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Rework of QFN - No Compromises!Flat packages such as QFNs (Quad Flat No-lead) or other MLFs (Micro Lead Frame) with outstanding thermal, inductive and capacitive characteristics are being increasingly incorporated into densely populated, space-saving assemblies. Unlike BGA components, QFNs however do not provide a solder ball array for SMD assembly but have to be soldered to the assembly with their contact pads attached directly to the metalized body (lead frames). This technology makes much higher demands compared to the handling of standard SMD components.
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Therefore the challenge is to apply fresh solder to the contact pad structure of the QFN component 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.
One possibility is to implement an additional work step with transfer plate and solder reflow. However, beside accuracy issues, exposing the QFN component to thermal stress not one but even two times must be considered a serious drawback. In comparison, paste printing the solder directly to the QFN component using a printing stencil is quicker, more precise and keeps the stress to the component at a minimum.
Therefore, direct paste-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 µm pitch for component sizes in the millimeter range, there is a limit to successful paste printing that relies on hand/eye dexterity. |
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When combined with other essential rework process steps, a 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. |
Video: Desoldering of QFN Component
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| 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 Direct Component Printing (DCP) 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: |
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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 squeegee 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). The printing result can be inspected with the patented vision alignment system. 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 (DCP) 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. Implementing nitrogen into the reflow process will improve the formation of an optimal solder meniscus.
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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.
Direct component-printing with the FINETECH Direct Component Printing (DCP) Module turns out to be the better solution.
Benefits:
- Very effective, uncomplicated and time-saving method to improve QFN or other MLF rework yield
- Applicable for volume rework of QFN or other MLF components with fine pitch
- Paste application as part of the rework cycle can be performed within any rework system equipped with the DCP Module
- No additional working step necessary (i.e. solder reflow from transfer plate): helps avoiding accuracy issues and thermal stress issues
- No second reflow process necessary: thermal stress to the QFN or other MLF component corresponds to the one of the original assembly - no compromises!
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