Flat no-leads package

Integrated circuit package with contacts on all 4 sides, on the underside of the package

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 plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. Flat no lead packages usually, but not always, include an exposed thermally conductive 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. The QFN package is similar to the quad flat package (QFP), and a ball grid array (BGA).

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. 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 thou 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.

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 are 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.

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, thermal and electrical performance are important.

Improved packaging technologies and component miniaturization can often lead to new or unexpected design, manufacturing, and reliability issues. This has been the case with QFN packages, especially when it comes to adoption by new non consumer electronic OEMs.

Some key QFN design considerations are pad and stencil design. When it comes to bond pad design two approaches can be taken: solder mask defined (SMD) or non solder mask defined (NSMD). A NSMD approach typically leads to more reliable joints, since the solder is able to bond to both the top and sides of the copper pad. The copper etching process also generally has tighter control than the solder masking process, resulting in more consistent joints. This does have the potential to affect the thermal and electrical performance of the joints, so it can be helpful to consult the package manufacturer for optimal performance parameters. SMD pads can be used to reduce the chances of solder bridging, however this may affect overall reliability of the joints. Stencil design is another key parameter in QFN design process. Proper aperture design and stencil thickness can help produce more consistent joints (i. e. minimal voiding, outgassing, and floating parts) with proper thickness, leading to improved reliability. There are also issues on the manufacturing side. For larger QFN components, moisture absorption during solder reflow can be a concern. If there is a large amount of moisture absorption into the package then heating during reflow can lead to excessive component warpage. This often results in the corners of the component lifting off the printed circuit board, causing improper joint formation. To reduce the risk of warpage issues during reflow a moisture sensitivity level of 3 or higher is recommended. Several other issues with QFN manufacturing include: part floating due to excessive solder paste under the center thermal pad, large solder voiding, poor reworkable characteristics, and optimization of the solder reflow profile.

Component packaging is often driven by the consumer electronics market with less consideration given to higher reliability industries such as automotive and aviation. It can therefore be challenging to integrate component package families, such as the QFN, into high reliability environments. QFN components are known to be susceptible to solder fatigue issues, especially thermomechanical fatigue due to thermal cycling. The significantly lower standoff in QFN packages can lead to higher thermomechanical strains due to coefficient of thermal expansion (CTE) mismatch as compared to leaded packages. For example, under accelerated thermal cycling conditions between 40 °C to 125 °C, various quad flat package (QFP) components can last over 10,000 thermal cycles whereas QFN components tend to fail at around 1,000 3,000 cycles. however this has primarily focused on die and 1st level interconnects. IPC 9071A attempted to address this by focusing on 2nd level interconnects (i. e. package to PCB substrate). The challenge with this standard is that it has been more adopted by OEMs than component manufacturers, who tend to view it as an application specific issue. As a result there has been much experimental testing and finite element analysis across various QFN package variants to characterize their reliability and solder fatigue behavior. Serebreni et al. proposed a semi analytical model to assess the reliability QFN solder joints under thermal cycling. This model generates effective mechanical properties for the QFN package, and calculates the shear stress and strain using a model proposed by Chen and Nelson. The dissipated strain energy density is then determined from these values and used to predict characteristic cycles to failure using a 2 parameter Weibull curve.

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, inspect solder joint quality, or probe lead(s).