QFN

A QFN or Quad Flat No leads or a MLF package is an integrated circuit package which falls under Surface mount technology packages used on printed circuit boards. This kind of package is similar to the Quad Flat Package, but the leads do not extend out from the sides of the package. This is a near CSP plastic encapsulated package made with a planar copper lead frame substrate. The perimeter lands on the bottom of the package provides with electrical contact to the PCB. The package comes with an exposed thermal pad which helps in thermal enhancement by having the die exposed to the PCB surface providing an efficient heat transfer path when soldered directly to the PCB. By using vias in the thermal pad one can achieve a direct heat removal path away from the package die thus increasing the thermal performance.

QFN Cross section

The figure shows the cross sectional view of a QFN package with a lead frame and wire bonding technology. There are two types of body designs, Puch singulation and saw singulation [http://www.freescale.com/files/analog/doc/app_note/AN1902.pdf] . In the figure a saw singulated body with an attached thermal head pad is shown. The lead frame are made up of copper alloy and a thermally conductive adhesive is used for attaching the Silicon die to the exposed heat pad. The silicon die is attached to the frame using a wire-bonding technology with wire made up of gold, ranging anywhere from 1 to 2mils in diameter.

Different Types of QFNs:

There are currently two general types of QFN packages in the market; an Air-Cavity QFN with air within the package, and a fully encapsulated Plastic-Moulded QFN with minimal air voids and no cavity within the package.

Each type of QFN has its own use. Due to its structure, the plastic-moulded QFN is usually limited to applications up to ~2-3GHz. It is usually composed of just 2 parts, a plastic compound and copper leadframe, and does not come with a lid.

In contrast, the Air-Cavity QFN is usually made up of 3 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 20-25GHz.

Advantages:

This package offers 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.

References

External links

* [http://www.amkor.com/Products/all_products/MLF.cfm QFN package called MicroLeadFrame]
* [http://www.onsemi.com/pub/Collateral/AND8086-D.PDF Board mounting notes for QFN packages]
* [http://www.finetech.de/enid/qfnrework Information about rework of QFN]