With the rapid development of modern electronic products, QFN packaging has been widely used in various electronic devices due to its low parasitic effect and compact size. However, as the demand for higher performance increases, the integration density on PCB has also risen, leading to more severe thermal challenges. To address this issue, Dual Row QFN packaging has emerged, providing designers with a more flexible and efficient packaging solution. So, what exactly is Dual Row QFN? How can its soldering quality be optimized? This article will delve into the structural characteristics, advantages, and key strategies to enhance the soldering quality of Dual Row QFN, helping you fully utilize this advanced packaging technology.
What is Dual Row QFN
Before exploring Dual Row QFN, let’s first understand the basic concept of QFN packaging. QFN, also known as Quad Flat No-Lead package, replaces traditional leads with pads located at the bottom of the chip. These pads are usually arranged in a single row around the perimeter of the package, ensuring good signal transmission performance.
Dual Row QFN or DR-QFN, on the other hand, is an improved version of the traditional QFN, featuring dual-row staggered pad design in the pad area, where pads are arranged both on the outer and inner sides of the chip bottom. This significantly increases the number of pins without expanding the package size. This packaging type retains all the advantages of traditional QFN while offering superior signal integrity, thermal performance, and functional integration.
However, it’s important to note that the unique pad structure of Dual Row QFN results in smaller pad spacing and denser solder joints, making it more challenging to control solder paste volume and coplanarity during soldering. This ultimately increases the risk of soldering defects.
Let’s now analyze specific case studies to explore optimization strategies for soldering Dual Row QFN components.
Case Study
In this case, we conducted a trial production of the QCA8337 Dual Row QFN (0.5 mm pitch), with dimensions as shown below (unit: mm).
After performing SPI (Solder Paste Inspection) on 20 trial-produced PCB, it was found that 18 boards reported insufficient solder paste on the pads (with some pad solder paste volume falling below 70%), but no excessive solder paste was detected. Post-soldering X-ray inspection revealed that the outer pads had insufficient solder volume, while the inner pads were relatively full, with some joints exhibiting bridging.
Overall, the yield rate was low, with only 60% pass rate during the soldering stage, far below the standard requirements. Therefore, process optimization was necessary to improve the yield rate.
Case Analysis
During the soldering process of Dual Row QFN packages, the quality of solder paste printing directly affects the reliability of solder joints. The stencil’s thickness and aperture size are key factors in determining the amount of solder paste. Excessively thick stencils or large apertures may result in excessive solder paste, leading to bridging, while small apertures may result in insufficient solder paste, causing weak joints or insufficient solder strength.
However, not all stencil apertures can smoothly achieve optimal solder paste release. To achieve ideal printing results, the design of the stencil apertures must meet certain conditions. The figure below illustrates the stencil aperture. Ideally, when F_s > F_t (where F_s is the adhesive force between the solder paste and the PCB pad, and F_t is the friction between the solder paste and the aperture wall), good printing results can be achieved. Therefore, the stencil aperture walls should be smooth to reduce friction and improve solder paste release efficiency. Similarly, when B > A (where B is the contact area between the solder paste and the PCB pad, and A is the contact area between the solder paste and the stencil aperture wall), printing results improve. However, the aperture wall area should not be too small to avoid insufficient solder paste. Clearly, the aperture wall area is directly related to the stencil thickness. Therefore, stencil thickness, aperture size, and wall smoothness collectively determine the solder paste printing quality.
In actual production, the adhesive force between the solder paste and the PCB pad, as well as the friction between the solder paste and the aperture wall, are difficult to measure directly. Therefore, aspect ratio and area ratio are commonly used to evaluate the stencil’s printing performance.
Aspect Ratio:
Ratio of aperture width to stencil thickness, typically used to evaluate printing performance for elongated apertures. The formula is:
Where:
- b is the aperture width.
- d is the stencil thickness.
Area Ratio:
Ratio of aperture area to aperture wall area, typically used for square or circular apertures. The formula is:
Where:
- L is the aperture length.
Since the density and wettability of leaded and lead-free solder pastes differ, different printing performance standards must be followed based on the type of solder paste used in a specific project:
- For leaded solder paste, aspect ratio ≥ 1.6, area ratio ≥ 0.66.
- For lead-free solder paste, aspect ratio ≥ 1.7, area ratio ≥ 0.7.
In this case, leaded solder paste was used, and the aperture shape was not elongated, with the aspect ratio already meeting the standard. Therefore, the area ratio was selected as the primary evaluation parameter.
Experimental Process
Original Design
In the initial trial production, the original stencil aperture design was as follows:
- Pad width set at a 1:1 ratio.
- Outer pad: 0.05 mm inner cut, 0.10 mm outer extension.
- Inner pad: Length maintained at 1:1.
- Aperture size: d – 0.12 mm.
- Outer pad: L – 0.562b – 0.2 mm, area ratio 0.66.
- Inner pad: L – 0.512b – 0.2 mm, area ratio 0.6.
Optimization Scheme 1
Aperture Design:
- Pad width set to 0.22 mm.
- Outer pad: 0.05 mm inner cut, 0.10 mm outer extension; inner pad length kept at 1:1.
- Aperture size: d – 0.12 mm.
- Outer pad: L – 0.562b – 0.22 mm, area ratio 0.66, with 10% more solder paste than the original design.
- Inner pad: L – 0.512b – 0.22 mm, area ratio 0.64, with 10% more solder paste than the original design.
Experimental Results:
After increasing the pad width by 0.02 mm, the outer pad maintained a good shape. However, X-ray images of the inner pad showed elliptical shadows, indicating excess solder that led to bridging.
