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Rework on Small, Leadless Devices
December 31, 1969 |Estimated reading time: 5 minutes
A rework process was developed to simplify rework of leadless devices, which will soon be taking over leaded as well as area-array devices. Many times, tooling and capital equipment are required for this type of rework. This method does not rely on capital equipment nor highly skilled operators. A description of the rework process is provided.
By Bob Wettermann, CIT, BEST Inc.
The quad flat pack no-lead (QFN) is a lead-less package with ≤1 mm profile, moderate thermal dissipation, and good electrical performance. Its SMT device leads are located on the bottom and sides. QFNs generally present a challenge to rework as they are typically smaller than QFPs. Their use has steadily increased in recent years; QFNs are projected to supersede both area array and leaded QFP type packages,1 especially as smaller, multifunctional handheld devices proliferate. These leadless packages are used in tight spaces where conventional solder paste printing rework methods struggle. Other techniques – device printing or paste dispensing – require capital equipment to control the rework process, adding a sizeable investment. In one rework process for leadless devices, the packages can be manually placed without relying on capital equipment.
Automated Rework Systems
Component pasting is a sequential process, each step designed to minimize the risk of a smeared solder paste pattern. A typical pad size for a 0.5-mm-pitch QFN is 0.25 × 0.40 mm; therefore, paste printing using a micro metal rework stencil requires patience, skill, and experience.
Figure 1A. Solder paste rolled into apertures.
The component to be reworked is fixtured into a holding module, where an appropriate micro stencil is placed into the alignment arm of the rework system. The stencil and component are checked for pad-to-aperture alignment.
Figure 1B. QFN bumped after paste application.
After the rework operator lowers the stencil into place, both the stencil and component are locked into position with vacuum pressure. Solder paste is rolled into the apertures, then the stencil alignment arm is reattached to the stencil, which is lifted from the surface of the component. The component is flipped, exposing the body to the pick-up nozzle. Split-vision optics again help align the component with the substrate pads. The component is placed and reflowed. The process for printing onto the substrate is similar to printing onto the component. Split-vision optics, vacuum capability, custom stencils, and a reflow heat source are all required on the rework system.
Figure 1C. Solder paste applied on PCB.
Several disadvantages hamper use of micro stencils for QFN rework. Generally, the lead time for new pattern and fixtures fabrication is impractical. The stencils can be fragile and bend easily, preventing a uniform snap-off from the land areas. They need to be residue-free, adding cleaning time before each use.
Paste Dispense onto Substrate
Another option is a solder paste dispensing system. A programmable gantry or board holder dispenses paste onto specific locations on the substrate. The substrate is locked into a holding fixture while the system dispenses a consistent, preprogrammed volume of paste at designated sites. Depending on the rework system, the program can also include pick-and-place with holding stage and vacuum nozzle.
There are some advantages and disadvantages to this technique. Dispensing onto the substrate offers flexibility, allowing the same equipment to be used for a variety of component configurations without custom stencils or fixtures. Paste volume is consistent. However, significant time is invested in programming each component and substrate configuration. Dispensing can also be a time-consuming operation limited for high-I/O devices. Paste viscosity and nozzle size limit throughput.
Manual Soldering
In some cases, QFN rework is achieved with a handheld hot air rework tool, which reflows the center ground plane. The QFN’s I/O pads must have a solderable toe surface, allowing soldering iron access to each individual termination, for rework to be sucessful. This technique is often problematic. Handheld hot air units lack the necessary control. Manual soldering depends on the technician’s skill and on component design.
Stencil Bumps
Stay-in-place stencil bump QFN rework2 eliminates many of the concerns and drawbacks of the other methods. The stay-in-place stencil bump method uses polyimide stencils, one temporary stencil to match the component and one to match the substrate. Stencils are placed over the land patterns of the device or the device and the PCB, controlling the solder print area. Component prep comprises cleaning pads of any residues with solder wick and isopropyl alcohol. A precision polyimide stencil is aligned and applied to the bottom of the component (Figure 1A). Solder paste is applied with a manual squeegee. The component can then be reflowed according to the paste and component manufacturer’s profile. Post-reflow, the polyimide stencil is removed from the component, leaving solder bumps instead of paste deposits or flat pads (1B). Inspect the solder bumps for uniformity and consistency before applying paste or flux to the substrate, which must prepped with solder wick and isopropyl alcohol. Align and place the semi-permanent polyimide stencil to the substrate. Roll the solder paste through the apertures of the stencil and wipe any excess paste from the stencil surface with a lint-free cloth (1C). Manually place the QFN into the solder-pasted apertures on the substrate stencil. As the solder bumps on the QFN align with the apertures, the rework technician will be able to “feel” the solder bumps on the device slide into the wells of the stencil apertures. Reflow the component according to the appropriate profile. After reflow, clean and inspect3 the assembly as usual.
There are several advantages to this method. There is no need for capital investment in a split-vision rework or dispense system, which can cost $20,000 or more. This approach does not require a highly skilled rework operator. It offers high throughput. The rework process for QFNs is more reliable, as the semi-permanent substrate stencil aids in maintaining electrical clearance of the paste and solder connections during reflow.
Stay-in-place solder bump QFN rework also simplifies the rework process. Magnification, a steady hand, paste flux, solder paste, a squeegee, and a programmable heat source are the only requirements to rework any size QFN. SMT
REFERENCES:
Contact the author for a list of references.
Bob Wettermann, CIT, president, BEST Inc., may be contacted at bwet@solder.net.