Disadvantages

The small size of the exposed contacts, and the large area of exposed thermal pad makes it easy for small parts, such as 3x3 mm DFN packages, to float on the pool of molten solder under the thermal pad during assembly. This causes the parts to make no contact to the printed circuit board pads in some instances. Due to the excellent thermal characteristics of this mounting package, it is very hard to rework the device, as hot air reflow typically does not offer enough heat to the thermal pad without damage to surrounding board material or parts. Oxidation of the exposed chip contact pads after being exposed to a reflow oven during initial assembly makes solder wetting to them during rework quite difficult. Additionally there is no clearance for a soldering pencil to reflow pads under the chip if touch up is desired. Sometimes contact can be made up the sides of the DFN package contact pads, but this does not work well in practice.

Comparison to other packages

The QFN package is similar to the Quad Flat Package, but the leads do not extend out from the package sides. It is hence difficult to hand-solder a QFN package.

Variants

Different manufacturers use different names for this package: ML (micro-leadframe) versus FN (flat no-lead), in addition there are versions with pads on all four sides (quad) and pads on just two sides (dual), and various thickness varying between 0.9–1.0 mm for normal packages and 0.4 mm for extremely thin. Abbreviations include:

MicroLeadFrame package

Micro leadframe package (MLP) is a family of integrated circuit QFN packages, used in surface mounted electronic circuits designs. It is available in 3 versions which are MLPQ (Q stands for quad), MLPM (M stands for micro), and MLPD (D stands for dual). These package generally have an exposed die attach pad to improve thermal performance. This package is similar to chip scale packages (CSP) in construction. MLPD are designed to provide a footprint-compatible replacement for small-outline integrated circuit (SOIC) packages.

MicroLeadFrame (MLF) is a near CSP plastic encapsulated package with a copper leadframe substrate. This package uses perimeter lands on the bottom of the package to provide electrical contact to the printed circuit board. The die attach paddle is exposed on the bottom of the package surface to provide an efficient heat path when soldered directly to the circuit board. This also enables stable ground by use of down bonds or by electrical connection through a conductive die attach material.

A more recent design variation which allows for higher density connections is the Dual Row MicroLeadFrame (DRMLF) package. This is an MLF package with two rows of lands for devices requiring up to 164 I/O. Typical applications include hard disk drives, USB controllers, and Wireless LAN.

See also

• Chip carrier Chip packaging and package types list

References

External links

• Board mounting notes for QFN packages
• Amkor MicroLeadFrame (MLF)Information
• ChipScale Review magazine, July - August 2000.]