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BGA welding technology for OEM factory technicians
In today’s electronics manufacturing, the quality control of Surface Mount Technology (SMT) has become quite advanced. However, BGA (Ball Grid Array) soldering remains a critical challenge. The overall process capability of an OEM factory largely depends on the precision and reliability of BGA soldering. With the continuous evolution of electronic components—offering higher performance, smaller size, and more advanced features—SMT processes are constantly under pressure to adapt. These changes in component design and product form present ongoing challenges for manufacturers.
One specific type of chip that poses unique difficulties is the "flower ball pad bow chip." As electronic packages evolve toward higher density and thinner designs, these chips become smaller and lighter while offering more powerful functionality. This is especially true for high-performance CPU chips used in mobile devices and ultrabooks, where both miniaturization and high integration are essential. These chips often have thousands of solder joints, with pads designed to be very small—typically less than 0.35mm in diameter—and arranged irregularly. Additionally, these chips tend to experience significant warping of their material body, leading to what is known as the "flower ball pad bow" phenomenon. Figure 1 illustrates this issue.
Figure 1: Material body warping
During the SMT phase, several key control points must be carefully managed. These include the reflow profile settings, stencil opening design, and solder paste selection. First, ensuring good coplanarity after reflow is crucial. The chip and PCB must fit perfectly to avoid issues like relative warping. Second, controlling the stencil opening is vital to prevent problems such as bridging, the pillow effect (HoP), and non-wetting open joints (NWO). Lastly, selecting the right solder paste can significantly improve the yield rate.
2.1 Reflow Temperature Control
For conventional lead-free BGA, SAC305 or SAC405 solder balls are commonly used, with peak temperatures typically ranging between 228°C and 250°C. However, for flower ball pad bow chips, which have very thin substrates, excessive peak temperature can cause deformation at the corners, leading to HoP or NWO. Conversely, if the temperature is too low, warping may not be fully corrected, resulting in bridging or virtual soldering. Therefore, a moderate peak temperature is recommended, along with the use of a multi-zone reflow oven and a gentle temperature curve to reduce thermal shock.
2.2 Stencil Opening Adjustment
Due to the thinness of the package, the substrate warps during reflow, causing large relative displacement between the solder balls and the PCB pads. To compensate, the stencil opening needs to be adjusted. In areas where the package is severely warped, increasing the stencil aperture helps increase the solder paste volume, compensating for the lack of paste and ensuring good solder joint formation. In contrast, in areas where the PCB is compressed, reducing the aperture prevents excess paste from causing bridging. For other regions, a 1:1 aperture ratio is acceptable.
2.3 Solder Paste Selection
BGA soldering involves thermal deformation, which can affect the solder paste's performance. If the thermal expansion coefficient (CTE) of the carrier plate differs significantly from the chip or packaging material, it can lead to deformation. For small-diameter BGA balls (e.g., 0.35mm), the limited solder paste volume makes flux volatility a concern. A high-adhesion solder paste with strong flux activity is recommended. It ensures better adhesion during deformation and maintains flux activity, helping to prevent HoP and NWO.
3. Verification Process
When dealing with new or custom-designed components, suppliers often provide recommendations for temperature profiles and stencil openings. However, even with strict adherence to these guidelines, achieving satisfactory results is not guaranteed. Differences in product design, component configuration, equipment, and quality requirements necessitate on-site adjustments. Field engineers must independently verify and optimize process parameters based on real-world conditions.
3.1 Chip Data
The DPS product uses a flower ball pad bow chip, specifically the CORE I7-6600U 2C 4M 2.6G SR2F1 (Figure 2). The package size is 42 × 24 mm, and it uses a BGA1356 package. The BGA ball diameter is 0.35 mm, but the pad design varies in shape and layout, as shown in Table 1.
Figure 2: CORE I7-6600U chip original pad
3.2 Formulation and Implementation Plan
Based on the three main SMT control points for flower ball pad bow chips, a process parameter setting plan was developed:
First, adjusting the reflow temperature curve to address chip deformation. The peak temperature was set to 240°C, using a 12-zone reflow oven with a controlled heating and cooling rate of 2–3°C/sec.
Second, optimizing the stencil opening method to increase solder paste volume. A stepped stencil with 0.12mm thickness was used, with different apertures for IC components (PITCH=0.4mm) and 0201 RC components. The four corners and center areas were modified to ensure sufficient paste without causing bridging.
Third, selecting a high-adhesion solder paste with No. 4 powder particles and nano-film technology. The stencil time was kept within 2 hours to maintain flux activity.
After implementing the plan, the BGA and PCB achieved good coplanarity, and the solder joints were well-formed. No bridging or virtual soldering was observed, as seen in the X-RAY inspection images (Figures 4 and 5).
Figure 4: I7-6600U image local
Figure 5: I7-6600U image partial magnification
3.3 Post-Verification
Verification 1: When producing SOM products, the same type of chip (CORE I7-4650U) was used. The same stepped stencil and process were applied, and no virtual soldering was found after X-RAY inspection. Functional testing confirmed stable performance.
Verification 2: For XTDB products using Intel C612 FCBGA, similar pad designs were used. The same stencil opening scheme was implemented, and no soldering failures occurred.
Of course, achieving a stable process required multiple iterations and fine-tuning. However, the final process plan proved effective and remained consistent in direction.
**Conclusion**
In standard BGA soldering, solder paste selection and furnace temperature settings are key. For flower ball pad bow chips, additional attention is needed:
1. Control the reflow temperature to 240°C or above to ensure flatness and proper fitting.
2. Expand the stencil openings while avoiding bridging, especially around the BGA corners and inner rings.
3. Use a stepped stencil to manage solder paste transfer and avoid bridging.
4. Monitor the solder paste exposure time to maintain flux activity.
5. Ensure the PCB is well-supported to prevent deformation during SMT.
This analysis is shared with industry peers, aiming to enhance production quality through technical collaboration. Due to limitations in detection equipment and personal expertise, there may be gaps in this content. We welcome feedback and corrections from experts.