Optimization Scheme 2
Aperture Design:
- Aperture dimensions remained the same as in Scheme 1, but the stencil thickness was reduced to 0.10 mm.
- Aperture size: d – 0.1 mm.
- Outer pad: L – 0.562b – 0.22 mm, area ratio 0.79, with 8.3% less solder paste than the original design.
- Inner pad: L – 0.512b – 0.22 mm, area ratio 0.77, with 8.3% less solder paste than the original design.
Experimental Results:
After reducing the stencil thickness, the inner pad still showed outward shadow expansion in X-ray images, with a risk of bridging. Although the thinner stencil improved solder paste release slightly, the paste volume did not decrease significantly compared to the 0.12 mm stencil due to better release performance. Nevertheless, the outer pad showed better printing results after increasing its width to 0.22 mm.
Optimization Scheme 3
Aperture Design:
- Pad width maintained at 0.22 mm.
- Outer pad: 0.05 mm inner cut, 0.10 mm outer extension.
- Inner pad: 0.06 mm inner cut.
- Aperture size: d – 0.12 mm.
- Outer pad: L – 0.562b – 0.22 mm, area ratio 0.66, with 10% more solder paste than the original design.
- Inner pad: L – 0.452b – 0.22 mm, area ratio 0.62, with 2.9% less solder paste than the original design.
Experimental Results:
During PCB assembly, considering the solder requirements of other components, the stencil thickness was kept at 0.12 mm. By increasing the inner cut of the inner pad to 0.06 mm, the solder paste volume was effectively reduced. X-ray images showed that the pad profile returned to a rectangular shape, eliminating the elliptical shadow, but excess solder was still observed at the inner ends, leading to a risk of bridging.
Optimization Scheme 4
Aperture Design:
- Pad width maintained at 0.22 mm.
- Outer pad: 0.05 mm inner cut, 0.10 mm outer extension.
- Inner pad: 0.09 mm inner cut.
- Aperture size: d – 0.12 mm.
- Outer pad: L – 0.562b – 0.22 mm, area ratio 0.66, with 10% more solder paste than the original design.
- Inner pad: L – 0.422b – 0.22 mm, area ratio 0.6, with 9.3% less solder paste than the original design.
Experimental Results:
Increasing the inner cut of the inner pad to 0.09 mm further reduced the solder paste volume. X-ray images showed improved pad profiles with fewer elliptical shadows and a reduced likelihood of bridging, although the problem was not completely eliminated.
Optimization Scheme 5 (Final Solution)
Aperture Design:
- Outer pad: 0.05 mm inner cut, 0.10 mm outer extension, width 0.22 mm.
- Inner pad: 0.09 mm inner cut, width reduced to 0.20 mm.
- Aperture size: d – 0.12 mm.
- Outer pad: L – 0.562b – 0.22 mm, area ratio 0.66, with 10% more solder paste than the original design.
- Inner pad: L – 0.422b – 0.2 mm, area ratio 0.57, with 17.6% less solder paste than the original design.
Experimental Results:
Reducing the inner pad width to 0.20 mm and maintaining the 0.09 mm inner cut optimized the solder paste volume for both inner and outer pads. X-ray images displayed stable pad profiles and good soldering results, achieving the required pass rate for mass production. The final scheme was deemed suitable for large-scale production and was recommended for use with ENIG (Electroless Nickel Immersion Gold) surface treatment to further improve soldering quality and reliability.
Recommendations for Dual Row QFN Soldering Optimization
Improve Printing Accuracy
Due to the small solder ends of Dual Row QFN chips, the shape and position accuracy of the printed solder paste are crucial. After the first printing, it is recommended to inspect under a microscope to evaluate the printing accuracy and adjust the stencil machine compensation parameters accordingly. Avoid relying on naked-eye observation, as inexperienced operators may misjudge, affecting the final results.
Key Printing Parameter Settings
Printing parameters such as squeegee speed, release speed, and release distance should be set based on the fine-pitch pads on the PCB. For Dual Row QFN soldering:
Reduce squeegee speed and release speed to minimize impact on the back of the stencil.
Prevent solder paste pulling or collapse and ensure uniform solder paste filling.
Special attention should be given to squeegee pressure control, as excessive pressure can cause solder paste leakage from the back of the stencil, leading to flux migration and the potential for solder bridging during reflow.
Inner Pad Solder Volume Control
The inner pads of Dual Row QFN are highly sensitive to solder volume, and slight deviations can lead to bridging or weak joints. It is recommended to adjust the inner pad aperture cut amount based on X-ray inspection data to ensure appropriate solder paste volume.
Surface Treatment Process Selection
As shown in the X-ray images, PCB treated with HASL (Hot Air Solder Leveling) often exhibit uneven tin heights, leading to uneven solder volume distribution during soldering. Since this cannot be adjusted during the PCBA process, ENIG (Electroless Nickel Immersion Gold) is recommended for Dual Row QFN PCB to ensure uniform pad flatness and even solder distribution, thereby improving soldering quality.
Ending
Dual Row QFN packages, with their higher pin density and superior performance, offer a more flexible solution for modern high-density PCB designs. However, their unique pad structure imposes stricter requirements on the soldering process, particularly in solder paste printing accuracy, pad aperture design, and soldering parameter control. The optimized soldering scheme significantly improved solder joint quality, effectively reducing the risk of bridging and weak joints, ensuring reliable results for mass production. As packaging technologies continue to evolve, further process improvements for more complex packaging forms will be key to enhancing the reliability of electronic products.
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