Leadless SMD packages: great performance on a small surface area
For a number of years now we have seen leadless DFN packages (discrete flat no-leads) for discrete components, which, compared to conventional SMD packages, do away with contact and assembly terminals extending from the side. Instead, the terminals have been moved to the underside of the package.
The major benefit of this configuration is the reduced area for a given electrical function on the board because the space taken up by the terminals is eliminated. This DFN technology enables the use of very flat lead frames with large outer contact faces acting as a heatsink.
The result is a low installed height and, compared to conventional SMD packages, a much improved capability to dissipate heat from the interior. These DFN packages have already become established as a standard form in end applications where a small form factor and its accompanying design advantage contribute directly to added value. Smart phones are a striking example where the smallest possible form per function is of the upmost priority and a driving force in further development of package technology to both increase functional density and enhance the degree of freedom in designing an end application.
To date, the benefit of this area gain was partially offset by challenges in verifying assembly of the components on a board by optical control instruments. Unlike standard SMD packages, there are no visible solder pads on the side of the package which often serve to check the quality of soldering when placing these DFN packages.
Through the introduction of a new generation of leadless packages, it is now possible to combine the benefits of DFN with those of standard SMD packages. What is characteristic of these packages is that they feature contact pads not only on the underside but also on parts of the side faces. In addition to the purely physical presence of metal pads on the side, as already found in many DFN packages, effective use of this feature requires completely reliable side wetting in conventional soldering. Such wetting capability is not usually sufficiently reliable on purely copper surfaces. The side-pads first have to be tin-plated in the same way as the undersides, then complete wetting can be implemented by conventional soldering processes.
The tin-plated side-pads enable 100 % reliable wetting
in conventional soldering
The new DFN1006D-2 (SOD882D) and DFN1608D-2 (SOD1608) packages from NXP Semiconductors are the first of their kind on which the side-pads are tin-plated. Tin wetting at the sides enables conventional optical verification of assembly quality plus greater mechanical strength and reduced tilting effect. Tilting is not an objective quality constraint on otherwise inconspicuous solder joints, but may be rated so in some cases.
When the area taken up by the DFN1006D-2 (SOD882D) package, in the given case 1 x 0.6 x 0.37 mm (L x B x H), is compared to that of a conventional SMD package, the gain in space becomes very obvious. Taking the maximum possible size of the silicon as a criterion, a comparable leaded SMD package with external contact and assembly terminals, such as the SOD523 (SC-79) at 1.2 x 0.8 x 0.6 mm, occupies more than twice the area of the PCB without generating any additional functional value.
Instead of the gain in space achieved by using a smaller DFN package, a performance gain is also possible. The DFN1608D-2 package allows up to 1.5 A of electrical current, twice that of the leaded package – in the same space as a standard SOD523 component. In addition, onboard height is reduced by more than half. (The PMEG4015EPK Schottky rectifier in the DFN1608D-2 and the PMEG3005EB in the SOD523 were used as the basis for this comparison).
DFN is a quantum leap in circuit design, especially for boards incorporated in small, flat electronic devices such as tablet PCs and smart phones. In cases like these, LED backlighting and LED flashlights are applications that require small but powerful Schottky diodes to improve the efficiency and increase service life of the battery. With the second generation of leadless packages, the last supposed drawback of restricted visual assembly control has been eliminated, and mechanical stability (shear strength) has also been enhanced.
When it comes to manufacturing costs, DFN technology also compares favorably with conventional SMD packages. Direct cost benefits can be expected in high-volume manufacturing conditions, especially with small packages. Alternatively, there are indirect cost benefits by replacing a large and material-intensive, conventional package with a thermally equivalent but smaller DFN package, for example.
Summarizing, the DFN format scores well on the aspects looked at here – design, function and cost. The DFN format has a lot to recommend in becoming a general standard for plastic SMD components in and beyond the communication segment.
About the author
Ralf Euler is currently head of product management for small-signal discrete components at NXP Semiconductors.
Previously, Euler was a product marketing manager for diodes and transistors at NXP. He also has several years of experience as a development engineer focusing on bipolar transistors at Philips Semiconductors. Ralf Euler holds a PhD (physics) from the University of Cologne (Germany).
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