Circuits Assembly - October 2008 - (Page 32)

Components Improving QFN Reliability By Joseph G. Ameen and Gilson V. Geralde The proper solder volume makes a dramatic difference. mall QFN packages (sometimes referred to as a micro leadframe package, or MLP) present processing challenges. The requirements for grounding the package bottom and making reliable connections to I/Os on the periphery on this same plane can be difficult. If too much solder is applied to the ground plane, the part will float, resulting in poor connections to the I/Os (Figure 1, online). If too little is applied, insufficient grounding will result. In addition, according to IPC standards, toe fillets are not required.1 This can severely limit the connection area. The toe surfaces are created when parts are diced from a lead frame. The lead frame thickness creates the toe (Figure 2, online). Because this surface is unprotected copper and usually oxidized, it is difficult to wet with solder. To improve device reliability, the proper amount of solder is needed to ground the bottom and simultaneously create a toe fillet. Several manufacturers and the literature2,3 offer suggestions for the ground area. This article focuses on the toe fillet and bottom connection, which create a kind of two-pronged connector, providing electrical continuity from both points. Toe Fillet Influences The first factor is the solder paste itself. Select a paste based on established criteria. For example, select a paste with the appropriate type (solder 32 Circuits Assembly OCTOBER 2008 S ball size) and one that does not slump within the process window (Figure 3). Profile the reflow oven in accordance with the QFN supplier’s recommendation. In this example, a paste supplier recommended a maximum of 230°C and an ideal temperature below 220°C. We set the oven around the 220°C maximized limit (Figure 4). Follow stencil guidelines so the pad-pulling tension to the retention wall tension ratio of the stencil is at least 0.6, and preferably higher. Based on a typical QFN’s pad sizes, the calculation indicates a 0.004"-0.005" stencil thickness works best. Inspect the board pads that will receive solder paste. Ensure the solder mask is aligned with the I/O pads and the opening in the mask is not excessively large. This will eliminate wicking of solder over and down the pads’ sides (Figure 5). Solder mask misalignment or oversizing also could create I/O pads of varying lengths (Figure 6), making it difficult to control solder volume for even toe wetting. The goal is a uniform surface area that will accept a uniform solder volume. A good board supplier can help obtain the desired results (Figure 7). When the Figure 3. A well-defined nonsolder paste, slumping solder. circuitsassembly.com http://www.circuitsassembly.com

Table of Contents Feed for the Digital Edition of Circuits Assembly - October 2008

Circuits Assembly - September 2008 Contents Letters Caveat Lector Industry News Market Watch Talking Heads Global Sourcing Screen Printing Better Manufacturing 'Checking Up' on Medical Electronics Solder Ball Attachment Using Laser Soldering Improving QFN Reliability Reflow Soldering Tech Tips Test and Inspection Process Doctor Pb-Free Lessons Learned Alternative Energies Eastern Advances Product Spotlight Ad Index Assembly Insider Technical Abstracts

Circuits Assembly - October 2008

For optimal viewing of this digital publication, please enable JavaScript and then refresh the page. If you would like to try to load the digital publication without using Flash Player detection, please click here.