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Snap, Print, & Flip: Paste Printing for QFN Rework
December 31, 1969 |Estimated reading time: 5 minutes
As the industry looks to implement smaller components, MLFs such as QFNs that enable higher PCB component densities, are being incorporated into space-constrained products. Conventional solder paste methods are not viable options. This article examines how paste printing can be used in the rework process.
By Robert Avila and Dominik Horn
Progressively, the industry is looking to and implementing smaller components. Micro-leadframe (MLF) components, such as QFNs, are being incorporated into area-sensitive products - with contact pads attached directly to the bodies using leadframe technology (Figure 1). These components enable higher PCB densities.
Sound surprisingly familiar? Yes, we have heard it before - BGA, chip-scale packages (CSPs), and flip chips. But now there is a difference, especially from the rework perspective, for example, solder paste and its application during the rework process. Leadless components do not provide solder coating; therefore, fresh solder paste must be applied prior to placement and reflow. Typically, these packages are used in tight spaces where it is difficult to use conventional solder pasting methods (stencil and blade) because there is not enough real estate to place the stencil and draw paste over it. Direct dispensing of solder paste is not a viable option either. It is expensive, slow, and rarely available in rework environments. Therefore, pasting the component is the preferred method. Thin-film stencils, fixturing, and skillful hands can sometimes achieve wonders, but aren’t we supposed to be getting our hands out of the way? In this scenario, someone is usually left juggling a pasted component upside-down, and trying to position it onto a nozzle prior to reflow without disturbing the pasted work. Reworking MLF or QFN components repeatedly with high yield requires more of a hands-off approach that can be integrated easily into the rework tool. What is needed is an equipment module that ensures the paste process does not limit rework yield. Because stencils are available with less than 300-mm pitch for component sizes in the millimeter range, there is a limit to successful paste printing that relies on hand/eye dexterity.
Figure 1. QFN with solder paste.
When combined with other essential rework process steps, one direct component printing method* ensures an all-in-one solution for QFN and MLF components. Integrating residual solder removal and component reflow with the correct thermal profiling for the freshly pasted component will provide a smoother path to high-yield rework. This is another example of when successful rework becomes a value process, and is no longer considered an overhead step.
Pasting a component is a sequential process. Each step is designed to minimize the risk of paste smudging. However, consider an aspect that could be viewed as a downside to the process. A semi-custom fixture is required to hold the MLF into the print module. These relatively inexpensive and simple-to-manufacture fixtures accept components that vary in size by a few millimeters. It is fair to say that if the anticipated rework schedule consists of one-off-type rework, with little prospect of repetition, the need for this fixturing could be taxing. In this case, more manually intensive hand pasting could be the way to go - especially if the pads are large and the pitch is not considered fine. However, the advantages outweigh the inconvenience, especially for a volume-rework environment. As the following steps indicate, direct component printing is a sequentially stepped procedure using a single module integrated into the rework tool. At the end of the sequence, the component is picked from the stencil frame by the same nozzle to be used during the subsequent hot-gas reflow process. Steps of the process are shown in Figures 2-5.
Figure 2. Component snap and flip.
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Figure 3. Paste spread with spatula.
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Figure 4. Reflow arm lowered to remove component.
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Figure 5. Solder paste distribution.
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The component to be pasted is placed in the component carrier, snapped into the module, and flipped 180° (Figure 2). A stencil-handling tool is loaded into the reflow arm of the system and locked into place. This tool is used to pick up the stencil, align it, and drop it into place over the part to be printed. It can be used for any component and varies with the overall size of the stencil frame. Stencil openings and component pads are viewed through the split-vision optics and aligned. The stencil is lowered toward the component surface, vacuum clamped to the module, and freed from the pick-up tool. Solder paste is spread with a spatula in typical fashion (Figure 3). Once the paste is printed, the stencil surface is realigned to the pick-up tool, vacuum applied, and then lifted from the component surface prior to being removed from the tool (Figure 4). Typical solder paste distribution can be seen in Figure 5. The reflow nozzle for the specific MLF is inserted, and the module is flipped back 180° so that the backside of the die faces the pick-up tool. The component is picked from the fixture so that the freshly pasted surface faces the MLF pads. Using split-vision optics, the pads are aligned to the substrate and the component is placed prior to reflow.
The direct component printing module is integral to the process. Split-vision optics are used not only to align the stencil to the LAN array (to ensure accurate paste placement on the component), but also to align the component to the substrate. This ensures that once it is picked from the module, the process consisting of align, place, and reflow can continue automatically.
Figure 6. Poor pasting effect.
This is just part of the solution to successfully replace MLF components. Prior to component reflow, the board/substrate must be cleaned of residual solder - preferably by non-contact means. For smaller, fine-pitch components, an integrated removal module ensures minimal risk to the integrity of the substrate that could be subject to damage from conventional wicking. With the board cleaned and the component pasted, all that remains is to place the component using touch-down force control and reflow using a thermal-management philosophy that ensures reflow without disturbing nearby components. The effects of poor paste-printing methods can be seen in Figure 6, where incomplete pad coverage is visible. Such circumstances create the possibility for open joints. In Figure 7, however, uniform paste coverage yields improved overall reflow.
Figure 7. Uniform paste coverage.
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Conclusion
Although workers with dexterity and better-than-average hand/eye coordination can complete most processes in the rework world, eventually there’s a limit. When the solution comes at a small capital outlay with minimal tooling costs, it may be best to take the line of lesser risk and improve rework yields.
* DCP module, Finetech USA, Tempe, Ariz.
Robert Avila, application engineer, Finetech USA, may be contacted at (480) 893-1630; e-mail: robert@finetechusa.com; Website: www.finetechusa.com. Dominik Horn, product manager, Finetech, may be contacted at 49 309 366 810; e-mail: dominik.horn@finetech.de; Website: www.finetech.de.