Keeping Up With the Joneses: The Evolution of Leadframes
01/01/2005
We all know array packaging’s benefits: higher I/O density, better design flexibility, a smaller footprint, and good thermal and electrical performance. But, while array may look like a compelling solution at first glance, don’t count out recent advances made in leadframe packaging - including quad flat no lead (QFN) and other enhanced leadless leadframe-based packages. Before you make a decision to migrate from leadframe to array, I would like to explain how leadframe has evolved and still may be the most practical and cost-effective solution for many applications.
The QFN package has increased I/O counts vs. footprint ratio over the standard leadframe packages considerably (in package sizes less than 7 x 7 mm), and requires less than half of the space required for a TSSOP package with the same lead count. You can still achieve a higher I/O density on an array package, but QFN packages provide an attractive solution where I/O counts are less than 100.
In terms of design, QFN leadframes can be customized to improve design performance. Design features such as flexible lead configurations, fused leads, and a grounded die attach pad (DAP) for thermal performance and wire down bonds offer performance enhancements over standard leadframe packages. There also are variations of QFN that have the ability to include die and passives in one package, making them ideal for SiP applications. One variation can also be an exact replacement for existing dual package footprints.
When it comes to material cost set, QFN packages use a standard bill of materials, similar to leaded packages. Leadframes for QFNs are designed and fabricated in the same way as a standard leaded package. By using a QFN leadframe vs. a substrate, a considerable cost savings can be achieved. Since leadframes for QFN-type packages are mounted to a polyimide film, it can be processed just like a thin laminate in matrix format, using the standard die attach, wire bond, mold, and singulation processes.
Finally, there’s performance. QFN packages eliminate the gullwing leads on standard leaded packages that can act as noise generators in high-frequency applications or cause mechanical defects because of coplanarity problems. QFNs have an exposed die pad on the bottom of the packages that provides a direct thermal path for heat removal. They also meet the industry’s most stringent moisture sensitivity level (MSL) standards, as well as support Japanese requirements for pre-plated leadframes and lead-free solders.
QFNs also can be used for flip chip applications. The electrical performance is superior, because the wire bond has been eliminated. Lower inductance and resistance values can be achieved, as well as excellent package reliability.
While QFN illustrates how leadframe packaging has evolved, new enhanced leadless packages demonstrate how leadframe is positioned for the future. Unlike QFN, enhanced leadless packages have a greater I/O density and can be configured with multiple rows of leadframe posts and power/ground ring options. Unlike the standard QFN package, these design features allow for improved package performance and design flexibility.
These new leadless packages have the same low-cost material set as standard QFN packages. Depending on the process used, there may be no metal cutting, which provides a robust singulation process, longer saw blade life, and more units per hour. An efficient assembly process and the ability to strip test will also ensure a cost-effective package.
The latest leadless packages offer improved electrical and thermal performance, thanks to a shorter path from chip to board, and die being attached to an exposed metal DAP. Similar to QFN, these packages allow for wire bonding directly to a leadframe post. After the etch process, the post is a short, direct path to the mating PCB. The flip chip version of these packages improves performance by eliminating wire bonds, and provides a direct signal path from the bump to the leadframe post, and then on to the mating PCB.
There’s no doubt that array packaging has many advantages, but manufacturers looking for ways to cut production costs should not overlook the evolving advantages of the latest leadframe packaging. Leadless and enhanced leadless packages offer increased I/O counts and design flexibility, improved electrical and thermal performance, and superior package reliability. Taking advantage of an existing infrastructure and using standard material sets, leadless packages are an attractive alternative for many applications, as cost and performance continue to drive package selection.
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GREG PHIPPS, design manager, may be contacted at Advanced Interconnect Technologies, 1284 Forgewood Ave., Sunnyvale, CA 94089; (408) 734-3222; e-mail: [email protected].